Stanford Research Systems SIM918 Operating And Service Manual
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.
Copyrightc Stanford Research Systems, Inc., 2006. All rights reserved.
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
i
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
V
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
8
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
◦
C
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
bias
in
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:
V
= 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
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
1
1
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)
R
F
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:
V
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
2
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.
2
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.
3
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
3
FPLC equals the principal rejection frequency of an internal analog-to-digital
converter used to measure the input offset trim. See the command OFST.
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