User's Manual

MicroStrain, Inc.
Differential Input Channel
The SG-Link® OEM has one differential input channel. This channel is designed to
accommodate strain gauges with 350 ohm resistance or greater. The SG-Link® OEM by
default is configured to support a full Wheatstone bridge installation. The SG-Link®
OEM can also be ordered from the factory configured to support half and quarter bridge
installations. It is important to know how your SG-Link® OEM has been configured
before proceeding with use. You may wish to check your invoice or you may inspect the
circuit board assembly for factory-applied completion resistors. The location of these
completion resistors are described in Appendix 6 of this manual.
The SG-Link® OEM powers the differential input channel with +3 volts DC at up to 50
mA for bridge excitation. A special feature of the excitation is that it will revert to a
‘pulsed’ mode when sampling rates are below 100 Hz in order to conserve power.
Programmable gain is user-adjustable and allows the bridge circuit to be amplified to suit
the characteristics of the particular strain gauges in use; this gain can be set from 20 to
2560 (See Programmable Gain section of this manual for more details). Programmable
offset is also user-adjustable and allows for the ‘zeroing’ of strain gauge output. Strain
gauge bridges can have significant offset due to a number of factors including initial
ohmic error in the gauge itself, installation of the gauge on a non-flat surface, initial
offset error in the electronics, and small errors in the bridge completion resistors. Details
of programmable gain and offset use may be found in the Agile-Link™ software manual.
Further conditioning (noise reduction) is provided by an anti-aliasing filter (- 3dB down
@ 500 Hz; low pass filters with cutoff @ 500 Hz).
The SG-Link® OEM differential input channel additionally contains a precision 499
Kohm (499000 ohm) resistor shunt resistor. This resistor is employed by Agile-Link™
software to perform a precise shunt calibration of the SG-Link® OEM and the
Wheatstone bridge installation. Alternatively, users may either use Agile-Link™
software to perform a shunt calibration using an external precision resistor, or perform a
shunt calibration using a method of their own design.
The 3 volt signal is digitized by the SG-Link® OEM’s 12-bit A/D converter, creating a 0
to 4096 ‘bit’ digital range. As explained elsewhere, the Agile-Link™ system operates
digitally by resolving all sensor voltages to ‘bits’, transmitting the bits by radio to the
base station, carrying the bits into host software, and finally returning these bits to volts
at the software level. Agile-Link™ software further provides an on-board function (Strain
Wizard) which allows engineering units (microstrain) to be output and displayed directly
from the software. Please refer to the Strain Wizard Help in the Agile-Link™ software
for detailed use.
In the pages following you will find wiring diagrams which detail the specifics for
installing quarter, half and full bridges. Please also refer to the Terminal Strip section for
other important details and practice.
A great way to become familiar with your new equipment is to take 15 minutes and
connect your SG-Link® OEM to the SG-Link® Tester Board. The SG-Link® Tester
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