Tektronix Keithley AIM4 Isolated High-Level Analog Input Module Rev. B User manual

AIM4

Isolated High-Level Analog Input Module

The AIM4 Isolated High-Level Analog Input Module provides isolation and signal con­ditioning for four channels of voltage input (from *5OMV to *5V) or current loop in­put. Because the input channels are completely isolated from other channels and from
the system ground, the AIM4 module accepts signals with common mode voltages as
high as *50W, and provides protection to #JOV (normal mode). Channel-to-channel
isolation is 500V.

The AIM4 module provides switch selected gains of xl or x100. By changing resistor
and potentiometer values, individual local gains can be selected for each channel.

The AIM4 module may be placed in slots 3-10 of the baseboard (slots 2-10 if AMMl is
used). To install the module, remove the top cover and insert the module in the desired
slot with the component side facing the power supply. Place the AlM4 module as far
away from the power supply as possible to minimize noise and thermal effects.

CAUTION: Always turn off the power before installing or removeing modules. To
minimize the possibility of EM1 radiation, never operate the system with top
removed. Connect the + and -

inputs of unused channels together.

cover

User-Configured Components

Gain amplification for each channel can be changed by setting switches SW.-S104 to the
desired positions (xl or x100). As shipped the AIM4 is configured for xl gain (see
Figure 1).

Installing resistors in the appropriate locations provides current to voltage conversion,
allowing connection of current loop inputs.

All inputs are connected to a bank of screw terminals, which provides positive and
negative (high and low) terminals for each of the four input channels. See Table 1 for
summary of user-configured components.

Table 1. User-Configured Components on the AIM4
Name Designation Function

Switches lOl-104 SlOl, Sl.02, SlO3, S104 Gain setting channel 0, 1, 2, 3 (xl or x100)
Potentiometer Rlffl, R105, RlO9, Rll3 Trim gain for each channel
Resistors User Installed

Screw Terminals 162

Optional current voltage resistors.
Innut saew terminals. channels O-3

Document Number: 500-91401 Rev. B

AM41

AIM42

Figure 1. AIM4 Module Configuration

Connections
Terminals for connections to the AIM4 module are marked on the module. Typical con-

nections are shown in Figure 2. The use of shielded cable is recommended to minimize
noise pick up and the possibility of EMI radiation. One end of the shield should be

connected to the AIM4 ground and the other end left disconnected. The shield should

not be used as a signal path.

WARNING: Dangerous user-supplied voltages may be present on the input terminals.

CAUTION: Maximum input with xl gain is f5V. Maximum input with xl00 gain is

*5OmV.

SHIELD

Figure 2. Typical AIM4 Connections

Gain Adjustment

Switches SlOl-S104 set the gain applied to channels O-3 to either xl or x100. To set the
gain, place the switch in the desired position. Note that the module is configured for
xl gain as shipped. Remember that the maximum input voltage is i5V (xl gain) or

st5OmV (x100 gain). The maximum signal output range of the AIM4 module is k5V. Do
not apply gains when the resulting voltage will exceed &5V or measurement errors will
result.

By changing the values of user-installed resistors, alternate gain factors can be applied
to signals connected to the AIM4 module. In this way, the gain for each channel can be
determined.

The selection of a resistor should be based on the following formula:
G =1 + 10,000/R

Where G is the gain applied to the signal and R is the value in ohms of the installed
resistor.

Gain Trimming
Potentiometers RlOl, RI.05, R109 and Rll3 are provided for trimming the gain resistors

R102, R106, RllO and Rl14. If new gain resistors are reinstalled, different potentiometers
may be required.

AIM43

The relationship of the potentiometers and the gain resistor on the total resistance is
described by the formula:

Where RT is the total resistance (the value which ultimately affects the gain factor), R,
is the value in ohms of the gain resistor, and RP is the value in ohms of the poten­tiometer (trimmed to the desired value).

Current Inputs
With the installation of optional resistors in user-installed locations, the AIM4 module

can be configured to accept floating current loop inputs. The resistors are installed bet­ween the high and low input terminals converting the current range of the signal to an
appropriate voltage range.

The selection of the resistor is based on Ohms law:
E=I.R

Voltage (volts) = Current (amps) * Resistance (ohms)

Because the AIM4 module has an input range of *5V (with xl gain), these resistors
must convert the expected current range into a voltage range within these limits.
Therefore, volts should be set to 5, and current set to the upper limit of the expected
current range.

With a signal expected to range from 4 to 2OmA, for example, Ohm’s law yields the
following equation:

5 (volts) =

.02 (amps) * R (resistance)

R =250

As a result, a 25O!J resistor should be instaIled for that signal range. The resulting
voltage range of 1 to 5V can be further amplified by the PGA (on the AlMl module) if
the input range of the A/D converter is set to *lOV, or 0-1OV.

If there are several loads on the loop (if, for example, an analog chart recorder is being
driven at the same time), the compliance voltage at the current loop transmitter may be
exceeded. If this is the case, a smaller resistor (possibly 25Q) can be installed, and more
gain applied to the signal, either globally at the PGA, or locally by changing resistors.

Arm-4

Additional loads in series with the current loop can be connected on either side of the
isolated input.

Maintaining Accuracy
Each of the four inputs on the AIM4 module has a separate isolated input stage, each

of which feeds a signal to a common output stage. Each input stage has a separate ad-

justment potentiometer, making it possible to null the input offset. A single adjustment

takes care of any output offset. To maintain AIM4 accuracy, the following procedure
should be used:

1. Connect a jumper between the + and - inputs of channel 0.

2. Set the gain switch for channel 0 to the xl position and adjust the output offset con­trol (R119) for a zero reading.

3. Set the gain switch to the xl00 position.

4. Adjust the channel 0 input potentiometer (R104) so that a value of OV is read back
from the module.

5. Set the gain switch to the xl00 position for each of the other channels.

6. Short the inputs to each of the other three channels and adjust the respective input
offset potentiometers for a zero reading. (RlO channel 1, Rll2 = channel 2, R116 =
channel 3).

7. The gain potentiometer for each channel can now be adjusted for xl00 gain (50mV in­put = 5v output).

RlOl = Channel 0 Gain
R105 = Channel 1 Gain
Rlo9 =
Rll3 = Channel 3 Gain

Channel 2 Gain

Input Attenuation
The AIM4 module may be modified to apply a 1OO:l attenuation factor to the input

signal, thus allowing the module to measure voltages up to rtlOOV. To modify the
AIM4, install a 100 resistor in the user installed location for the channel in question.
Remove the input inductor for that channel (El02 for channel 0, El04 for channel 1,
El06 for channel 2, and El.08 for channel 3). In its place, install 99OkQ resistor. To main­tain system accuracy, both resistors should have a maximum tolerance of *O.l%*

When reading data back from a modified channel, you must correct for the attenuation
factor in order to obtain a correct reading. When using commands listed in this manual,

simply multiply the voltage reading obtained by a factor of 100. When using the Soft500

system, a similar correction factor must be applied. These correction factors are ex­clusive of other factors necessary because of certain parameters within the system (such

as global gain applied by the AIM1 module).

*May be obtained from Keithley Data Acquisition and Control. Order part number

R-263-1Ok for the lOQ resistor. Order R-282-90k and R-282-900k and
connect in series to make up 99OkQ.

Commands

The AIM4 module commands are summarized in Table 2. The locations of slot-

dependent commands are presented in Table 3.

AIM45

Table 2. Commands used with AIM4
Command Location

SELECT CHANNEL CMDA (Slot-denendentj

Table 3. Locations for Slot-dependent CMDA
Slot Location

Slot 3
Slot 4
Slot 5
Slot 6
Slot 7
Slot 8
Slot 9
Slot lo

SELECT CHANNEL
Location: Slot-dependent CMDA

EE

CFE88
CFF8A
CFFSC
CEF8E
CFF90
CEF92

The SELECT

select individual channels for A/D conversion. When selecting a channel, the number of
that channel is written into the CMDA location- On the AM4 module four channels,
numbered O-3, can be selected with the SELECT CHANNEL command.

SELECT CHANNEL should precede the SELECT SLOT command, which should be
issued with the number of the slot in which the AIM4 module is installed. SELECT
SLOT affects the global multiplexer on the AIM1 and is described in the reference sec­tion for that module.

When measuring the same channel repeatedly, the SELECT
not be reissued for every measurement. Similarly, when several channels on the same
module are read in succession, the SELECT SLOT command need only be issued once
at the start of the sequence. Both SELECT SLOT and SELECT CHANNEL must be

issued before starting any AID conversions, or the data will be invalid. See Table 4 for
values written to the SELECT CHANNEL location.

Table 4. Values Written to SELECT CHANNEL
Function Binary Hex

Channel
Channel
Channel
Channel

CHANNEL command location is used by alI analog input modules to

CHANNEL command need

Decimal

0 00000 HO
1 00001 Hl
2 ooolo I32
3 00011 H3

0
1
2
3

AlM4-6

AIM4 Module Calibration

Use the procedure below along with the information in Figure 3 to calibrate the AIM4
module. Use shielded cable for calibration connections, connect the shield to module
ground only.

1. Place the A&I4 module in slot 4.

2. Connect the DMM high input to the AIM4 analog output (WlOl). Connect the
DMM low level to module ground.

3. Connect jumper wires between the + and - terminals of all channels.

4. POKE the SELECT

0.

5. Place the channel 0 gain switch (SlOl) to the xl position and adjust the output off­set control (Rll.8) for a reading of 0.000 +l count on the DMM.

6. Place the channel 0 gain switch in the xl00 position and adjust the channel 0 input
offset control (RlO4) for a reading of 0.000 fl count on the DMh4.

7.

Set the gain switches of the other channels to the xl00 positions.

8. Adjust the input offset controls for the other three channels in a similar manner.
POKE the SELECT CHANNEL location with the values for the desired channel (1 =
channel 1, 2 = channel 2, 3 = channel 3). Adjust the input offset control for a
reading of O.OOO*l count on the DIvlM (RlO8 = channel 1, RX2 = channel 2, Rll.6 =
channel 3).

9. Remove the shorting jumper from the channel 0 terminals, but leave all gain swit­ches in the xl00 positions. Leave all other inputs shorted.

Xl. Connect the calibrator high terminal to the + input of channel 0. Connect the

calibrator low terminal to the - input of channel 0.

11. POKE the SELECT CHANNEL location with a value of 0. Set the calibrator output
to a value of 5OmV

12. Adjust the channel 0 gain control for a reading

33. Repeat steps lo-12 for the three remaining channels. Remove the jumper only from
the channel being calibrated. The calibrator must be connected to the channel being
calibrated, and the SELECT CHANNEL location must be POKED with the correct

value for that channel. Adjust the corresponding channel gain control for a reading
of 5V &In-iv on the DMM with a 50mV input from the calibrator. The inputs for the
channels not being calibrated must be kept shorted.

CHANNEL location (CEF86) with a value of 0 to select channel

of 5V

*lnW on the DMM.

AlM4-7

~zz-j

$$O

“35

-%tE

OUTPUT OFFSET
CHAN 3 INPUT OFFSET

CHAN 2 INPUT OFFSET
CHAN 2 GAIN

-WpCCU

CHAN 1 INPUT OFFSET
CHAN 1 GAIN

6-200K
b-m

CHAN 0 INPUT OFFSET
CHAN 0 GAIN

6--

-

AIM48

Figure 3. AIM4 Calibration Adjustments

Theory of Operation

Refer to drawing number 500-166 for the following discussion.

The primary component on the AIM4 module is a 4 channel isolated input circuit, UlOl

(Analog Devices 2B54A). This circuit provides isolated signal conditioning and
multiplexing for input signals ranging from *5OmV to *5V. The output of UlOlO is
routed to the AN OUT path on the Series 500 baseboard.

Each of the four input channels is electrically isolated from system ground and from
other input channels. Circuits U102 and 103 are responsible for channel selection from
among the four channels. Ul.03 stores the value of DO and Dl, as set by the SELECT
CHANNEL command (signal line CMDA); U102 decodes the output of Ul.03, producing
four outputs, designated YO-y3 that drive the channel selection circuitry in UlOl. U103
is a quad transparent data latch (74LS75), and U102 is a one-of-four binary to decimal
decoder (74LSl39).

Potentiometers RlO4, RlO8, Rll2, and Rll6 trim the input offset of Channels O-3 respec­tively, and Rll.8 provides an output offset adjustment for UlOl.

Gain for each channel is set by a switch, a fixed resistor, and a potentiometer. Switches
SlOl-S104 select xl/x100 gain for channels O-3 respectively, while the parallel combina­tion of the resistor and potentiometer determine the gain of the channel in xl00 mode.
Rl20, R106, RllO and R114 are the gain resistors for channels O-3 respectively, while
potentiometers RlOl, R105, Rl.09 and Rl16 are respective trim adjustments for channels
o-3.

AIM4 Specifications

Input channels: 4 isolated from each other and ground

Input characteristics:

Gain: xl, x100, switch selectable for each channel. User configurable for other gains

with optional resistor.

Input range:

xl, *5V max
xl.00, *5OmV max

Accuracy:

Gain:

xl, *0.2%

x100, *O-2% adjustable to 1 lsb
Gain non-linearity: f0.02% max
Input offset: rt5OmV max, adjustable to zero
Output offset: &i2nW max, adjustable to zero

Temperature coefficient:

xl, *0.0025%/“c
x100, *0.0050%/“c

Input offset: *5pV/“C
Output offset: *5O@/“C

Input noise voltage: 1ccV p-p, O.OlHz to lOOHz, Rs <lJsQ

Input bias current: + 8nA max
Input resistance: lOOMQ
Protection: DOV RMS max normal mode, f I= 6OHz
Isolation: 500V peak, channel to channel or channel to ground
Common mode rejection: 145dB, xlO0, R, <loO$ f I = 6OHz
Normal mode rejection: 55dB, x100, f z=5OHz
Settling time after channel selection: 2.5msec to O.Ol%, assuming settled input

AIMlAo

i-2lXU

-

AIM4 COMPONENT LAYOUT

AIM4WAIM4-12

I;-

IllIIIU I I*1

I

AIM4 SCHkMATlC DIAGRAM

AIM4WAIM414