Stanford Research Systems SIM918 Operating And Service Manual

Download

Operation and Service Manual

Stanford Research Systems

Precision Current Preampliﬁer

SIM918

Revision 1.8 • December 13, 2006

Certiﬁcation

Stanford Research Systems certiﬁes that this product met its published speciﬁcations at the time of shipment.

Warranty

This Stanford Research Systems product is warranted against defects in materials and workmanship for a period of one (1) year from the date of shipment.

Service

For warranty service or repair, this product must be returned to a Stanford Research Systems authorized service facility. Contact Stanford Research Systems or an authorized representative before returning this product for repair.

Information in this document is subject to change without notice.

Stanford Research Systems, Inc. 1290–D Reamwood Avenue Sunnyvale, CA 94089 USA Phone: (408) 744-9040 • Fax: (408) 744-9049

www.thinkSRS.com • e-mail: info@thinkSRS.com

Printed in U.S.A. Document number 9-01592-903

SIM918 Precision Current Preampliﬁer

Contents

General Information iii

Safety and Preparation for Use . . . . . . . . . . . . . . . . iii

Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Speciﬁcations . . . . . . . . . . . . . . . . . . . . . . . . . . vii

1 Getting Started 1 – 1

1.1 Introduction to the Instrument . . . . . . . . . . . . . 1 – 2

1.2 Front-Panel Operation . . . . . . . . . . . . . . . . . . 1 – 5

1.3 Connections . . . . . . . . . . . . . . . . . . . . . . . . 1 – 8

1.4 Power-On . . . . . . . . . . . . . . . . . . . . . . . . . 1 –9

1.5 Restoring the Default Conﬁguration . . . . . . . . . . 1 – 9

1.6 SIM Interface . . . . . . . . . . . . . . . . . . . . . . . . 1 – 10

2 Description of Operation 2 – 1

2.1 About Transimpedance Ampliﬁers . . . . . . . . . . . 2 – 2

2.2 Bias and Ground . . . . . . . . . . . . . . . . . . . . . 2– 4

2.3 Output . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 – 5

2.4 Autozero Trim . . . . . . . . . . . . . . . . . . . . . . . 2 – 5

2.5 Phase-Locked Loop . . . . . . . . . . . . . . . . . . . . 2 – 6

2.6 Autocalibration . . . . . . . . . . . . . . . . . . . . . . 2– 6

2.7 Clock Stopping . . . . . . . . . . . . . . . . . . . . . . 2 – 7

2.8 Quiescent Operation . . . . . . . . . . . . . . . . . . . 2 – 8

3 Remote Operation 3 – 1

3.1 Index of Common Commands . . . . . . . . . . . . . . 3 – 2

3.2 Alphabetic List of Commands . . . . . . . . . . . . . . 3– 4

3.3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3– 6

3.4 Commands . . . . . . . . . . . . . . . . . . . . . . . . . 3 – 7

3.5 Status Model . . . . . . . . . . . . . . . . . . . . . . . . 3 – 24

4 Circuit Description 4 – 1

4.1 Schematic Diagrams . . . . . . . . . . . . . . . . . . . 4 – 2

A Index A – 1

ii Contents

SIM918 Precision Current Preampliﬁer

General Information

The SIM918 Precision Current Preampliﬁer, part of Stanford Research Systems’ Small Instrumentation Modules family, converts an input electric current into a proportional voltage output while maintaining zero potential diﬀerence between the input terminal and a bias terminal.

The main ampliﬁer stage presents a transimpedance RF, equal to the current gain of the preampliﬁer, to an input current iin. The bias voltage V voltage at the output of the instrument is

Safety and Preparation for Use

is subtracted from the output of the stage, so that the

bias

out

= (V

− iin× RF) − V

bias

= −iin× RF.

bias

Connections

WARNING

CAUTION

Biomedical applications

WARNING

No dangerous voltages are generated by the module. However, the outer shield of the front-panel Input coaxial (BNC) connector in the SIM918 can be switched to the rear-panel Program input. If a dangerous voltage is applied to the Program terminal, it may be present on the outer shell of the Input connector, and may cause injury or death.

Do not exceed ±60 volts to the Earth at the center terminal of the rear-panel Shield Program Voltage BNC connector.

Do not exceed ±15 volts to the Earth at the center terminal of the front-panel Input and Bias BNC connectors, or at the center terminal of the rear-panel Ref Clock Sync BNC connector.

Under certain conditions, the SIM918 may prove to be unsafe for applications involving human subjects. Incorrect grounding, component failure, and excessive common-mode input voltages are examples of conditions in which the instrument may expose the subject

iii

iv General Information

to large input currents. Therefore, Stanford Research Systems does not recommend the SIM918 for such applications.

Caution regarding use with photomultipliers

CAUTION

Service

Preparation for use

The front-end ampliﬁer of this instrument is easily damaged if a photomultiplier is used improperly with the preampliﬁer. When left completely unterminated, a cable connected to a PMT can charge to several hundred volts in a relatively short time. If this cable is connected to the curent input of the SIM918, the stored charge may damage the front-end JFET. To avoid this problem, provide a leakage path of about 100 kΩ to ground inside the base of the PMT to prevent charge accumulation.

Do not install substitute parts or perform unauthorized modiﬁcations to this instrument.

The SIM918 is a single-wide module designed to be used inside the SIM900 Mainframe. Do not turn on the power to the mainframe or apply voltage or current inputs to the module until the module is completely inserted into the mainframe and locked in place.

SIM918 Precision Current Preampliﬁer

General Information v

SIM918 Precision Current Preampliﬁer

vi General Information

Symbol Description

Alternating current

Caution - risk of electric shock

Frame or chassis terminal

Caution - refer to accompanying documents

Earth (ground) terminal

Battery

Fuse

On (supply)

Off (supply)

Symbols you may Find on SRS Products

SIM918 Precision Current Preampliﬁer

General Information vii

Notation

The following notation will be used throughout this manual:

WARNING

CAUTION

A warning means that injury or death is possible if the instructions are not obeyed.

A caution means that damage to the instrument or other equipment is possible.

Typesetting conventions used in this manual are:

• Front-panel buttons are set as [GAIN ]; [GAIN ] is shorthand for “[GAIN ] & [GAIN ]”.

• Front-panel indicators are set as OVLD.

• Signal names are set as ¬STATUS.

• Signal levels are set as HIGH.

• Remote command names are set as *IDN? .

• Literal text other than command names is set as OFF.

• Special ASCII characters are set as hCRi.

Remote command examples will all be set in monospaced font. In these examples, data sent by the host computer to the SIM918 are set as straight teletype font, while responses received by the host computer from the SIM918 are set as slanted teletype font.

SIM918 Precision Current Preampliﬁer

viii General Information

Speciﬁcations

Performance characteristics

Min Typ Max Units

Gain Selection 106, 107, 10

Accuracy, 106V/A ±0.1 %

107V/A ±0.1 % 108V/A ±1 %

Stability, 106V/A ±10 ppm/◦C

107V/A ±50 ppm/◦C 108V/A ±100 ppm/◦C

Current input Selection On, open

Oﬀset voltage [1–3] ±10 µV

Resistance 1 Ω

Capacitance 18 pF

Bias current, DC [3] 0.5 2.0 pA

AC [1, 4] 3.5 pA rms

Current noise at 1 kHz [5], 106V/A 130 fA/√Hz

107V/A 42 fA/√Hz 108V/A 15 fA/√Hz

Voltage noise [1, 4] 25 µV rms

−3 dB bandwidth [5], 106V/A 22 kHz 107V/A 12 kHz 108V/A 4 kHz

Terminals Isolated BNC [6]

BNC shield Ground, bias, program /open

V/A

Bias input Selection On, ground

Program input Voltage −60 +60 V

Reference clock sync Selection Input, output

Voltage [7] −5.0 +5.0 V

Resistance 10 MΩ

−3 dB bandwidth 0.2 Hz Terminals Isolated BNC [6]

BNC shield Ground, ﬂoat

Resistance 3 GΩ

Terminals Grounded BNC [8], rear

Interface Rear BNC [8], TTL [9]

Input frequency [10] 0.90 1.10 Hz

Output frequency 1.0 Hz

SIM918 Precision Current Preampliﬁer

General Information ix

Min Typ Max Units

Autozero Selection On, hold

Source Internal, external reference clock

Switching frequency 0.50 Hz

Output Voltage [7] −10.0 +10.0 V

Maximum current ±100 mA

Short circuit duration Indeﬁnite

Resistance 100 Ω

Oﬀset voltage [2] ±50 µV

Common-mode rejection, DC 80 dB

Terminals Grounded BNC [6]

Operating Temperature [11] 0 40

◦

Power +5, ±15 V DC

Supply current, +5 V 100 mA

±15 V 150 mA

Conditions:

[1] With autozero on. [2] Following an autocalibration at (23 ± 5)◦C within 24 hours. [3] 100 s average. [4] 0.1 Hz to 10 Hz. [5] For a 100 pF source capacitance and an inﬁnite source resistance.

Higher values of source capacitance or a ﬁnite source resistance will

degrade these speciﬁcations. [6] Amphenol 31–10–4052 or similar. [7] An overload will be detected and the instrument is not guaranteed

to perform properly if these limits are exceeded, or if |V

exceeds the limits. Continuous application of a bias voltage V

excess of ±15 V will damage the instrument. [8] Tyco 227169–4 or similar. [9] Rising-edge sensitive.

[10] External reference clock capture range. The instrument is not guar-

anteed to perform properly if these limits are exceeded.

[11] Non-condensing.

−iin×RF|

bias

General characteristics

Dimensions 1.500W × 3.600H × 7.000D

SIM918 Precision Current Preampliﬁer

Interface Serial (RS–232) through SIM interface

Connectors BNC (3 front [4], 2 rear [6]); DB–15 (male) SIM interface

Weight 1.7 lbs

x General Information

SIM918 Precision Current Preampliﬁer

1 Getting Started

In This Chapter

This chapter gives you the necessary information to get started quickly with your SIM918 Precision Current Preampliﬁer.

1.1 Introduction to the Instrument . . . . . . . . . . . . . 1– 2

1.1.1 Current ampliﬁers and autozero . . . . . . . . 1 – 2

1.1.2 Clocks . . . . . . . . . . . . . . . . . . . . . . . 1 – 2

1.1.3 Cabling and grounding . . . . . . . . . . . . . 1–3

1.1.4 Autocalibration . . . . . . . . . . . . . . . . . . 1– 3

1.1.5 Remote interface and status . . . . . . . . . . . 1– 3

1.1.6 Block diagram . . . . . . . . . . . . . . . . . . . 1 – 4

1.1.7 Front and rear panels . . . . . . . . . . . . . . . 1 – 5

1.2 Front-Panel Operation . . . . . . . . . . . . . . . . . . 1 – 5

1.2.1 Gain . . . . . . . . . . . . . . . . . . . . . . . . 1–5

1.2.2 Autozero . . . . . . . . . . . . . . . . . . . . . . 1– 5

1.2.3 Input . . . . . . . . . . . . . . . . . . . . . . . . 1 – 7

1.2.4 Bias . . . . . . . . . . . . . . . . . . . . . . . . . 1 – 7

1.2.5 Output overload . . . . . . . . . . . . . . . . . 1 – 8

1.3 Connections . . . . . . . . . . . . . . . . . . . . . . . . 1 – 8

1.4 Power-On . . . . . . . . . . . . . . . . . . . . . . . . . 1 – 9

1.5 Restoring the Default Conﬁguration . . . . . . . . . 1 – 9

1.6 SIM Interface . . . . . . . . . . . . . . . . . . . . . . . 1 – 10

1.6.1 SIM interface connector . . . . . . . . . . . . . 1 – 10

1.6.2 Direct interfacing . . . . . . . . . . . . . . . . . 1– 10

1 – 1

1 – 2 Getting Started

1.1 Introduction to the Instrument

1.1.1 Current ampliﬁers and autozero

A current, or transimpedance, ampliﬁer converts electric current into a proportional output voltage. Unlike a simple resistor, the ampliﬁer presents a low-impedance terminal to the input current. In the SIM918 Precision Current Preampliﬁer, the electric potential of the input terminal, Vin, is accurately made equal to the user-provided potential at the bias terminal, V nitude of the resulting input oﬀset voltage is nearly zero:

= Vin− V

ofs

In all transimpedance ampliﬁers, the input potential is kept near that of the bias through the action of negative feedback. When the bias voltage is at ground, the input terminal is often said to present a virtual ground, or a virtual null. Without autozeroing, this vir-virtual ground tual ground drifts, in some cases by many millivolts. This error in the electric potential of the input terminal may be unacceptable in precision measurements.

bias

, or to ground. The absolute mag-oﬀset voltage

, |V

| < 10 µV.

ofs

1.1.2 Clocks

In the SIM918, an autozero circuit measures V

every 2 seconds and

ofs

makes the adjustment necessary to keep the oﬀset voltage at zero. The autozero feature can be engaged or inhibited remotely or from the front panel, giving the user ﬂexibility in sensitive applications. With autozero inhibited, the preampliﬁer retains microvolt input accuracy for many hours. When engaged, it takes the autozero only a few cycles of a reference clock to restore the oﬀset to within its speciﬁed limits.

The gain, or transimpedance, of the preampliﬁer can be set to RF= 106, 107, or 108V/A, remotely and from the front panel. Along with voltage accuracy, the SIM918 oﬀers a low input bias current and a current noise that is close to the lower limit imposed by the Johnson noise of the transimpedance.

The autozero circuit switches between measuring the input oﬀset voltage, and the oﬀset voltage of the zeroing ampliﬁer itself, at one half the frequency of an internal or external reference clock. The in-reference clock ternal clock signal (typically 1.0 pulse per second, pps, i.e. 1.0 Hz)one pps can be selected, remotely or from the front panel, to be output on

In the unfortunate but established terminology of electronics, the word bias conveys diﬀerent meanings. The bias current is the input current present in the instrument in the absence of a current from an external source.

SIM918 Precision Current Preampliﬁer

1.1 Introduction to the Instrument 1 – 3

a rear-panel connector. Alternatively, the same connector can be used to input a clock signal at (1.0 ± 10%) pps (i.e. 1.10 Hz–0.90 Hz), synchronizing the switching to an external source.

The reference clock in the SIM918 operates independently of the oscillator used to clock the digital control circuitry. The latter is designed with a special clock-stopping architecture. The microcontroller is turned on only in the following cases: when the settings are being changed; autozero is turned on or oﬀ; and during autocalibration, remote communications, or when an overload condition or an external reference clock event occurs. This guarantees that no digital noise contaminates low-level analog signals.

With autozero oﬀ and in the absense of an external clock input, the preampliﬁer enters a completely quiescent state: no reference clock transitions are present that can disturb the measurement of a low-level electric current.

1.1.3 Cabling and grounding

The SIM918 provides maximum ﬂexibility for cabling and grounding. The input connection can be opened, and the bias voltage can be connected to signal ground.

The shield of the Input BNC can be switched between signal ground, the bias voltage, or the rear-panel Program input (which can be left ﬂoating, if desired). With the Program input, a user can supply an excitation potential to an experiment via the shield conductor of the input cable, while the excited current ﬂows through the center conductor to the SIM918. The shield of the Bias BNC can be independently grounded or ﬂoated.

The input and bias selections, and those of their shields, can be made via the push of a front-panel button or remotely.

1.1.4 Autocalibration

A user-commanded autocalibration procedure allows one to eliminate the eﬀects of thermal drifts in the autozero circuit, and to reduce output oﬀset voltage.

1.1.5 Remote interface and status

A remote computer can access the module through the SIM900 Main-remote interface frame, using RS–232 or GPIB. All instrument settings can be queried via the remote interface. The SIM918 can be operated outside the SIM900 Mainframe by powering it with its required DC voltages.

SIM918 Precision Current Preampliﬁer

1 – 4 Getting Started

LPF

0.5 Hz

(±1)

INPUT

OUTPUT

BIAS

−

Main amp

Difference amp

Zeroing amp

offset adj

If the maximum bias voltage is exceeded, or the chosen gain setting causes the output voltage to exceed its maximum, the appropriate overload LED turns on. If the module cannot lock to an external reference clock signal, an LED indicates an unlocked state. If armed, the module also generates a status signal to alert the user of the overload or unlocked condition.

1.1.6 Block diagram

The output of the main ampliﬁer (transimpedance stage) is referenced to the bias voltage. A diﬀerence ampliﬁer subtracts the bias voltage, so the output of the instrument is directly proportional to the input current iinand the gain RF:

out

= (V

− iin× RF) − V

bias

= −iin× RF. (1.1)

bias

A block diagram of the preampliﬁer is shown below in Figure 1.1.

Figure 1.1: The SIM918 block diagram.

SIM918 Precision Current Preampliﬁer

1.2 Front-Panel Operation 1 – 5

1.1.7 Front and rear panels

1.2 Front-Panel Operation

1.2.1 Gain

The gain RFof the preampliﬁer, in volts per microampere, is indicated on the front panel of the instrument via a green annunciator LED. Press one of the [GAIN ] buttons to change the gain. If [GAIN ] is pressed when RF= 1 V/µA, the press has no eﬀect. If [GAIN ] is pressed when RF= 100 V/µA, the press has no eﬀect.

A simultaneous press of [GAIN ] has a special meaning. This press initiates autocalibration (Section 2.6).

1.2.2 Autozero

1.2.2.1 Engaging the autozero circuit

The autozero circuit is turned ON by the press of a front-panel button. There will be a pause of up to 3.3 seconds (a wait for a positive-going edge of the reference clock). At the end of the pause, the green

Note the minus sign in Eq. (1.1); the output voltage is positive for a current that ﬂows out of the input terminal.

Figure 1.2: The SIM918 front and rear panels.

SIM918 Precision Current Preampliﬁer

1 – 6 Getting Started

annunciator LED will turn on and the zeroing circuit will become active.

The same button turns autozeroing oﬀ. There will be a less than 1 s pause in order for the present control output of the autozero circuit to be sampled and stored. At the end of the pause, the LED indicator will turn oﬀ and all switching inside the SIM918 will cease. The sampled control output (trim) will remain applied to the transimpedancestage ampliﬁer, zeroing it to the best of precision available at the time the autozero circuitry is inhibited.

1.2.2.2 Reference clock detection

The autozero circuit switches at one half the frequency of an internal or external reference clock. If a periodic TTL-level signal is applied to the rear-panel Ref Clock Sync connector, and the connector is not selected for output (see the next section), the preampliﬁer will recognize the external clock and attempt to lock to the signal. The green External LED will illuminate for the duration of the external clock input.

1.2.2.3 Output 1 pps sync

If the frequency of the external clock is stable and is between 0.90 Hzcapture range and 1.10 Hz, the module will successfully lock to the signal. It typ-lock acquisition time ically takes 250 s (just over 4 minutes) to acquire a lock. The yellow Unlocked LED is illuminated whenever the SIM918 is not in a locked state. For further discussion of locking, see Chapter 2.5.

For the duration of an unlocked state, the switches in the autozero circuit are not guaranteed to have correct duty cycles. Therefore, the speciﬁed input oﬀset accuracy is not guaranteed while Unlocked.

The internal reference clock is used when an external clock signal is not present. In this state, neither the External LED nor the Un- locked LED is illuminated.

The rear-panel Ref Clock Sync connector can be used to output the internal reference clock. The signal at the output is TTL, typically at 1.0 Hz (1.0 pps). The [Output 1 pps sync] button toggles the direction of the signal at the rear-panel connector. The output direction is indicated by a green LED. An inactive Output 1 pps sync indicates that the connector may be used to input an external clock.

If [Output 1 pps sync] is pressed while an external reference clock signal is present at the connector, clock output will fail and a DeviceDependent Error (Section 3.5.3) will be issued. If an external signal is applied to the Ref Clock Sync terminal while the connector is selected for output, the external signal will not be recognized.

SIM918 Precision Current Preampliﬁer

1.2 Front-Panel Operation 1 – 7

1.2.3 Input

The [INPUT Open] button opens and closes a relay in the path of the input current. A green LED indicates a disconnected input terminal. The input capacitance of the SIM918 is at its lowest with input open, and is speciﬁed in the table on Page viii.

1.2.3.1 Input shield

Successive presses of the [INPUT Shield] button connect the outer shell of the Input BNC to the rear-panel Shield Program Voltage terminal, a buﬀered copy of the bias voltage, and to signal ground. The state of the input shield connection is indicated by one of three LEDs: the yellow INPUT Shield Prog, the yellow INPUT Shield Bias, or the green INPUT Shield GND.

To ﬂoat the shield of the Input connector, leave the Shield Program Voltage BNC open and select INPUT Shield Prog.

1.2.4 Bias

1.2.4.1 Bias overload

The [BIAS GND] button toggles the source of the bias between the voltage at the center terminal of the Bias BNC and the signal ground of the instrument. If the bias source is set to ground, the green BIAS GND light is on.

With Bias grounded, the diﬀerence ampliﬁer (Figure 1.1) is switched out and the output of the instrument is taken directly from the transimpedance stage. With this conﬁguration, there is no common-mode error and the output-oﬀset error is reduced.

When Bias is connected to a user voltage, the voltage is buﬀered internally before being distributed to other parts of the preampliﬁer. The oﬀset error of the bias buﬀer is included in the input oﬀset accuracy speciﬁcations in the table on Page viii.

An overload condition is recognized and the BIAS OVLD LED is activated if the absolute value of the voltage applied to the Bias input exceeds certain limits. These limits are typically ±5.0 V, andbias overload limits are between

−5.2 V ≤ V

≤ −4.9 V, 4.9 V ≤ V

min

max

≤ 5.2 V.

SIM918 Precision Current Preampliﬁer

The overload LED stays on for a minimum of 50 ms; after this time it turns oﬀ if the overload condition has ceased.

1 – 8 Getting Started

1.2.4.2 Bias shield

Successive presses of the [BIAS Shield] button ﬂoat the outer shell of the Bias BNC and connect it to ground. The state of the bias shield connection is indicated by one of two LEDs: the yellow BIAS Shield Float or the green BIAS Shield GND.

Note that it is the electric potential at the Bias terminal, not the potential diﬀerence across the Bias connector, that the autozero circuit uses as the reference for the input voltage.

1.2.5 Output overload

An overload condition is recognized and the OUTPUT OVLD LED is activated if the absolute value |iin×RF|exceeds certain limits. These limits are typically ±10.0 V, and are betweenoutput overload limits

1.3 Connections

Panel BNC Terminal Signal Direction

Front Input Center Input current Input

Rear Shield Program Voltage Center Shield program voltage Input

−10.4 V ≤ V

≤ −9.9 V, 9.9 V ≤ V

min

max

≤ 10.4 V.

The overloaded state is also recognized, and OUTPUT OVLD activated, if the raw output of the transimpedance stage, |V

−iin×RF|,

bias

exceeds these limits. To distinguish between the two output overload possibilities, use the OVLD? query. The overload LED stays on for a minimum of 50 ms; after this time it turns oﬀ if the overload condition has ceased.

There are ﬁve BNC connectors in the SIM918, three on the front panel and two at the rear.

Shield Shield program voltage, Output

bias voltage, signal ground

Bias Center Bias voltage Input

Shield Float, power ground

Output Center Output voltage Output

Shield Signal ground Output

Shield Chassis ground

Ref Clock Sync Center Reference clock Input, output

Shield Chassis ground Input, output

Table 1.1: BNC connections in the SIM918.

For a further discussion of grounding, see Section 2.2.1. The SIM interface connector is discussed in Section 1.6.1.

SIM918 Precision Current Preampliﬁer

1.4 Power-On 1 – 9

1.4 Power-On

The instrument retains the following settings in non-volatile memory:

1. The power line frequency (FPLC): 60 Hz or 50 Hz.

2. The gain.

3. Autozero on/oﬀ.

4. Input selection (on, open).

5. Input shield selection (program, bias, ground.)

6. Bias selection (on, ground).

7. Bias shield selection (ﬂoat, ground.)

8. Whether or not the phase-locked loop (Section 2.5) stays active when autozero is oﬀ.

9. Calibration values.

The power-on conﬁguration of the remote interface is detailed in Section 3.3.1.

1.5 Restoring the Default Conﬁguration

The default conﬁguration of the SIM918 is:

1. Gain 106V/A.

2. Autozero on.

3. Input connected.

4. Input shield at ground.

5. Bias at ground.

6. Bias shield at ground.

7. Reference clock direction is input.

8. The phase-locked loop (Section 2.5) is inactive when autozero is oﬀ.

To reset the module into this conﬁguration, turn the SIM900 Mainframe power on while holding a front-panel button of the SIM918 for at least 2.0 seconds. The same conﬁguration can also be reached from the remote interface by issuing the *RST command.

SIM918 Precision Current Preampliﬁer

FPLC equals the principal rejection frequency of an internal analog-to-digital converter used to measure the input oﬀset trim. See the command OFST.

1 – 10 Getting Started

1.6 SIM Interface

The primary connection to the SIM918 Precision Current Preampliﬁer is the rear-panel DB–15 SIM interface connector. Typically, the SIM918 is mated to a SIM900 Mainframe via this connection, either through one of the internal mainframe slots or the remote cable interface.

It is also possible to operate the SIM918 directly, without using the SIM900 Mainframe. This section provides details on the interface.

1.6.1 SIM interface connector

The DB–15 SIM interface connector carries all the power and communication lines to the instrument. The connector signals are speciﬁed in Table 1.2.

Direction

Pin Signal Src ⇒ Dest Description

1 SIGNAL GND MF ⇒ SIM Signal ground 2 ¬STATUS SIM ⇒ MF Status/service request (GND = asserted, +5 V= idle) 3 RTS MF ⇒ SIM HW handshake (unused in SIM918) 4 CTS SIM ⇒ MF HW handshake (unused in SIM918) 5 ¬REF 10MHZ MF ⇒ SIM 10 MHz reference (no connection in SIM918) 6 −5V MF ⇒ SIM Power supply (no connection in SIM918) 7 −15V MF ⇒ SIM Power supply 8 PS RTN MF ⇒ SIM Power ground

9 CHASSIS GND Chassis ground 10 TXD MF ⇒ SIM Async data (start bit = “0”= +5 V; “1” = GND) 11 RXD SIM ⇒ MF Async data (start bit = “0”= +5 V; “1” = GND) 12 REF 10MHZ MF ⇒ SIM 10 MHz reference (no connection in SIM918) 13 +5V MF ⇒ SIM Power supply 14 +15V MF ⇒ SIM Power supply 15 +24V MF ⇒ SIM Power supply (no connection in SIM918)

1.6.2 Direct interfacing

Table 1.2: SIM interface connector pin assignments, DB–15.

The SIM918 is intended for operation in the SIM900 Mainframe, but users may wish to directly interface the module to their own systems without the use of additional hardware.

The mating connector needed is a standard DB–15 receptacle, such as Tyco part number 747909–2 (or equivalent). Clean, well-regulated supply voltages of ±15.0 V DC, +5.0 V DC must be provided, following the pinout speciﬁed in Table 1.2 and the minimum currents in the table on Page ix. Ground must be provided on Pins 1 and 8, with chassis ground on Pin 9. The ¬STATUS signal may be monitored

SIM918 Precision Current Preampliﬁer

1.6 SIM Interface 1 – 11

on Pin 2 for a low-going TTL-compatible output indicating a status message. See Section 3.5 for the description of status messages.

CAUTION

The SIM918 has no internal protection against reverse polarity, missing supply, or overvoltage on the +5 V and the ±15 V power-supply pins. Supply voltages above 5.5 V on Pin 13, above +16 V on Pin 14, or below −16 V on Pin 7 are likely to damage the instrument. SRS recommends using the SIM918 together with the SIM900 Mainframe for most applications.

1.6.2.1 Direct interface cabling

If the user intends to directly wire the SIM918 independent of the SIM900 Mainframe, communication is usually possible by directly connecting the appropriate interface lines from the SIM918 DB–15 plug to the RS–232 serial port of a personal computer.4Connect RXD from the SIM918 directly to RxD on the PC, TXD directly to TxD. In other words, a null-modem-style cable is not needed.

To interface directly to the DB–9 male (DTE) RS–232 port typically found on contemporary personal computers, a cable must be made with a female DB–15 socket to mate with the SIM918, and a female DB–9 socket to mate with the PC’s serial port. Separate leads from the DB–15 need to go to the power supply, making what is sometimes know as a “hydra” cable. The pin connections are given in Table 1.3.

DB–15/F to SIM918 Name

DB–9/F

10 ←→ 3 TxD 11 ←→ 2 RxD

5 Computer Ground

to Power Supply

7 ←→ −15 V DC 13 ←→ +5 V DC 14 ←→ +15 V DC

1 ←→ Signal Ground (separate wire to Ground)

8 ←→ Power Ground (separate wire to Ground)

9 ←→ Chassis Ground (separate wire to Ground)

Table 1.3: SIM918 direct interface cable pin assignments.

The distinct Signal Ground and Power Ground, and the chassisnote about grounds ground, are not directly connected within the SIM918. The power

Although the serial interface lines on the DB–15 do not satisfy the minimum voltage levels of the RS–232 standard, these lines are typically compatible with desktop personal computers.

SIM918 Precision Current Preampliﬁer

1 – 12 Getting Started

ground carries the return currents of digital control signals, powerintensive analog ampliﬁers, and the power supplies, whereas the output voltage references to a stable signal ground (Section 2.2.1). When operating in the SIM900, the three grounds are tied together in the SIM900 Mainframe. Signal Ground and Power Ground are connected through back-to-back Schottky diodes, so they cannot be more than ∼ ±0.35 V apart. The three ground lines should be separately wired to a single, low-impedance ground source at the power supply.

1.6.2.2 Serial settings

The initial serial port settings at power-on are: baud rate 9600, 8 bits, no parity, 1 stop bit, and no ﬂow control. The baud rate of the SIM918 cannot be changed. Flow control is not implemented in the SIM918. The parity may be changed with the PARI command.

SIM918 Precision Current Preampliﬁer

2 Description of Operation

This chapter provides a number of additional details of the operation of the SIM918.

In This Chapter

2.1 About Transimpedance Ampliﬁers . . . . . . . . . . 2 – 2

2.2 Bias and Ground . . . . . . . . . . . . . . . . . . . . . 2– 4

2.3 Output . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 – 5

2.4 Autozero Trim . . . . . . . . . . . . . . . . . . . . . . . 2 – 5

2.5 Phase-Locked Loop . . . . . . . . . . . . . . . . . . . . 2 – 6

2.6 Autocalibration . . . . . . . . . . . . . . . . . . . . . . 2 – 6

2.7 Clock Stopping . . . . . . . . . . . . . . . . . . . . . . 2 – 7

2.8 Quiescent Operation . . . . . . . . . . . . . . . . . . . 2 – 8

2.1.1 Input capacitance and stability . . . . . . . . . 2– 2

2.1.2 Choosing the right gain . . . . . . . . . . . . . 2 – 3

2.2.1 Grounds . . . . . . . . . . . . . . . . . . . . . . 2 – 4

2.2.2 Bias . . . . . . . . . . . . . . . . . . . . . . . . . 2 – 4

2 – 1

2 – 2 Description of Operation

(F)

out, TA

Bias

Input BNC

Op-amp

2.1 About Transimpedance Ampliﬁers

A transimpedance ampliﬁer is an operational ampliﬁer with a resistortransimpedance ampliﬁer in the feedback path. More generally, the feedback impedance ZFalways has a resistive and a reactive component. The two inputs to the operational ampliﬁer have a very high impedance; and the feedback in the ampliﬁer acts to keep the two inputs at the same electric potential. Therefore, the input current iinis forced through the feedback impedance, and the ampliﬁer produces an output voltage

The ampliﬁer acts to translate the input current into a proportional output voltage, with the feedback impedance being the coeﬃcient of proportionality. Hence the term transimpedance.

The main ampliﬁer of the SIM918 Precision Current Preampliﬁer is a transimpedance ampliﬁer based on a composite, JFET-input design.

2.1.1 Input capacitance and stability

The input impedance of JFET devices is extremely large. However, the impedance experienced, as a whole, by a current input to the feedback ampliﬁer (i.e. the change in the input voltage1divided by the input current) has two other, much more signiﬁcant contributions in parallel. The ﬁrst one is the eﬀect of the output on the input via feedback due to ﬁnite open-loop voltage gain of the operational ampliﬁer, and the second one is input capacitance.

out, TA

= V

− iin× ZF.

bias

Figure 2.1: A transimpedance ampliﬁer.

The ﬁrst term equals the feedback impedance divided by the openloop gain of the operational ampliﬁer2:

(F)

= ZF/A

Without the action of the autozero circuit.

Within the current bandwidth of the transimpedance ampliﬁer.

(2.1)

SIM918 Precision Current Preampliﬁer

2.1 About Transimpedance Ampliﬁers 2 – 3

and is below 1 Ω at DC. The open-loop gain is very high at DC, and typically decreases as inverse frequency with a corresponding 90 phase lag:

AOL= −j fT/ f

where fTis the gain-bandwidth product of the operational ampliﬁer (∼ 10 MHz in the SIM918). Substituting into Equation (2.1), observe

= j

(F)

If the feedback element is purely resistive, the term due to ZFbehaves exactly as if a large inductor were connected between the input and the bias terminals. This contribution to the input impedance increases linearly with frequency, with a corresponding 90◦phase

lead.

The input terminal always has some amount of parasitic capacitance to the bias terminal, and to ground. The input capacitance speciﬁcation in the table on Page viii applies for no cable connected to the Input BNC. A coaxial cable adds ∼ 100 pF/m of capacitance. Even one meter of coax will dramatically increase the input capacitance.

The input capacitance, in parallel with the eﬀective inductance, can form a resonant tank circuit if the capacitance is large enough that the resulting resonant frequency lies within the bandwidth of the transimpedance ampliﬁer. The SIM918 has adequate compensation (feedback capacitance) to prevent oscillation for up to 100 pF of additional input capacitance.4To avoid a feedback oscillation, follow these steps:

◦

1. Place the preampliﬁer as close as possible to the signal being measured, and use the shortest cable length necessary to connect them.

2. Reduce all stray capacitance to bias or ground at the output of the experiment under measurement.

3. Use a lower gain setting, which reduces the eﬀective input inductance.

Other detrimental eﬀects of excess input capacitance include reduced current bandwidth, poor step response (overshoot and ringing), and increased output noise.

2.1.2 Choosing the right gain

It is important to consider the output resistance (in general, impedance) of the current source being measured. The transimpedance

The input voltage leads the input current.

Longer cable lengths are tolerated at lower gain settings.

SIM918 Precision Current Preampliﬁer

2 – 4 Description of Operation

2.2 Bias and Ground

2.2.1 Grounds

stage ampliﬁes its input voltage noise by the factor5of (1+RF/R where R

is the source resistance. This noise adds in quadrature

source

with the current noise of the stage.To prevent the voltage noise term from dominating the overall output noise, set the current gain to

RF. R

source

The output of the SIM918 is referenced to ground. To maintain the DC accuracy of the instrument, there are two separate ground references. Power Ground (Pin 8 of the SIM interface connector) provides a current return path for digital control signals, powerintensive analog ampliﬁers, and the power supplies. Signal Ground (Pin 1 of the interface connector) serves as the reference point for analog voltages. The outer shell of the Output BNC connector is tied to Signal Ground. The output current of the preampliﬁer returns to the power supply through Signal Ground. When INPUT Shield GND is selected, the shell of the Input BNC is also tied to Signal Ground.

2.2.2 Bias

The outer shells of the rear-panel Shield Program Voltage and Ref Clock Sync BNCs are connected to chassis ground, Pin 9 of the DB–15 SIM interface connector. The separate power, signal, and chassis grounds are not directly connected within the preampliﬁer. When operating in the SIM900 Mainframe, the three grounds are tied together inside the mainframe, and through the mainframe to the Earth. The signal and power grounds are connected inside the SIM918 through back-to-back Schottky diodes, so they cannot be more than ∼ ±0.35 V apart.

The bias potential is received by an ultralow-oﬀset voltage buﬀer. It is this buﬀered voltage that appears on the shield of the Input BNC when INPUT Shield Bias is selected.

The shield of the Bias connector is not used as a reference for the bias voltage. When Bias Shield GND is selected, the shield is at Signal Ground. The limits on the bias voltage in Section 1.2.4.1 are relative to this ground. A voltage exceeding these limits by more than 1 V will be clamped , through diodes, to ±5.5 V relative to Power Ground. The 10 MΩ input resistor is connected between Bias and Signal Ground.

At frequencies low enough that the resistive components of the two impedances are dominant.

SIM918 Precision Current Preampliﬁer

2.3 Output 2 – 5

To reduce output noise of the SIM918, the Bias input is limited to a bandwidth speciﬁed in the table on Page viii. Beyond this frequency, the transimpedance stage cannot follow variations in the bias voltage, but the output diﬀerence ampliﬁer (discussed in the next section) does. Hence the common-mode rejection of the instrument is greatly reduced at frequencies above DC.

The bias sensing circuitry is always active, and will signal BIAS OVLD when the applied input exceeds the voltage limits in Section 1.2.4.1, even if Bias is set to GND.

2.3 Output

When Bias is switched to GND, the output of the instrument is taken directly from the transimpedance stage; otherwise, from a diﬀerence ampliﬁer (Figure 1.1) that subtracts the bias voltage from the output of the transimpedance stage. Both outputs have equal drive capacity.

The output impedance of the SIM918 Precision Current Preampliﬁer is 100 Ω. The preampliﬁer can drive load impedances from ∞ to 0 Ω for the full ±10 V range of output voltage. When driving a 50 Ω load, the gain will be one third of that displayed on the front panel.

2.4 Autozero Trim

The output signal is ﬁltered by a passive LRC, with f

−3 dB

= 25 kHz.

The ﬁlter eliminates broad-spectrum noise, while adding a negligible amount of overshoot in the step response. The R in the ﬁlter contributes to the output resistance.

The output diﬀerence ampliﬁer, when engaged (Bias not at [GND]), introduces an oﬀset error that can be greater than the maximum input oﬀset error of the preampliﬁer. The error is reduced by autocalibration (Section 2.6). The output oﬀset can also be trimmed from the remote interface by using the command OFST 1.

The autozero control loop is fully analog.6Its settling time, to within the maximum input oﬀset voltage speciﬁcation in the table on Page viii, is 40 s. Two adjustments can be made to loop parameters via the remote command OFST. The ﬁrst one to consider, OFST 3, adjusts the zero point of the loop itself, i.e. the voltage to which the control loop drives the input oﬀset when autozero is ON.

The command OFST 2 sets the code in a digital-to-analog converter, the output of which adds together with the output of the loop to form the overall control output of the autozero circuit. When autozero is engaged, the control loop will compensate for changes made

SIM918 Precision Current Preampliﬁer

With discrete time steps.

2 – 6 Description of Operation

via OFST 2, driving the input oﬀset voltage back to a value determined by OFST 3. With autozero oﬀ, OFST 3 has no eﬀect and OFST 2 changes the input oﬀset directly.

The best value for OFST 2 is readjusted each time autozero is turned from on to oﬀ. If the preampliﬁer is to be operated under conditions that tolerate absolutely no clock transitions, the recommended course of action is to turn autozero on, let the control loop drive the output to zero and settle, and turn autozero oﬀ for the duration of the experiment (Section 2.8).

The value for OFST 3 is reestablished by autocalibration (Section 2.6).

2.5 Phase-Locked Loop

The switches in the autozero circuit receive a clock signal from an internal phase-locked loop. If an external reference clock is available at the rear-panel Ref Clock Sync connector, autozero is on, and the connector is not selected for output, the PLL will attempt to lock to the clock signal. The synchronization will be successful for external clock frequencies between 0.90 Hz and 1.10 Hz.

2.6 Autocalibration

In the internal reference clock mode, the PLL oscillator runs freely, generating a 1.0 Hz square wave with rising edges at the beginning of each autozero half-cycle. The voltage-controlled oscillator in the loop operates at 240 Hz for 60 Hz power line frequency (FPLC), and at 200 Hz for FPLC = 50 Hz.

The PLL can be automatically inhibited, and the voltage-controlled oscillator turned oﬀ, when the reference clock is External and autozero is oﬀ. The behavior is set by the remote command APLL. With APLL OFF, the oscillator turns oﬀ. In this mode, under external ref- erence clock, the instrument will undergo the full capture and lock transient after autozero is switched on. To avoid the 4 minute capture delay, set APLL ON. The module restores the last known APLL mode upon power-on.

When autozero is on, the PLL oscillator is always running, regardless of the reference clock source. If the reference clock is internal and autozero is oﬀ, the PLL oscillator is oﬀ. There is no capture delay under internal clock.

To ensure the speciﬁed oﬀset accuracy, the preampliﬁer must be selfcalibrated within the 24 hours preceding a measurement. A valid autocalibration must take place at (23 ± 5)◦C with the module warmed up for at least 2 hours at (23±5)◦C. If the module is being used inside

SIM918 Precision Current Preampliﬁer

2.7 Clock Stopping 2 – 7

the SIM900 Mainframe, the autocalibration must also be inside the mainframe. Otherwise, perform the autocalibration with the same connection to an independent supply as you use for the operation.

Disconnect all inputs and outputs to the SIM918 while performing the autocalibration. Connect the center and shield terminals of the Bias BNC together externally, e.g. with a grounding cap. To calibrate, issue the com-

mand ACAL, or press both [GAIN ] at the same time. Depending on the ﬁrmware revision, the calibration may take up to 20 minutes to complete. During the autocalibration, all LEDs are lit. At the end of the calibration, the module returns to its pre-ACAL settings, except the reference clock direction is reset to input.

If autocalibration is unsuccessful, for example because an external current is applied to Input, the calibration constants revert to their original values and the command LDDE? will return Code 2. If an external reference clock is detected, the autocalibration terminates immediately with LDDE? 2.

Autocalibration does not aﬀect gain accuracy.

2.7 Clock Stopping

The microprocessor clock of the SIM918 stops if the module is idle, “freezing” the digital circuitry. The following actions “wake up” the clock:

1. A power-on.

2. A press of a front-panel button.

3. Activity (send or receive) at the remote interface.

4. An overload.

5. A change in external reference clock status: several rising edges at the rear-panel connector while in internal clock mode, or cessation of clocking while in external mode.

6. Loss of PLL lock.

The clock runs for as long as is necessary to complete a change of settings requested from the front panel, or to communicate the output of a query through the remote interface. However, the clock will remain active for as long as the overload or unlocked condition exists, and for the full duration of an autocalibration.

SIM918 Precision Current Preampliﬁer

This default behavior can be modiﬁed with the remote command AWAK. Setting AWAK ON will prevent the clock from stopping. The module returns to AWAK OFF upon power-on.

2 – 8 Description of Operation

Note that the operation of the PLL oscillator is completely independent of the microprocessor clock.

2.8 Quiescent Operation

Follow these steps to operate the SIM918 Precision Current Preampliﬁer in sensitive measurements that can tolerate absolutely no module clock transitions:

1. Reset the preampliﬁer (Section 1.5).

2. Set Input to Open.

3. Wait for at least 40 s.

4. Turn autozero oﬀ.

5. Select the desired gain.

6. Close the input.

7. Perform the measurement.

After this sequence is complete, the control output of the autozero circuit holds at a value that initially drives the input oﬀset within its speciﬁcation. Without active autozeroing, the input oﬀset may drift as time progresses, so Steps 1–6 may need to be repeated.

SIM918 Precision Current Preampliﬁer

3 Remote Operation

In This Chapter

This chapter describes operating the SIM918 over the serial interface.

3.1 Index of Common Commands . . . . . . . . . . . . . 3 – 2

3.2 Alphabetic List of Commands . . . . . . . . . . . . . 3 – 4

3.3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3– 6

3.3.1 Power-on conﬁguration . . . . . . . . . . . . . 3– 6

3.3.2 Buﬀers . . . . . . . . . . . . . . . . . . . . . . . 3 – 6

3.3.3 Device Clear . . . . . . . . . . . . . . . . . . . . 3– 7

3.4 Commands . . . . . . . . . . . . . . . . . . . . . . . . 3– 7

3.4.1 Command syntax . . . . . . . . . . . . . . . . . 3– 7

3.4.2 Notation . . . . . . . . . . . . . . . . . . . . . . 3 – 9

3.4.3 Examples . . . . . . . . . . . . . . . . . . . . . 3– 9

3.4.4 General commands . . . . . . . . . . . . . . . . 3 – 10

3.4.5 Conﬁguration commands . . . . . . . . . . . . 3– 11

3.4.6 Calibration commands . . . . . . . . . . . . . . 3 – 14

3.4.7 Status commands . . . . . . . . . . . . . . . . . 3 – 16

3.4.8 Interface commands . . . . . . . . . . . . . . . 3 – 19

3.4.9 Serial communication commands . . . . . . . 3 – 22

3.5 Status Model . . . . . . . . . . . . . . . . . . . . . . . 3 – 24

3.5.1 Status Byte (SB) . . . . . . . . . . . . . . . . . . 3 – 25

3.5.2 Service Request Enable (SRE) . . . . . . . . . . 3 – 25

3.5.3 Standard Event Status (ESR) . . . . . . . . . . 3 – 26

3.5.4 Standard Event Status Enable (ESE) . . . . . . 3– 26

3.5.5 Communication Error Status (CESR) . . . . . . 3– 26

3.5.6 Communication Error Status Enable (CESE) . 3– 27

3.5.7 Overload Status (OLSR) . . . . . . . . . . . . . 3 – 27

3.5.8 Overload Status Enable (OLSE) . . . . . . . . . 3 – 28

3.5.9 Reference Clock Status (RCSR) . . . . . . . . . 3– 28

3.5.10 Reference Clock Status Enable (RCSE) . . . . . 3 – 29

3 – 1

3 – 2 Remote Operation

3.1 Index of Common Commands

Symbol Deﬁnition

i Bit number (0–7) j Unsigned integer (0–65535) m Unsigned integer (1–3) y, z Literal token

(?) Required for queries; illegal for set commands

var Parameter always required

{var} Required parameter for set commands; illegal for queries [var] Optional parameter for both set and query forms

General

HELP(?) 3 – 10 Instrument Help AWAK(?) {z} 3 – 11 Keep Clock Awake

Conﬁguration

FPLC(?) {j} 3 – 12 Power Line Cycle Frequency GAIN(?) {m} 3 – 12 Gain INPT(?) {z} 3 – 12 Input BIAS(?) {z} 3 – 12 Bias SHLD(?) y {, z} 3 – 13 Shield CHOP(?) {z} 3 – 13 Autozero SYNC(?) {z} 3 – 13 Reference Clock Direction FREQ? 3 – 13 Reference Clock Frequency PHAS? 3 – 14 Autozero Phase APLL(?) {z} 3 – 14 Keep PLL Active

Calibration

ACAL 3 – 14 Autocalibration READ? m 3 – 15 Read Microvoltmeter OFST(?) m {, j} 3 – 15 Oﬀset Trim

Status

*CLS 3 – 16 Clear Status *STB? [i] 3 – 16 Status Byte *SRE(?) [i,] {j} 3 – 16 Service Request Enable *ESR? [i] 3 – 16 Standard Event Status *ESE(?) [i,] {j} 3 – 16 Standard Event Status Enable CESR? [i] 3 – 17 Communication Error Status CESE(?) [i,] {j} 3 – 17 Communication Error Status Enable OLSR? [i] 3 – 17 Overload Status OLSE(?) [i,] {j} 3 – 17 Overload Status Enable

SIM918 Precision Current Preampliﬁer

3.1 Index of Common Commands 3 – 3

RCSR? [i] 3 – 17 Reference Clock Status RCSE(?) [i,] {j} 3 – 17 Reference Clock Status Enable PSTA(?) {z} 3 – 18 Pulse ¬STATUS Mode LBTN? 3 – 18 Last Button OVLD? 3 – 18 Overload RCLK? 3 – 19 Reference Clock State

Interface

*RST 3 – 19 Reset *IDN? 3 – 20 Identify *TST? 3 – 20 Self Test *OPC(?) 3 – 20 Operation Complete CONS(?) {z} 3 – 20 Console Mode LEXE? 3 – 21 Execution Error LCME? 3 – 21 Command Error LDDE? 3 – 22 Device Error TOKN(?) {z} 3 – 22 Token Mode TERM(?) {z} 3 – 22 Response Termination

Serial Communications

PARI(?) {z} 3 – 23 Parity

SIM918 Precision Current Preampliﬁer

3 – 4 Remote Operation

3.2 Alphabetic List of Commands

*CLS 3 – 16 Clear Status *ESE(?) [i,] {j} 3 – 16 Standard Event Status Enable *ESR? [i] 3 – 16 Standard Event Status *IDN? 3 – 20 Identify *OPC(?) 3 – 20 Operation Complete *RST 3 – 19 Reset *SRE(?) [i,] {j} 3 – 16 Service Request Enable *STB? [i] 3 – 16 Status Byte *TST? 3 – 20 Self Test

ACAL 3 – 14 Autocalibration APLL(?) {z} 3 – 14 Keep PLL Active AWAK(?) {z} 3 – 11 Keep Clock Awake

BIAS(?) {z} 3 – 12 Bias

CESE(?) [i,] {j} 3 – 17 Communication Error Status Enable CESR? [i] 3 – 17 Communication Error Status CHOP(?) {z} 3 – 13 Autozero CONS(?) {z} 3 – 20 Console Mode

FPLC(?) {j} 3 – 12 Power Line Cycle Frequency FREQ? 3 – 13 Reference Clock Frequency

GAIN(?) {m} 3 – 12 Gain

HELP(?) 3 – 10 Instrument Help

INPT(?) {z} 3 – 12 Input

LBTN? 3 – 18 Last Button LCME? 3 – 21 Command Error

SIM918 Precision Current Preampliﬁer

3.2 Alphabetic List of Commands 3 – 5

LDDE? 3 – 22 Device Error LEXE? 3 – 21 Execution Error

OFST(?) m {, j} 3 – 15 Oﬀset Trim OLSE(?) [i,] {j} 3 – 17 Overload Status Enable OLSR? [i] 3 – 17 Overload Status OVLD? 3 – 18 Overload

PARI(?) {z} 3 – 23 Parity PHAS? 3 – 14 Autozero Phase PSTA(?) {z} 3 – 18 Pulse ¬STATUS Mode

RCLK? 3 – 19 Reference Clock State RCSE(?) [i,] {j} 3 – 17 Reference Clock Status Enable RCSR? [i] 3 – 17 Reference Clock Status READ? m 3 – 15 Read Microvoltmeter

SHLD(?) y {, z} 3 – 13 Shield SYNC(?) {z} 3 – 13 Reference Clock Direction

TERM(?) {z} 3 – 22 Response Termination TOKN(?) {z} 3 – 22 Token Mode

SIM918 Precision Current Preampliﬁer

3 – 6 Remote Operation

3.3 Introduction

Remote operation of the SIM918 is through a simple command language documented in this chapter. Both set and query forms of most commands are supported, allowing the user complete control of the ampliﬁer from a remote computer, either through the SIM900 Mainframe or directly via RS–232 (see Section 1.6.2.1).

See Table 1.2 for the speciﬁcation of the DB–15 SIM Interface Connector.

3.3.1 Power-on conﬁguration

The initial settings for the remote interface are 9600 baud with no parity and no ﬂow control, and with local echo disabled (CONS OFF).

The following values are retained in non-volatile memory:

1. The power line frequency (FPLC).

2. The gain.

3. Autozero on/oﬀ.

3.3.2 Buffers

4. Input selection (on, open).

5. Input shield selection (program, bias, ground.)

6. Bias selection (on, ground).

7. Bias shield selection (ﬂoat, ground.)

8. Whether or not the phase-locked loop stays active when autozero is oﬀ.

9. Calibration values.

Upon power-on, those settings are restored to their values before the power was turned oﬀ.

Where appropriate, the default or power-on value for parameters is listed in boldface in the command descriptions.

The SIM918 stores incoming bytes from the host interface in a 64byte input buﬀer. Characters accumulate in the input buﬀer until a command terminator (either hCRi or hLFi) is received, at which point the message is parsed and executed. Query responses from the SIM918 are buﬀered in a 64-byte output queue.

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 7

If the input buﬀer overﬂows, then all data in both the input buﬀer and the output queue are discarded, and an error is recorded in the CESR and ESR status registers.

3.3.3 Device Clear

The SIM918 host interface can be asynchronously reset to its poweron conﬁguration by sending an RS–232-style hbreakisignal. From the SIM900 Mainframe, this is accomplished with the SRST command; if directly interfacing via RS–232, then use a serial break signal. After receiving the Device Clear, the CONS mode is turned OFF. Note that this only resets the communication interface; the basic function of the SIM918 is left unchanged; to reset the preampliﬁer, use *RST.

The Device Clear signal will also terminate the output of the HELP? command from the SIM918.

3.4 Commands

This section provides syntax and operational descriptions for remote commands.

3.4.1 Command syntax

The four letter mnemonic (shown in CAPS) in each command sequence speciﬁes the command. The rest of the sequence consists of parameters.

Commands may take either set or query form, depending on whether the “?” character follows the mnemonic. Set only commands are listed without the “?”, query only commands show the “?” after the mnemonic, and optionally query commands are marked with a “(?)”.

Parameters shown in { } and [ ] are not always required. Parameters in { } are required to set a value, and should be omitted for queries. Parameters in [ ] are optional in both set and query commands. Parameters listed without surrounding characters are always required.

Do not send ( ) or { } or [ ] as part of the command.

Multiple parameters are separated by commas. Multiple commands may be sent on one command line by separating them with semicolons (;) so long as the input buﬀer does not overﬂow. Commands are terminated by either hCRi or hLFi characters. Null commands and whitespaces are ignored. Execution of the command does not begin until the command terminator is received.

SIM918 Precision Current Preampliﬁer

Token parameters (generically shown as y and z in the commandtokens descriptions) can be speciﬁed either as a keyword or as an integer

3 – 8 Remote Operation

value. Command descriptions list the valid keyword options, with each keyword followed by its corresponding integer value. For example, to set the response termination sequence to hCRi+hLFi, the following two commands are equivalent:

TERM CRLF —or— TERM 3

For queries that return token values, the return format (keyword or integer) is speciﬁed with the TOKN command.

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 9

3.4.2 Notation

The following table summarizes the notation used in the command descriptions:

Symbol Deﬁnition

i Bit number (0–7) j Unsigned integer (0–65535) m Unsigned integer (1–3) y, z Literal token

(?) Required for queries; illegal for set commands

var Parameter always required

{var} Required parameter for set commands; illegal for queries [var] Optional parameter for both set and query forms

3.4.3 Examples

Each command is provided with a simple example illustrating its usage. In these examples, all data sent by the host computer to the SIM918 are set as straight teletype font, while responses received by the host computer from the SIM918 are set as slanted teletype font.

The usage examples vary with respect to set/query, optional parameters, and token formats. These examples are not exhaustive, and are intended to provide a convenient starting point for user programming.

SIM918 Precision Current Preampliﬁer

3 – 10 Remote Operation

3.4.4 General commands

Instrument HelpHELP(?)

Outputs a condensed version of Section 3.4 to the remote interface.

HELP may be used with or without the query sign, with the same eﬀects.

HELP?Example:

Notation: i is bit number (0..7); j is a 16-bit unsigned integer (0..65535); m is a small unsigned integer (1..3); y, z are tokens (?) question required for queries, illegal for set commands; [] = parameter is optional for both set and query forms; {} = parameter is required to set, illegal for queries; parameter without brackets is always required; the brackets themselves should not be sent.

General commands: HELP? - Send this text. AWAK(?) {z} - Keep the module clock awake.

Configuration commands: FPLC(?) {j} - Power line rejection frequency (50, 60). GAIN(?) {m} - Set/query gain. INPT(?) {z} - Input (OPEN, CLOSE). BIAS(?) {z} - Bias (GND, ON). SHLD(?) y {,z} - Shield (INPUT, BIAS)

(GND, BIAS, FLOAT, PROG). CHOP(?) {z} - Autozero OFF/ON. SYNC(?) {z} - Reference clock IN/OUT. FREQ? - Reference clock frequency (Hz). PHAS? - Switch position (Zero Amp, Zero Zero). APLL(?) {z} - Keep PLL active when autozero is off.

Calibration commands: ACAL - One-time autocalibration. READ? m - Check intermediate/output voltage (uV). OFST(?) m {,j} - Set/query offset trim.

Status commands: *CLS - Clear Status. *STB? [i] - Query the Status Byte. *SRE(?) [i,] {j} - Service Request Enable.

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 11

*ESR? [i] - Query Standard Event Status register. *ESE(?) [i,] {j} - Standard Event Status Enable. CESR? [i] - Query the Communications Error Status. CESE(?) [i,] {j} - Communications Error Status Enable. OLSR? [i] - Query Overload Status register. OLSE(?) [i,] {j} - Overload Status Enable. RCSR? [i] - Query Reference Clock Status register. RCSE(?) [i,] {j} - Reference Clock Status Enable. PSTA(?) {z} - Pulse Status or change its level. LBTN? - Which button last pressed? OVLD? - Bias, intermediate, or output overloaded? RCLK? - Reference clock int./ext., locked?

Interface commands: *RST - Reset to known state. *IDN? - Identify. *TST? - Does nothing. *OPC(?) - Operation complete. CONS(?) {z} - Console OFF/ON. LEXE? - Last Execution Error. LCME? - Last Communications Error. LDDE? - Last Device Error. TOKN(?) {z} - Turn token mode OFF/ON. TERM(?) {z} - Cmd line end (NONE, CR, LF, CRLF, LFCR).

Serial interface command (baud rate is always 9600): PARI(?) {z} - Parity (NONE, EVEN, ODD, MARK, SPACE).

Keep Clock AwakeAWAK(?) {z}

Set (query) the SIM918 keep-awake mode {to z = (OFF 0, ON 1)}.

Ordinarily, the clock oscillator for the SIM918 microcontroller is held in a stopped state, and only enabled during processing of events (Section 2.7). Setting AWAK ON forces the clock to stay running, and is useful only for diagnostic purposes.

AWAK ONExample:

3.4.5 Conﬁguration commands

SIM918 Precision Current Preampliﬁer

3 – 12 Remote Operation

Power Line Cycle FrequencyFPLC(?) {j}

Set (query) the power-line rejection frequency {to j = (50, 60)}, in Hz.

The FPLC value is retained in non-volatile memory, and is not modiﬁed by a power-on reset.

FPLC 60Example:

GainGAIN(?) {m}

Set (query) the preampliﬁer gain {to m}.

Value Gain, V/A

0 2.0 × 10 1 1.000 × 10

2 1.000 × 10 3 1.00 × 10

A special case m = 0 connects the input terminal to the output of the transimpedance stage through RF= 20 kΩ. This state is indicated by all GAIN LEDs switched oﬀ. With BIAS GND, the conﬁguration reproduces the input oﬀset voltage at the output of the instrument. This voltage may be measured with READ? 3.

The conﬁguration m = 0 is volatile, and is reset to m = 1 upon power-on.

GAIN?Example:

InputINPT(?) {z}

Set (query) the preampliﬁer input connection {to z = (OPEN 0, CLOSE 1)}.

INPT CLOSEExample:

BiasBIAS(?) {z}

Set (query) the preampliﬁer bias connection {to z = (GND 0, ON 1)}.

BIAS?Example:

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 13

ShieldSHLD(?) y {, z}

Set (query) the preampliﬁer BNC shield connection for y = (INPUT 0, BIAS 1) {to z = (GND 0, BIAS 1, FLOAT 2, PROG 3)}.

The following combinations are valid:

BNC y Shield z

INPUT GND

BIAS PROG

BIAS GND

FLOAT

SHLD INPUT,BIASExample: SHLD BIAS,FLOAT

AutozeroCHOP(?) {z}

Set (query) the preampliﬁer autozero selection {to z = (OFF 0, ON 1)}. There will be a wait before the chosen regime takes eﬀect (Section 1.2.2.1).

CHOP?Example:

Reference Clock DirectionSYNC(?) {z}

Set (query) the direction of the signal at the SIM918 rear-panel Ref Clock Sync connector {to z = (IN 0, OUT 1)}. The direction is reset to IN upon power-on.

SYNC 1Example:

Reference Clock FrequencyFREQ?

Query the reference clock frequency of the preampliﬁer, in hertz. The nominal frequency of the internal reference clock is 1.0 Hz. The command can also be used to measure the frequency of an external reference clock.

If autozero is oﬀ and the reference clock is internal, or if autozero is oﬀ and APLL is set to OFF under external reference clock, there are no reference clock transitions inside the SIM918 and FREQ? will time out with Execution Error 16.

FREQ?Example:

1.023

SIM918 Precision Current Preampliﬁer

3 – 14 Remote Operation

Autozero PhasePHAS?

Query the autozero switch position in the preampliﬁer. The responses are ZA 0 (while zeroing the input voltage of the main ampliﬁer itself) and ZZ 1 (while zeroing the oﬀset volage of the zeroing ampliﬁer). These responses alternate with every cycle of the reference clock.

The switches are parked in the ZA state when autozero is oﬀ.

TOKN ON; PHAS?Example:

Keep PLL ActiveAPLL(?) {z}

Set (query) the “keep PLL active when autozero is oﬀ” mode of the preampliﬁer {to z = (OFF 0, ON 1)}.

This setting only applies to the external reference clock direction. When APLL is OFF, turning oﬀ the autozero function (CHOP OFF) will fully halt the PLL oscillator (Section 2.5), ensuring that no digital clock transitions occur. One consequence of APLL OFF is that the oscillator will require up to 250 s to reestablish the lock to an external reference when returning to CHOP ON.

Conversely, with APLL ON the internal oscillator will continue to track an external 1 pps reference clock during CHOP OFF periods. In this case, there is no re-lock time needed when returning to CHOP ON.

The PLL oscillator always turns oﬀ with CHOP OFF in the internal reference clock mode.

The APLL setting is retained in non-volatile memory, and is not modiﬁed by a power-on reset.

APLL 1Example:

3.4.6 Calibration commands

AutocalibrationACAL

Perform a self-calibration (Section 2.6). Make sure to disconnect all inputs and outputs to the SIM918. Remote commands are not processed until ACAL is complete.

ACALExample: LDDE?

checks for success of an autocalibration.

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 15

Read MicrovoltmeterREAD? m

Query instrument voltage m, in microvolts. READ? 1 queries the voltage at the overall Output terminal, and READ? 3, the voltage at the output of the transimpedance stage. These diagnostics are useful in manually adjusting the output and input oﬀsets with the commands OFST 1 and OFST 2/OFST 3, respectively.

The command READ? 2 measures the control output of an internal digital-to-analog converter. This output adds together with the output of the autozero control loop to form the overall control output of the autozero circuit, and is adjusted with OFST 2.

Disconnect all inputs and outputs to the SIM918 before issuing READ? . Connect the center and shield terminals of the Bias BNC together externally, e.g. with a grounding cap. There will be a wait of several seconds for the

command to execute while internal switches are conﬁgured and the voltages are sampled and averaged. Autozero is ON while READ? executes, and the reference clock is used during the measurement. The results of READ? will be unpredictable if the reference clock is external and Unlocked. Remote commands are not processed until READ? is complete.

READ? 1Example:

-12

Oﬀset TrimOFST(?) m {, j}

Set (query) oﬀset trim m {to j}. The trims are established by autocalibration. If needed, the oﬀsets in the SIM918 (Section 2.4) may be adjusted manually as follows:

Trim m Range of j Adjusts oﬀset When By

1 -128– +126 Output BIAS ON 3.9 µV/count 2 -32768– +32767 Input CHOP OFF 0.45 µV/count 3 -128– +126 Input CHOP ON 0.45 µV/count

A greater value of OFST 1 will make the output voltage more positive. A greater value of OFST 2 or OFST 3 will make the input oﬀset more positive, i.e. will make the input voltage exceed the bias voltage by more microvolts.

OFST? 3Example:

-13

READ? 3

OFST 3,-42

SIM918 Precision Current Preampliﬁer

3 – 16 Remote Operation

3.4.7 Status commands

The Status commands query and conﬁgure registers associated with status reporting of the SIM918. See Section 3.5 for the status model.

Clear Status*CLS

*CLS immediately clears the ESR, CESR, RCSR, and OLSR status registers.

*CLSExample:

Status Byte*STB? [i]

Query the Status Byte register [Bit i].

Execution of the *STB? query (without the optional Bit i) always causes the ¬STATUS signal to be deasserted. Note that *STB? i will

not clear ¬STATUS, even if Bit i is the only bit presently causing the ¬STATUS signal.

*STB?Example:

Service Request Enable*SRE(?) [i,] {j}

Set (query) the Service Request Enable register [Bit i] {to j}.

*SRE 0,1Example:

Standard Event Status*ESR? [i]

Query the Standard Event Status Register [Bit i].

Upon execution of *ESR? , the returned bit(s) of the ESR register are cleared.

*ESR?Example:

Standard Event Status Enable*ESE(?) [i,] {j}

Set (query) the Standard Event Status Enable register [Bit i] {to j}.

*ESE 6,1Example: ESE?

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 17

Communication Error StatusCESR? [i]

Query the Communication Error Status Register [Bit i].

Upon executing a CESR? query, the returned bit(s) of the CESR register are cleared.

CESR?Example:

Communication Error Status EnableCESE(?) [i,] {j}

Set (query) the Communication Error Status Enable register [Bit i] {to j}.

CESE?Example:

Overload StatusOLSR? [i]

Query the Overload Status Register [Bit i].

Upon executing an OLSR? query, the returned bit(s) of the OLSR register are cleared.

OLSR?Example:

Overload Status EnableOLSE(?) [i,] {j}

Set (query) the Overload Status Enable register [Bit i] {to j}.

OLSE 4Example:

Reference Clock StatusRCSR? [i]

Query the Reference Clock Status Register [Bit i].

Upon executing an RCSR? query, the returned bit(s) of the RCSR register are cleared.

RCSR?Example:

Reference Clock Status EnableRCSE(?) [i,] {j}

Set (query) the Reference Clock Status Enable register [Bit i] {to j}.

SIM918 Precision Current Preampliﬁer

RCSE 3,1Example:

3 – 18 Remote Operation

Pulse ¬STATUS ModePSTA(?) {z}

Set (query) the Pulse ¬STATUS mode {to z = (OFF 0, ON 1)}.

When PSTA ON is set, all new service requests will only pulse the ¬STATUS signal LOW (for a minimum of 1 µs). The default behavior is to latch ¬STATUS LOW until a *STB? query is received.

A reset does not alter PSTA. The value in boldface above is the power-on value.

PSTA OFFExample:

Last ButtonLBTN?

Query the number of the last button pressed. The response is

LBTN? Last button

1 [GAIN ] 2 [GAIN ] 3 [AUTOZERO] 4 [Output 1 pps sync] 5 [INPUT Open] 6 [INPUT Shield] 7 [BIAS GND] 8 [BIAS Shield] 9 Both [GAIN ] and [GAIN ] (autocalibrate)

10 A button held upon power-on (reset)

The value 0 is returned if no button was pressed since the last LBTN? .

A query of LBTN? always clears the button code, so a subsequent LBTN? will return 0.

LBTN?Example:

OverloadOVLD?

Query the current overload condition. The response is

OVLD? Overloaded

1 Bias 2 Output 4 Bias + Output

Combination overloads are reported by summing the values of the individual overload ﬂags. This command complements the OLSR status register described in Section 3.5.7, and the three overload ﬂags correspond one-to-one with bits in OLSR. However, once cleared

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 19

by OLSR? or *CLS, the overload status bits will stay cleared even though the overload condition may persist and remain reported by OVLD?.

OVLD?Example:

implies that the bias is not overloaded; the transimpedance stage

− iin× RF) is overloaded; and the output is overloaded.

bias

Reference Clock StateRCLK?

Query the current source and lock state of the reference clock of the preampliﬁer. The responses are INTERNAL 0, EXTERNAL 1, and UNLOCKED 3.

This command complements the RCSR status register described in Section 3.5.9, but there is no one-to-one correspondence between the response of RCLK? and bits in RCSR. Once cleared by RCSR? or *CLS, the reference clock status bits will stay cleared even though the reference clock state may persist and remain reported by RCLK? .

RCLK?Example:

UNLOCKED

3.4.8 Interface commands

The Interface commands provide control over the interface between the SIM918 and the host computer.

Reset*RST

Reset the SIM918 to its default conﬁguration.

*RST sets the following:

1. Gain to 106V/A.

2. Autozero on.

3. Input connected.

4. Input shield to ground.

5. Bias to ground.

6. Bias shield to ground.

7. Reference clock direction to input.

8. The phase-locked loop to inactive when autozero is oﬀ (APLL

OFF).

SIM918 Precision Current Preampliﬁer

9. Clock oscillator to stop during idle time (AWAK OFF).

3 – 20 Remote Operation

10. The token mode to OFF.

*RST does not aﬀect PSTA, CONS, TERM, and all service-enable registers (*SRE, *ESE, CESE, RCSE, or OLSE).

*RSTExample: CONS?

Identify*IDN?

Query the device identiﬁcation string.

The identiﬁcation string is formatted as:

Stanford Research Systems,SIM918,s/n******,ver#.###

where SIM918 is the model number, ****** is a 6-digit serial number, and #.### is the ﬁrmware revision level.

*IDN?Example:

Stanford Research Systems,SIM918,s/n005432,ver2.1

Self Test*TST?

There is no internal self-test in the SIM918 after the power-on, so this query always returns 0.

*TST?Example:

Operation Complete*OPC(?)

Sets the OPC ﬂag in the ESR register.

The query form *OPC? writes a 1 into the output queue when complete, but does not aﬀect the ESR register.

*OPC?Example:

Console ModeCONS(?) {z}

Set (query) the console mode {to z = (OFF 0, ON 1)}.

CONS causes each character received at the input buﬀer to be copied to the output queue.

A reset does not alter CONS. The value in boldface above is the power-on value. CONS is set to OFF upon Device Clear.

CONS ONExample:

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 21

Execution ErrorLEXE?

Query the Last Execution Error code. A query of LEXE? always clears the error code, so a subsequent LEXE? will return 0. Valid codes are:

Value Deﬁnition

0 No execution error since last LEXE? 1 Illegal value 2 Wrong token 3 Invalid bit

16 Reference clock inactive

*STB? 12; LEXE?; LEXE?Example:

3 0

The error (3, ”Invalid bit”) is because *STB? only allows bit-speciﬁc queries of 0–7. The second read of LEXE? returns 0.

Command ErrorLCME?

Query the Last Command Error code. A query of LCME? always clears the error code, so a subsequent LCME? will return 0. Valid codes are:

Value Deﬁnition

0 No command error since last LCME? 1 Illegal command 2 Undeﬁned command 3 Illegal query 4 Illegal set 5 Missing parameter(s) 6 Extra parameter(s) 7 Null parameter(s)

8 Parameter buﬀer overﬂow 10 Bad integer 11 Bad integer token 12 Bad token value 14 Unknown token

*IDNExample: LCME?

The error (4, ”Illegal set”) is due to the missing “?”.

SIM918 Precision Current Preampliﬁer

3 – 22 Remote Operation

Device ErrorLDDE?

Query the Last Device-Dependent Error code. A query of LDDE? always clears the error code, so a subsequent LDDE? will return 0. Valid codes are:

Value Deﬁnition

0 No execution error since last LEXE?

1 Reference clock conﬂict

2 Unable to autocalibrate

ACALExample: LDDE?

indicates a successful autocalibration.

Token ModeTOKN(?) {z}

Set (query) the token query mode {to z = (OFF 0, ON 1)}.

If TOKN ON is set, then queries to the SIM918 that return tokens will return a text keyword; otherwise they return a decimal integer value. Thus, the only possible responses to the TOKN? query are ON and 0.

TOKN OFFExample:

Response TerminationTERM(?) {z}

Set (query) the htermi sequence {to z = (NONE 0, CR 1, LF 2, CRLF 3, or LFCR 4)}.

The htermi sequence is appended to all query responses sent by the module, and is constructed of ASCII character(s) 13 (carriage return) and 10 (line feed). The token mnemonic gives the sequence of characters.

A reset does not alter TERM. The value in boldface above is the power-on value.

TOKN ON; TERM?Example:

CRLF

3.4.9 Serial communication commands

Note that the SIM918 can only support a single baud rate of 9600, and does not support ﬂow control. A reset does not change the serial interface settings; use Device Clear.

SIM918 Precision Current Preampliﬁer

3.4 Commands 3 – 23

ParityPARI(?) {z}

Set (query) the parity {to z = (NONE 0, ODD 1, EVEN 2, MARK 3,

SPACE 4)}. The value in boldface is the power-on value.

TOKN ON; PARI?Example:

EVEN

SIM918 Precision Current Preampliﬁer

3 – 24 Remote Operation

CESB

MSS

ESB

IDLE

undef

RCSB

OLSB

Status Byte

SB SRE

OPC: Operation Complete

INP: Input Buffer Error

DDE: Device Error

EXE: Execution Error

CME: Command Error

URQ: User Request

PON: Power On

QYE: Query Error

ESR ESE

Standard Event Status

PARITY: Parity Error

FRAME: Framing Error

HWOVRN: Hardware Input Overrun

OVR: Input Buffer Overrun

RTSH: RTS Halted

CTSH: CTS Halted

DCAS: Device Clear

NOISE: Noise Error

CESR CESE

Communication Error Status

Bias

Output

undef

Bias + Output

OLSR OLSE

Overload Status

STATUS

Leave

Arrive

Lock

undef

Unlock

RCSR RCSE

Reference Clock Status

3.5 Status Model

The SIM918 status registers follow the hierarchical IEEE–488.2 for-status registers mat. A block diagram of the status register array is given in Figure 3.1.

Event Registers : These read-only registers record the occurrence of deﬁned

Enable Registers : These read/write registers deﬁne a bitwise mask for their cor-

Figure 3.1: Status register model for the SIM918 Precision Current Preampliﬁer.

There are two categories of registers in the SIM918 status model:

events. If the event occurs, the corresponding bit is set to 1. Upon querying an event register, all set bits within it are cleared. These are sometimes known as “sticky bits,” since once set, a bit can only be cleared by reading its value. Event register names end with SR.

responding event register. If a bit position is set in an event register while the same bit position is also set in the enable register, then the corresponding summary bit message is set. Enable register names end with SE.

SIM918 Precision Current Preampliﬁer

3.5 Status Model 3 – 25

At power-on, all status registers are cleared.

3.5.1 Status Byte (SB)

The Status Byte is the top-level summary of the SIM918 status model. When masked by the Service Request Enable register, a bit set in the Status Byte causes the ¬STATUS signal to be asserted on the rearpanel SIM interface connector.

Weight Bit Flag

1 0 OLSB 2 1 RCSB 4 2 undef (0)

8 3 undef (0) 16 4 IDLE 32 5 ESB 64 6 MSS

128 7 CESB

OLSB : Overload Summary Bit. Indicates whether one or more of the

enabled ﬂags in the Overload Status Register has become true.

RCSB : Reference Clock Summary Bit. Indicates whether one or more

of the enabled ﬂags in the Reference Clock Status Register has become true.

IDLE : Indicates that the input buﬀer is empty and the command

parser is idle. Can be used to help synchronize SIM918 query responses.

ESB : Event Status Bit. Indicates whether one or more of the enabled

events in the Standard Event Status Register is true.

MSS : Master Summary Status. Indicates whether one or more of the

enabled status messages in the Status Byte register is true.

CESB : Communication Error Summary Bit. Indicates whether one or

more of the enabled ﬂags in the Communication Error Status Register has become true.

3.5.2 Service Request Enable (SRE)

Each bit in the SRE corresponds one-to-one with a bit in the SB register, and acts as a bitwise AND of the SB ﬂags to generate MSS. Bit 6 of the SRE is undeﬁned—setting it has no eﬀect, and reading it always returns 0. This register is set and queried with the *SRE(?) command.

SIM918 Precision Current Preampliﬁer

At power-on, this register is cleared.

3 – 26 Remote Operation

3.5.3 Standard Event Status (ESR)

The Standard Event Status Register consists of 8 event ﬂags. These event ﬂags are all “sticky bits” that are set by the corresponding events, and cleared only by reading or with the *CLS command. Reading a single bit (with the *ESR? i query) clears only Bit i.

Weight Bit Flag

1 0 OPC

2 1 INP

4 2 QYE

8 3 DDE 16 4 EXE 32 5 CME 64 6 URQ

128 7 PON

OPC : Operation Complete. Set by the *OPC command.

INP : Input buﬀer error. Indicates data has been discarded from the

input buﬀer.

QYE : Query Error. Indicates data in the output queue has been lost.

DDE : Device-Dependent Error. Indicates a failed autocalibration or

a reference clock conﬂict.

EXE : Execution Error. Indicates the error in a command that was

successfully parsed. Out-of-range parameters are an example.

CME : Command Error. Indicates a command parser-detected error.

URQ : User Request. Indicates that a front-panel button was pressed.

PON : Power On. Indicates that an oﬀ-to-on transition has occurred.

3.5.4 Standard Event Status Enable (ESE)

The ESE acts as a bitwise AND with the ESR register to produce the single-bit ESB message in the Status Byte Register (SB). The register can be set and queried with the *ESE(?) command.

At power-on, this register is cleared.

3.5.5 Communication Error Status (CESR)

The Communication Error Status Register consists of 8 event ﬂags; each of the ﬂags is set by the corresponding event, and cleared only by reading the register or with the *CLS command. Reading a single bit (with the CESR? i query) clears only Bit i.

SIM918 Precision Current Preampliﬁer

3.5 Status Model 3 – 27

Weight Bit Flag

1 0 PARITY

2 1 FRAME

4 2 NOISE

8 3 HWOVRN 16 4 OVR 32 5 RTSH 64 6 CTSH

128 7 DCAS

PARITY : Parity error. Set by serial parity mismatch on the incoming data

byte.

FRAME : Framing error. Set when an incoming serial data byte is missing

the STOP bit.

NOISE : Noise error. Set when an incoming serial data byte does not

present a steady logic level during each asynchronous bitperiod window.

HWOVRN : Hardware Overrun. Set when an incoming serial data byte is

lost due to internal processor latency. Causes the input buﬀer to be ﬂushed, and resets the command parser.

OVR : Input buﬀer Overrun. Set when the input buﬀer is overrun by

the incoming data. Causes the input buﬀer to be ﬂushed, and resets the command parser.

RTSH : RTS Holdoﬀ Event. Unused in the SIM918.

CTSH : CTS Holdoﬀ Event. Unused in the SIM918.

DCAS : Device Clear. Indicates that the SIM918 received the Device

Clear signal (an RS–232 hbreaki). Clears the input buﬀer and the output queue, and resets the command parser.

3.5.6 Communication Error Status Enable (CESE)

The CESE acts as a bitwise AND with the CESR register to produce the single-bit CESB message in the Status Byte Register (SB). The register can be set and queried with the CESE(?) command.

At power-on, this register is cleared.

3.5.7 Overload Status (OLSR)

The Overload Status Register consists of 3 event ﬂags; each of the ﬂags is set by the corresponding overload, and cleared only by reading the register or with the *CLS command. Reading a single bit (with the OLSR? i query) clears only Bit i.

SIM918 Precision Current Preampliﬁer

3 – 28 Remote Operation

Weight Bit Flag

1 0 Bias

2 1 Output

4 2 Bias + Output

8 3 undef (0) 16 4 undef (0) 32 5 undef (0) 64 6 undef (0)

128 7 undef (0)

Bias : Bias overload. Indicates that |V

tion 1.2.4.1).

Output : Output overload. Indicates that |V

tion 1.2.5).

Bias + Output : Transimpedance stage overload. Indicates that

bias

Reading this register (with the OLSR? query) clears all overload bits that are set. If the overload condition persists, the bits will remain cleared until the overload condition ceases and reoccurs. Use OVLD? to query the current state of the overload.

3.5.8 Overload Status Enable (OLSE)

The OLSE acts as a bitwise AND with the OLSR register to produce the single-bit OLSB message in the Status Byte Register (SB). The register can be set and queried with the OLSE(?) command.

At power-on, this register is cleared.

3.5.9 Reference Clock Status (RCSR)

− iin× RF| > 10.0 V.

| > 5.0 V (see also Sec-

bias

| > 10.0 V (see also Sec-

out

Leave : Reference clock stop detect. Indicates that the external refer-

The Reference Clock Status Register consists of 4 event ﬂags; each of the ﬂags is set by the corresponding clock event, and cleared only by reading the register or with the *CLS command. Reading a single bit (with the RCSR? i query) clears only Bit i.

Weight Bit Flag

1 0 Leave

2 1 Arrive

4 2 Unlock

8 3 Lock 16 4 undef (0) 32 5 undef (0) 64 6 undef (0)

128 7 undef (0)

ence clock signal has ceased.

SIM918 Precision Current Preampliﬁer

3.5 Status Model 3 – 29

Arrive : Reference clock start detect. Indicates that several peri-

odic clock edges have been newly present at the rear-panel Ref Clock Sync connector.

Unlock : Unlock detect. Indicates that the reference clock PLL (Sec-

tion 2.5) has transitioned from locked or idle to unlocked.

Lock : Lock detect. Indicates that the reference clock PLL has transi-

tioned from unlocked to locked.

Reading this register (with the RCSR? query) clears all event bits that are set. If the clock state persists (e.g. the clock remains unlocked), the bits will remain cleared until the state ceases and reoccurs. Use RCLK? to query the current state of the reference clock.

3.5.10 Reference Clock Status Enable (RCSE)

The RCSE acts as a bitwise AND with the RCSR register to produce the single-bit RCSB message in the Status Byte Register (SB). The register can be set and queried with the RCSE(?) command.

At power-on, this register is cleared.

SIM918 Precision Current Preampliﬁer

3 – 30 Remote Operation

SIM918 Precision Current Preampliﬁer

4 Circuit Description

In This Chapter

4.1 Schematic Diagrams . . . . . . . . . . . . . . . . . . . 4 – 2

4 – 1

4 – 2 Circuit Description

4.1 Schematic Diagrams

Circuit schematic diagrams follow this page.

SIM918 Precision Current Preampliﬁer

Appendix A Index

¬STATUS signal, 1 – 3, 1 – 10, 3 – 16, 17, 3 – 25 hbreaki signal, see Device Clear

Accuracy

gain, see Gain, accuracy oﬀset, 2 – 6

Autocalibration, 1 – 3, 2 – 6, 7, 3 – 14, 15, 3 –

18, 3 – 22, 3 – 26

Autozero, ix, 1 – 2, 3, 1 – 5, 2 – 6, 2 – 8, 3 – 14,

button, see Button, [AUTOZERO] clock source, see Reference clock, source LED, see LED, AUTOZERO phase, 3 – 14 selecting, ix, 1 – 2, 3, 1 – 9, 2 – 6, 3 – 6, 3 –

13, 14, 3 – 19

switching frequency, ix, 1 – 2, 1 – 6

Autozero circuit, 1 – 3, 1 – 5, 6, 1 – 8, 2 – 2, 2 –

5, 6, 2 – 8, 3 – 15

settling time, 2 – 5, 2 – 8

Bandwidth

bias, viii, 2 – 4

current, viii, 2 – 2, 3 Baud rate, 1 – 12, 3 – 6 Bias, iii, 2 – 4

bandwidth, see Bandwidth, bias

button, see Button, [BIAS GND]

connector, iii, viii, 1 – 7, 8, 2 – 7, 3 – 15

shield, viii, 1 – 3, 1 – 7–9, 2 – 4, 3 – 6, 3

– 12, 3 – 19

input resistance, viii, 2 – 4

LED, see LED, BIAS GND

overload, see Overload, bias

selecting, viii, 1 – 3, 1 – 7, 1 – 9, 2 – 5, 3 –

6, 3 – 12, 3 – 19

shield

button, see Button, [BIAS Shield] LED, see LED, BIAS Shield

voltage, iii, 1 – 2–4, 1 – 7, 8, 2 – 4, 5, 3 – 15

limits, viii, ix, 1 – 7, 2 – 4, 5

voltage buﬀer, 1 – 7, 2 – 4

oﬀset voltage, 1 – 7, 2 – 4 Block diagram, 1 – 4 BNC, iii, viii, ix, 1 – 3, 1 – 7, 8, 2 – 3, 4, 2 – 7, 3

– 12, 3 – 15

Buﬀer

bias, see Bias, voltage buﬀer input, 3 – 6, 3 – 25–27

overﬂow, 3 – 6, 7, 3 – 27

output, see Output queue

voltage, see Output, voltage buﬀer Button, 1 – 9, 3 – 18, 3 – 26

[AUTOZERO], 1 – 5, 3 – 18 [BIAS GND], 1 – 7, 2 – 5, 3 – 18 [BIAS Shield], 1 – 7, 3 – 18 [GAIN], 1 – 5, 2 – 7, 3 – 18 [INPUT Open], 1 – 6, 3 – 18 [INPUT Shield], 1 – 7, 3 – 18 [Output 1 pps sync], 1 – 6, 3 – 18

Circuit schematics, see Schematic diagrams Clock stopping, 1 – 3, 2 – 7, 3 – 11, 3 – 19 Command

error, 3 – 21, 3 – 26 parameters, 3 – 7, 3 – 21 separator, 3 – 7 terminator, 3 – 6, 7, 3 – 20, 3 – 22

Common-mode rejection (CMRR), ix, 1 – 7, 2

– 4 Compensation, 2 – 3 Console mode, 3 – 6, 7, 3 – 20 Current gain, see Gain Current input, see Input

DB–15, 1 – 10, 11, 2 – 4 DB–9

female, 1 – 11 male, 1 – 11

Default conﬁguration, see Reset Device Clear, 3 – 7, 3 – 20, 3 – 22, 3 – 27 Device error, 1 – 6, 3 – 21, 3 – 26

A – 1

A – 2 Index

Diﬀerence ampliﬁer, 1 – 4, 1 – 7, 2 – 4, 5 Dimensions, ix

Error

command, see Command, error

Execution error, 3 – 13, 3 – 21, 3 – 26

Firmware revision, 2 – 7, 3 – 20 Flow control, 1 – 12, 3 – 6, 3 – 22 FPLC, 1 – 9, 2 – 6, 3 – 6, 3 – 11 Front panel, 1 – 5, 1 – 8, 2 – 7

operation, 1 – 5

Full duplex, see Console mode

Gain, iii, viii, 1 – 2, 1 – 4, 5, 1 – 9, 2 – 3, 2 – 5, 3

– 6, 3 – 12, 3 – 19

accuracy, viii, 2 – 7, 3 – 12 button, see Button, [GAIN] LED, see LED, GAIN selecting, 2 – 8

stability, viii General information, iii Ground, iii, 1 – 2, 1 – 10, 2 – 4

chassis, 1 – 8, 1 – 10, 11, 2 – 4

Earth, 2 – 4

power, 1 – 7, 8, 1 – 10, 11, 2 – 4

signal, 1 – 3, 1 – 7, 8, 1 – 10, 11, 2 – 4

virtual, 1 – 2

Help, 3 – 10

Input, iii, iv, 2 – 3, 2 – 7, 3 – 12

bias, see Bias

bias current, 1 – 2

AC, viii DC, viii

button, see Button, [INPUT Open]

capacitance, viii, 1 – 6, 2 – 2

connector, iii, viii, 1 – 8

shield, iii, viii, 1 – 3, 1 – 7–9, 2 – 4, 3 –

6, 3 – 12, 3 – 19; see also Shield pro-

gram voltage current, iii, 1 – 2, 1 – 4–6, 1 – 8, 2 – 2 impedance, viii, 1 – 2, 2 – 2 LED, see LED, INPUT Open maximum cable length, 2 – 3

oﬀset voltage, iii, viii, 1 – 2, 1 – 6, 7, 2 – 5,

2 – 8, 3 – 12, 3 – 15

adjusting, see Trim, input oﬀset drift, 2 – 8 measuring, 3 – 12, 3 – 15 zeroing, 3 – 14

program, see Shield program voltage resistance, see Input, impedance selecting, viii, 1 – 3, 1 – 6, 1 – 9, 3 – 6, 3 –

12, 3 – 19

shield

button, see Button, [INPUT Shield] ﬂoat, 1 – 3, 1 – 7 LED, see LED, INPUT Shield

voltage, iii, 1 – 2, 1 – 8, 2 – 2, 3 – 15

Interface

direct, 1 – 10

cable, 1 – 11

remote, see Remote interface SIM, see SIM interface

JFET, iv, 2 – 2

LED

AUTOZERO, 1 – 5, 2 – 5 BIAS GND, 1 – 7, 2 – 5, 3 – 12 BIAS OVLD, 1 – 3, 1 – 7, 2 – 5 BIAS Shield

Float, 1 – 7

GND, 1 – 7, 2 – 4 External, 1 – 6, 2 – 6 GAIN, 1 – 5, 3 – 12 INPUT Open, 1 – 6 INPUT Shield

Bias, 1 – 7, 2 – 4

GND, 1 – 7, 2 – 4

Prog, 1 – 7 OUTPUT OVLD, 1 – 3, 1 – 8 Output 1 pps sync, 1 – 6 Unlocked, 1 – 3, 1 – 6, 3 – 15

all on, 2 – 7

Main ampliﬁer, see Transimpedance stage Mainframe, see SIM900

Noise, 2 – 3–5

current, viii, 1 – 2, 2 – 3

SIM918 Precision Current Preampliﬁer

Index A – 3

Johnson, 1 – 2 overall, 2 – 3 voltage, viii, 2 – 3

Non-volatile settings, 1 – 9, 3 – 6, 3 – 11, 12, 3

– 14 Notation, vii, 3 – 6, 3 – 9 Null modem, 1 – 11

Oﬀset

input, see Input, oﬀset voltage output, see Output, oﬀset voltage

Output, 2 – 5

connector, ix, 1 – 8

shield, 1 – 8, 2 – 4 current, ix, 2 – 4 ﬁlter, 2 – 5 maximum load, 2 – 5 oﬀset voltage, ix, 1 – 3, 1 – 7, 2 – 5, 3 – 15

adjusting, see Trim, output oﬀset

measuring, 3 – 15 overload, see Overload, output resistance, ix, 2 – 5 voltage, iii, 1 – 2–5, 1 – 7, 8, 1 – 11, 3 – 12,

3 – 15

limits, ix, 1 – 8, 2 – 5

Output queue, 3 – 6, 3 – 20, 3 – 26, 27 Output 1 pps sync

button, see Button, [Output 1 pps sync]

Overload, 1 – 3, 2 – 7, 3 – 18

bias, ix, 1 – 3, 1 – 7, 2 – 5, 3 – 18, 3 – 28

OVLD indicator, see LED, BIAS OVLD bias plus output, ix, 1 – 8, 3 – 18, 3 – 28 output, ix, 1 – 3, 1 – 8, 3 – 18, 3 – 28

OVLD indicator, see LED, OUTPUT

OVLD

Parity, 1 – 12, 3 – 6, 3 – 22, 3 – 27 Phase-locked loop (PLL), 2 – 6, 7, 3 – 29

keep active, 1 – 9, 2 – 6, 3 – 6, 3 – 13, 14, 3

– 19

oscillator, 2 – 6, 7, 3 – 14

Photomultiplier (PMT), iv Power

ground, see Ground, power requirements, ix, 1 – 10, 11

Power-on, 1 – 9, 2 – 6, 7, 3 – 6, 7, 3 – 11–14, 3 –

18, 3 – 20, 3 – 22, 3 – 25–29

Power line frequency, see FPLC Preparation for use, iv Program, see Shield program voltage

Query command, 3 – 7, 3 – 21, 3 – 25 Quiescent operation, 1 – 3, 2 – 6, 2 – 8, 3 – 14

Rear panel, 1 – 5, 1 – 8 Reference clock, viii, 1 – 2, 3, 1 – 5, 1 – 8, 3 –

14, 15, 3 – 19, 3 – 21

capture time, 1 – 6, 2 – 6, 3 – 14 capture range, see input, frequency limits connector, iii, viii, 1 – 2, 1 – 6, 1 – 8, 2 – 4,

2 – 6, 7, 3 – 13, 3 – 28

direction, 1 – 8, 2 – 7

conﬂict, 1 – 6, 3 – 22, 3 – 26 selecting, viii, 1 – 6, 1 – 9, 3 – 13, 3 – 19

external, ix, 1 – 2, 3, 1 – 6, 2 – 6, 7, 3 – 13,

14, 3 – 19, 3 – 28

Unlocked indicator, see LED, Unlocked LED indicator, see LED, External unlocked, 1 – 3, 1 – 6, 2 – 7, 3 – 14, 15, 3

– 19, 3 – 29

input, 1 – 2, 1 – 6

frequency, 3 – 13 frequency limits, viii, ix, 1 – 6, 2 – 6

internal, ix, 1 – 2, 1 – 6, 2 – 6, 7, 3 – 13, 14,

3 – 19

frequency, viii, 1 – 6, 2 – 6, 3 – 13

levels, viii, 1 – 6 lock acquisition time, see capture time output, 1 – 2, 1 – 6, 2 – 6

frequency, see internal, frequency

source, ix, 1 – 2, 1 – 6, 2 – 6

Registers, see Status, registers Remote interface, ix, 1 – 3, 1 – 9, 2 – 7, 3 – 1, 3

– 7, 3 – 10, 3 – 19

data format, 3 – 13

Reset, 1 – 9, 2 – 8, 3 – 18–20, 3 – 22

power-on button hold, 1 – 9, 3 – 18

RS–232, 1 – 3, 1 – 11, 3 – 6, 7, 3 – 22, 3 – 27

settings, 1 – 12, 3 – 22

Safety, iii

biomedical applications, iii

SIM918 Precision Current Preampliﬁer

A – 4 Index

Schematic diagrams, 4 – 2 Self-test, 3 – 20 Serial interface, see RS–232 Serial number, 3 – 20 Service, iv Set command, 3 – 7, 3 – 21 Shield

bias, see Bias, connector, shield input, see Input, connector, shield

Shield program voltage, iii, 1 – 3, 1 – 7, 8

connector, iii, viii, 1 – 7, 8, 2 – 4 input resistance, viii limits, viii

SIM900, iv, 1 – 3, 1 – 9–11, 2 – 4, 2 – 6, 3 – 6, 7 SIM interface, ix, 1 – 10, 3 – 25

connector, 1 – 10, 2 – 4

Source resistance, 2 – 3 Speciﬁcations, viii Stability

from oscillation, 2 – 2 gain, see Gain, stability

Status, 3 – 15, 3 – 24

registers, 3 – 16, 3 – 24

CESE, 3 – 17, 3 – 27

CESR, 3 – 6, 3 – 16, 3 – 25–27

ESE, 3 – 16, 3 – 20, 3 – 26

ESR, 3 – 6, 3 – 16, 3 – 20, 3 – 25

OLSE, 3 – 17, 3 – 20, 3 – 28

OLSR, 3 – 17, 18, 3 – 25, 3 – 27, 28

RCSE, 3 – 17, 3 – 29

RCSR, 3 – 17, 3 – 19, 3 – 25, 3 – 28, 29

SB, 3 – 16, 3 – 25–29

SRE, 3 – 16, 3 – 20, 3 – 25

Sticky bits, 3 – 24, 25

input oﬀset, 1 – 6, 1 – 9, 2 – 5, 3 – 15 output oﬀset, 2 – 5, 3 – 15

Warmup, 2 – 6 Weight, ix

Zeroing ampliﬁer

input oﬀset voltage, 1 – 2

zeroing, 3 – 14

Temperature, ix, 2 – 6 Token, 3 – 7, 8, 3 – 21

mode, 3 – 19, 3 – 22

Transimpedance, iii, 1 – 2, 2 – 2

ampliﬁer, 1 – 2, 2 – 2, 3

Transimpedance stage, iii, 1 – 4, 1 – 7, 2 – 2–5,

3 – 12, 3 – 19, 3 – 28 front-end ampliﬁer, iv, 1 – 6 output voltage, 1 – 8, 2 – 2, 2 – 5, 3 – 15

Trim

autozero control loop, 2 – 5, 3 – 15

SIM918 Precision Current Preampliﬁer

Stanford Research Systems SIM918 Operating And Service Manual

Specifications and Main Features

Frequently Asked Questions

User Manual