MODEL 20B
          GENERAL PURPOSE DIFFERENTIAL AMPLIFIER
                                             LAWSON LABS, INC.
                                             3217 Phoenixville Pike 
                                             Malvern, PA 19355

                                               610 725-8800
                                                    or
                                               800 321-5355

Rev. D
                       TABLE OF CONTENTS

INTRODUCTION                                        Page 2

1.  INTERCONNECTIONS                                Page 2

2.  CONNECTING INPUTS                               Page 3

3.  OPERATION                                       Page 5

4.  USING MULTIPLE MODEL 20Bs                       Page 7

5.  TROUBLESHOOTING                                 Page 9

6.  INPUT CODE SUMMARY                              Page 10

7.  INTERNAL ADJUSTMENT & CALIBRATION               Page 10

8.  SPECIFICATIONS                                  Page 11

WARRANTY                                            Page 12

INTRODUCTION

     The Lawson Labs Model 20B Differential Amplifier, in
combination with any Lawson Labs precision A/D card, allows a
personal computer to be used as a multi-channel millivoltmeter.
Despite the low per channel cost, the Model 20B delivers highly
accurate and stable data when properly applied.

     The inputs to the Model 20B are protected against transient
overvoltages of up to 120 volts or 65 volts continuous.

     Four Model 20Bs can be used with one Model 134 or Model 140
A/D card. Two Model 20Bs can be used with one Model 141 A/D card.
Systems up to 512 channels can be assembled using Model 66
Expansion modules. Much larger systems are possible. Contact the
factory for details.

SECTION 1.    INTERCONNECTIONS

     Seven wires will be needed to connect the Model 20B to the
A/D card. See the appropriate section in the A/D card's manual
for pin locations. The interconnecting wire can be of any
general-purpose type. We strongly recommend shielded wire for all
connections. Small diameters and long lengths of wire will not
interfere with system function except in extreme cases. For
example, 250 feet of 26AWG shielded wire will not significantly
affect the system. The "+" output should connect to one of the
plus input pins on the A/D's connector and the "-" output should
connect to the corresponding minus input at the A/D card. The
examples in this manual assume the first Model 20B is connected
to input #0. If another input is used, the control codes should
be adjusted as described in the A/D manual. The ground terminal
(GND) on the Model 20B should be connected to chassis ground at
the computer.

WARNING: Use a cover-securing screw (without paint) for chassis
ground.

     On some portable computers, no metal chassis contact is
accessible. In that case, use the center screw of the power
outlet as ground. We do not recommend using the ground pin on the
A/D card connector instead of chassis ground because it is
possible that under certain catastrophic conditions the ground
current might be large enough to cause damage to the computer.

     The control inputs to the Model 20B should now be connected
to the isolated control output lines on the A/D card connector.
Input A is connected to output A on the A/D, input B is connected
to output B on the A/D, and input C is connected to output C on
the A/D card. Connect the Guard pin on the A/D card connector to
the GND terminal on the Model 20B.

NOTE: There should now be two separate wires from the computer
connected to the GND terminal of the Model 20B.

FIGURE 1.    INTERCONNECTION DIAGRAM FOR MODEL 20B

SECTION 2.    CONNECTING INPUTS

     Any type of millivolt source may be used with the Model 20B.

NOTE: No input voltages need be connected to test operation. You
may want to connect one source only and then check its operation
before connecting additional inputs.

     The input connections for the amplifier are inside the case.
Unplug the power supply and then remove the four screws at the
extreme corners of the cover. Inside the 20B you will find seven
pairs of screw terminals and six ground terminals. For shipment,
the input terminals are connected together and to ground. The
ground wires prevent static discharges, which can be of extremely
high voltage, from damaging the circuitry. You will find a
diagram of the input terminal locations on the circuit card
itself. The terminals labeled just "+" and "-" are selected with
a channel code of 3. Terminals labeled 3 through 6 are selected
using codes 4 through 7.

FIGURE 2.    MODEL 20B AMPLIFIER INPUT CONNECTIONS

     Run the first input cable through one of the holes in the
case. Remove the grounding wire from terminal pair #1 and connect
the plus wire to the plus terminal and the minus wire to the
minus terminal. If a shield is present, you may want to connect
it to one of the ground terminals (labeled G). Remember that both
the plus and and minus inputs must be within 2 volts of ground
potential for proper operation. If the voltage source is
floating, then the minus input should be connected to ground. If
multiple inputs each have a minus wire connected to a remote
common point, then either the common point should be grounded or
one (and only one) of the minus leads should be jumpered to
ground at the Model 20B. See Figure 3.

CAUTION: Excessive torque can damage the ground terminals.

     Connect any other sources in the same fashion. Holes are
provided at the card's edge for strain relief. Secure the wires
to the card with lacing cord or cable ties for protection against
mechanical failure. Replace the cover and securing screws.

NOTE: If a source is removed from the input terminals, it must be
replaced by a grounding wire.

FIGURE 3.    EXAMPLES OF INPUT SOURCES

SECTION 3.    OPERATION                                       
                                                              
     After the interconnections have been completed, plug in the
power cord and turn on the computer. If you are using the Lawson
Labs PC64 Data Logging Software, run the program. Configure the
A/D channel to which you have connected the Model 20B as a
multiplexer and enter the G factor. You should now see millivolts
at the amplifier input displayed on the screen. Amplifiers with
gains of 100 or more are scaled to display in microvolts.

     The following detailed description is for programmers who
want to integrate Model 20B support into their application
programs. In the following example, the Model 20B is assumed to
be at A/D channel #0. The Model 134 example assumes all switches
are OFF. The Model 140 example assumes switch #4 is in the ON
position. The Model 141 example assumes switches #1, 4, 5 and 6
are ON and all others are OFF. If your address switches are set
differently, see your A/D manual for the correct ADC address.

     Enter the following program. Set BOARDCODE in line 2030 to
128 for a Model 141; set it to 0 for a Model 134 or 140. If you
are using a Model 141, refer to the A/D board manual and be sure
to calibrate the 141 card in bipolar mode before proceeding.

     The main program below begins at line 1000. It calls various
subroutines to perform the necessary operations. Subroutine 2000
sets the constants for your particular combination of Model 20B
and A/D board. PAUSE for the Model 141 should be set to the
settling time of the A/D board plus 0.1 second. The settling time
is listed in the 141 manual. The 20B itself requires only one
millisecond settling time. The 0.1 second pause is needed because
of the resolution of the TIMER function. Subroutine 3000 reads
and displays the offset from channel #0 of the Model 20B. The
format for display may be changed if desired to display fewer
digits. Subroutine 7000 provides the necessary delay for the
hardware. Subroutine 8000, which depends on A/D board type, does
the actual reading of the data and the conversion to millivolts.

1000 REM Sample program
1010 GOSUB 2000: REM Set constants for your system
1020 ADCHAN = 0: REM Assume A/D board channel #0
1030 GOSUB 3000: REM Read and display offset
1090 END

2000 REM Set constants
2010 PAUSE = 0.1: REM For the 141, enter settling time + 0.1
2020 ADC = [value]: REM Your A/D board address
2030 BOARDCODE = [value]: REM 128 for Model 141, 0 for others
2070 RETURN

3000 REM Read and display offset
3010 OUT ADC, BOARDCODE + ADCHAN * 16
3020 GOSUB 8000
3030 OFFSET = V
3040 PRINT USING "A/D Channel ##"; ADCHAN;
3050 PRINT USING "    Offset #####.#### mv"; OFFSET
3060 RETURN

7000 REM Delay for settling time of PAUSE sec.
7010 IF TIMER > 86395 THEN GOTO 7010
7020 START = TIMER
7030 IF TIMER < START + PAUSE THEN GOTO 7030
7040 RETURN

   Include this subroutine if you are using a Model 140.

8000 REM Returns Model 140 voltage in millivolts
8010 OUT ADC + 1, 0
8020 IF INP(ADC) AND 128 THEN 8020
8030 X = INP(ADC)
8040 V = X MOD 16 + (INP(ADC + 1) MOD 16) * 10  + (INP( ADC + 2)
     MOD 16) * 100 + (INP(ADC + 3) MOD 16) * 1000
8050 IF X AND 32 THEN V = V + 10000
8060 IF X AND 64 THEN V = -V
8070 V = V * .25
8080 RETURN

     Include this subroutine if you are using a Model 134. In
line 8030, you may set AVERAGE anywhere from 1 to 255. Larger
numbers reduce noise while smaller numbers increase speed.

8000 REM Returns Model 134 voltage in millivolts
8010 GOSUB 7000
8020 X = INP(ADC)
8030 TOTAL = 0: AVERAGE = 20
8040 FOR X = 1 TO AVERAGE
8050 IF INP(ADC - 1) AND 2 THEN 8050
8060 V = INP(ADC + 1) * 256! + INP(ADC)
8070 TOTAL = TOTAL + V
8080 NEXT X
8090 TOTAL = TOTAL / AVERAGE: V = (TOTAL - 32768) * .152588
8100 RETURN

     Include this subroutine if you are using a Model 141.

8000 REM Returns Model 141 voltage in millivolts
8010 GOSUB 7000
8020 OUT ADC + 1, 1
8030 H = (INP(ADC + 2) AND &H0F)
8040 M = INP(ADC + 1)
8050 L = INP(ADC)
8060 OUT ADC + 1, 0
8070 COUNT = L + M * 256 + H * 65536
8080 V = COUNT * 9.536743E-03 - 5000
8090 RETURN

     RUN the program. The offset voltage from the 20B is
displayed on the screen. The value of the offset for the
amplifier should be close to zero volts. If the short-term
variation exceeds a millivolt, check for loose connections or
improper grounding. Note that channel 0 on the Model 20B is a
dedicated offset channel.

  Add the following lines to your program. LASTCHAN in line
2060 is the number of millivolt sources attached to your Model
20B assuming the first source is connected to channel 1. The
voltage at the input terminals will be printed at line 5090.

1050 FOR MUXCHAN = 1 TO LASTCHAN
1060 GOSUB 5000: REM Read and display input
1070 NEXT MUXCHAN

2050 G = [value]: REM from Model 20B case
2060 LASTCHAN = [value]: REM number of sources attached

5000 REM Read and display input
5010 M20BCHAN = MUXCHAN
5030 OUT ADC, BOARDCODE + ADCHAN * 16 +  M20BCHAN
5040 GOSUB 8000
5050 V = (V - OFFSET) * G / 100
5070 PRINT USING "A/D Channel ##"; ADCHAN;
5080 PRINT USING "    20B Channel ##"; M20BCHAN;
5090 PRINT USING "    ###.###### mv"; V
5100 RETURN

SECTION 4.    USING MULTIPLE MODEL 20Bs

     It is possible to connect two, three, or four Model 20Bs.
When multiple Model 20Bs are connected, all must be powered for
the system to function properly.

FIGURE 4.    INTERCONNECTION DIAGRAM FOR MULTIPLE MODEL 20Bs

                         
     For multiple amplifier operation, add or change the
following lines marked by asterisks in the program. MAXCHAN is
the highest A/D channel number which has a Model 20B connected to
it. Insert all the applicable G constants identified by their
channel numbers.

  1000 REM Sample program
  1010 GOSUB 2000: REM Set constants for your system
* 1020 FOR ADCHAN = 0 TO MAXCHAN
  1030 GOSUB 3000: REM Read and display offset
* 1050 FOR MUXCHAN = 1 TO LASTCHAN(ADCHAN)
  1060 GOSUB 5000: REM Read and display input
  1070 NEXT MUXCHAN
* 1080 NEXT ADCHAN
  1090 END

  2000 REM Set constants
  2010 PAUSE = 0.1: REM For the 141, enter settling time + 0.1
  2020 ADC = [value]: REM Your A/D board address
  2030 BOARDCODE = [value]: REM 128 for Model 141, 0 for others
* 2050 G(0) = [value]: G(1) = [value]: REM etc.
* 2060 LASTCHAN(0) = [value]: LASTCHAN(1) = [value]: REM etc.
  2070 RETURN

  3000 REM Read and display offset
  3010 OUT ADC, BOARDCODE + ADCHAN * 16
  3020 GOSUB 8000
  3030 OFFSET = V
  3040 PRINT USING "A/D Channel ##"; ADCHAN;
  3050 PRINT USING "    Offset #####.#### mv"; OFFSET
  3060 RETURN

  5000 REM Read and display input
  5010 M20BCHAN = MUXCHAN
  5030 OUT ADC, BOARDCODE + ADCHAN * 16 + M20BCHAN
  5040 GOSUB 8000
* 5050 V = (V - OFFSET) * G(ADCHAN) / 100
  5070 PRINT USING "A/D Channel ##"; ADCHAN;
  5080 PRINT USING "    M20B Channel ##"; M20BCHAN;
  5090 PRINT USING "    ###.###### mv"; V
  5100 RETURN

     RUN the program again. The voltages for all A/D channels and
Model 20B channels will be displayed.

SECTION 5.    TROUBLESHOOTING

GENERAL:   If problems occur, first short the input terminals of
the amplifier. If that channel then reads near zero volts, then
the problem is probably with the source or its connections.

A.  If the voltage reads near zero when you think it shouldn't,
check that the cord is plugged in and that the outlet is live.
Either the amplifier input or output could be short-circuited.
Double-check that guard on the A/D card is connected to GND on
the amplifier.

B.  If the polarity is reversed, reverse the input leads.

C.  If the voltage readings seem random, check for a broken wire.
Check that unused input terminals are grounded. Also make sure
that the correct input channel is being selected.

D.  If the voltage readings are consistent but wrong, check for
redundant signal grounds.

E.  If the readings show excessive scatter, check for loose
connections. Make sure the GND terminal on the amplifier is
connected to chassis ground at the computer and that ungrounded
sources have a connection from minus in to G in the Model 20B.
Perhaps shielding for the amplifier output wires is needed.

F.  If more than one device is connected to the output port, all
must be turned on for any to work properly.

G.  If a particular source is reading erratically in a multiple
20B system, check the like-numbered sources on the other boxes.

H.  Erratic readings can be caused by ground loops or missing
grounds. Try removing redundant ground wires or adding a
connection from the minus input terminal to ground.

I.  Erratic readings can also be caused by an extremely fast
computer over-running the settling ability of the Model 20B. Try
adding a longer pause in the program after the channel selection
but before the A/D conversion is initiated.

SECTION 6.    INPUT CODE SUMMARY

CODE         BINARY               AMPLIFIER
           C    B    A            TERMINALS

 0         0    0    0            (OFFSET)
 1         0    0    1            INPUT #1
 2         0    1    0            INPUT #2
 3         0    1    1            INPUT "+" and "-"
 4         1    0    0            INPUT #3
 5         1    0    1            INPUT #4
 6         1    1    0            INPUT #5
 7         1    1    1            INPUT #6

NOTE: There are no input terminals on channel #0 of the Model
20B. That channel is used only for software offset correction.

SECTION 7.    INTERNAL ADJUSTMENT & CALIBRATION

     The only trim adjustment inside the Model 20B is an offset
trim. It is normally set at the factory to 0.0 volts for the
amplifier. The output should be set with input code #0 selected.
The offset will change slightly with temperature but the effects
are cancelled in software and the drift will not cause errors.
With the offset at 0.0 volts the Model 20B has approximately
equal range in the positive and negative directions. If more
range is needed in either direction, the offset can be moved away
from zero by rotating the shaft of the trimpot with a small
screwdriver.

     There is no hardware gain adjustment. If you want to fine
tune the gain, adjust the G factor to match a known input
voltage. If you wish to have the calibration checked, contact
Lawson Labs for details.

SECTION 8.    SPECIFICATIONS

INPUTS:                 7 fully differential channels

INPUT OVERVOLTAGE
PROTECTION:             120 volts transient
                        65 volts continuous

INPUT IMPEDANCE:        1000 megohms typical

LINEARITY:              0.05% of full scale

SETTLING TIME:          400 microseconds

DIFFERENTIAL MODE
INPUT RANGE:            +/- 100 mv nominal with amplifier
                        gain of 50

COMMON MODE
INPUT RANGE:            +/- 2 volts

CONTROL INPUTS:         6 volt logic, negative true. Compatible
                        with all Lawson Labs A/D cards.

POWER
REQUIREMENT:            105-135 VAC, 50-60HZ, 6 Watts

SIZE:                   6.25 x 3.75 x 2 inches

OPTIONS:                Premium low-noise amplifier
                        Custom gain setting (x2 to x400)
                        DC powered, panel mounting

               LIMITED WARRANTY

      All Lawson Labs, Inc. products are guaranteed against
defects in materials and workmanship for a period of one year
from the date of delivery. Products must be returned to Lawson
Labs for warranty service. Contact Lawson Labs, Inc. at
800 321-5355 for return authorization before returning anything
for service. The above warranty is in lieu of all warranties
express or implied. Lawson Labs will not be liable for indirect
or consequential damages caused by any defect in this product.
Some states do not allow the limitation of consequential damages,
so the above exclusion may not apply to you.