Industrologic SGAMP-2

Strain Gauge/Load Cell Amplifier

Reference Manual

version 11/11/13

Industrologic, Inc.
3201 Highgate Lane
St. Charles, MO

Phone: (636) 723-4000

Trademark and Copyright Information

SGAMP is a Trademark of Industrologic, Inc. 2000

This document is Copyright (c) 2013 by Industrologic, Inc.
All rights reserved.

No part of this manual may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or scanning, for any purpose other than the personal use by the purchaser of this product. Industrologic, Inc. reserves the right to revise this document at any time without obligation to notify any person of such revision. Industrologic, Inc. assumes no responsibility for any errors that may appear in this document.


Whereas effort has been made to make the information in this document as useful and accurate as possible, Industrologic, Inc. assumes no responsibility for the application, usefulness, or completeness, of the information contained herein. Under no circumstance will Industrologic, Inc. be responsible or liable for any damages or losses including indirect or consequential damages or losses arising from either the use of any information contained within this manual or the use of any products or services referenced within this manual.

Industrologic, Inc. reserves the right to change any product's features, specifications, documentation, warranties, fee schedules, and conditions at any time and without notice.

Industrologic products are provided as printed circuit board level assemblies. Some Industrologic products may include mechanical packaging assessories which are provide for the purchaser's convenience, and are not intended to be final assembly enclosures.

Industrologic products are provided with the understanding that the purchaser will determine fitness for any particular application as well as achieve compliance with any particular specifications or regulations as required or desired for that application.


Industrologic, Inc. products are warranted to be free from defects in workmanship or material under normal use and service for a period of one year from date of purchase. Industrologic's warranty is to the original purchaser only, but is extended to you by the Distributor or OEM from whom you purchased the product.

Industrologic's obligation under this Warranty is limited to replacement with a new or repaired unit to the original purchaser. The unit must be returned, and replacements obtained from Industrologic, Inc. or the Distributor or OEM from whom it was purchased.

This warranty will not apply if, in the judgment of Industrologic, Inc., damage or failure of the product has resulted from accident, alteration, misuse, abuse, connection to or operation with incompatible signals or power sources, or improper installation.

Under no circumstances shall Industrologic, Inc. be liable to purchaser or any third party for any loss of profits or other direct or indirect costs, expenses, losses, or consequential damages arising out of or as a result of any defects in or failure of Industrologic products or any part or parts thereof.


Return and Repair Policy

Products purchased in single piece quantities may be returned, if in resellable condition, for a full refund of the purchase price only, to the original purchaser, within 30 days of purchase.

Products must be returned by original purchaser with a Return Materials Authorization (RMA) number issued by Industrologic, Inc. or the Distributor or OEM from whom it was purchased, authorizing its return. Current repair or replacement fees and procedures will be related during the issue of the RMA number.

Warranty or repair units must be returned with postage and insurance paid by purchaser. Industrologic, Inc. is not responsible for damages or loss during shipment.

Industrologic SGAMP-2

Strain Gauge Amplifier

The SGAMP-2 Strain Gauge Amplifier is a miniature printed circuit board assembly designed to amplify strain gauges arranged in a full Wheatstone bridge configuration, and is suitable for many applications where a bridge or differential input amplifier is required.

picture of SGAMP-2

Summary of Features

Mechanical specifications

.062" FR-4 glass epoxy printed circuit board, 1.1" long by 0.7" wide by 0.4" thick

mechanical drawing of SGAMP-2

Electrical Specifications

For chip specific information refer to the Analog Devices AMP04 data sheet.

Operating Environment:
Temperature: 0-50 degrees Celsius (32-122 Fahrenheit)
Humidity: 30 to 80 percent, non-condensing

Power Supply Requirements:
Operating voltage: 8 to 30 volts DC regulated or unregulated (filtered)
Operating current: 5mA, plus bridge current

Bridge Input:
Excitation voltage: 5 volts
Bridge type: Full Wheatstone Bridge
The resistance of strain gauges is commonly in the range of 120 to 350 ohms for each element. Resistances that vary greatly from this range or that are not closely matched may make it difficult or impossible to adjust the amplifier offset.

Voltage Output:
By design, the amplifier chip output voltage swing is limited to the power supply voltage minus one volt. To allow the voltage swing to span the entire range of 0-5 volts when used with data acquisition devices with this range the amplifier chip is powered by the unregulated power supply rather than the on-board +5 volt regulator. (At the same time, this design allows the output voltage to be greater than the range desired at some circuit gains and bridge signal levels.)

Using a strain gauge arrangement other than a Wheatstone bridge

The normal arrangement for strain gauges attached to the input of the SGAMP-2 is a full Wheatstone bridge, using 4 strain gauges. Many "load cells" are commercially available that include this arrangement and are therefore very easy to use with the SGAMP-2.

If it is neccesary to use only one or two strain gauges, resistors may be used to replace some of the stain gauges, but the values of these resistors must be very close to that of the strain gauges, or it may not be possible to set the offset voltage of the SGAMP-2 to zero.

The manufacturers of strain gauges often provide "bridge completion resistors" for use with their strain gauges, in order to form a Wheatsone bridge made of resistances with close tolerances.

Schematic Diagram

schematic diagram of SGAMP-2

The SGAMP-2 circuit assembly is normally shipped with a fixed gain resistor (RG) of 100 ohms, giving the cicuit a gain of 1000. Pads are included on the printed circuit board for a trimmer potentiometer which can replace the fixed gain resistor to allow for a variable gain circuit. If RG is replaced with another resistor it is suggested that a 1% metal film or other type of precision resistor be used. The following chart lists some common resistor values that can be used for RG, along with their associated gains.

Resistor RG      Gain
50               2000            
100              1000
200              500
500              200
1000             100
2000             50

The SGAMP-2 circuit assembly is normally shipped with a filter capacitor (CF) of .0015uf, for a low pass filter frequency of 1061 HZ. The following chart lists some common capacitor values that can be used for CF, along with their associated frequencies.

Capacitor CF     Frequency (HZ)
pF       uF
100000  .1       16
47000   .047     33
33000   .033     48
22000   .022     72
15000   .015     106
10000   .01      159
6800    .0068    234
5600    .0056    284
4700    .0047    338
3300    .0033    482
2700    .0027    589
2200    .0022    723
1800    .0018    884
1500    .0015    1061
1000    .001F    1591
820              1940
680              2340
560              2842
470              3386
330              4822
220              7234
180              8841
150              10,610
120              13,262
100              15,915
47               33,862
39               40,808
33               48,228
22               72,342
15               106,103

Refer to the Analog Devices AMP04 data sheet for detailed information about cicuit gain and filtering.

Since the amplifier chip is powered by the unregulated power supply, the output voltage level can be greater than the range desired at some power supply voltages, circuit gains, and bridge signal levels. The data acquisition device monitoring the amplifier output signal must include protection against overvoltage if the amplifier might be operated in this region.

Using a trimmer pot for gain resistor RG

The SGAMP-2 can be ordered from Industrologic with a 1000 ohm trimmer pot rather than the 100 ohm fixed resistor for RG, the gain resistor. The standard 100 ohm resistor selects a gain of 1000, and the 1000 ohm trimmer allows a gain from 100 to 1000. The fixed gain SGAMP-2 can also be converted to a variable gain amplifier after purchase by replacing the gain resistor RG with a suitable trimmer pot.

Analog Device specifies the AMP04 to have a gain range of 1 to 1000, which would be values of 100K ohms to 100 ohms for RG. Since trimmer pots can be adjusted down to zero ohms, using a trimmer pot for RG will allow the resistance of the trimmer to be adjusted below 100 ohms, bringing the gain higher than the maximum of 1000 specified by Analog Devices.

Although Industrologic has observed the SGAMP-2 circuit to work at gains higher than 1000, it is not recommended, because the AMP04 may be operating in an area of degraded performance. To avoid gains this high, adjust the gain trimmer pot used for RG to the fully counter-clockwise position, and then turn it clockwise until the desired gain and circuit operation is reached.

The offset adjustment on the SGAMP-2 will be affected by having an adjustable gain. The two adjustments will be interactive, that is, they may need to be adjusted back and forth a number of times to find the ideal offset voltage related to the gain setting.

For example, if the offset trimmer pot is adjusted for a voltage slightly above zero volts, (which is often the case), increasing the gain with the gain trimmer pot will increase the offset voltage.

Conversely, if the gain setting is reduced, it will be necessary to check the offset voltage to be sure that it is still slightly above zero volts (if this is intended).

end of manual
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