Measurement Computing StrainBook/616 - Manual (Page 18)

Bridge Configuration
The strain gage, connected to a DB9 connector, connects to amplifiers through a channel-dedicated Bridge
Completion and Shunt Cal Network. This network consists of user installed resistors for bridge completion.
Several combinations of resistors and three different shunt values may be installed simultaneously.
External connector tie points and the programmable Input Configuration & Cal MUX determine the actual
configuration in use. Once the network is fully configured, most bridge configurations and resistances can
be accommodated without re-opening the StrainBook.
The shunt resistors allow each bridge to be put into a known imbalance condition for setting or verifying
channel calibration. Shunt calibration allows a full-scale gain to be set without physically loading the
bridge.
Reference Note:
Chapter 6 contains detailed information regarding bridge configuration and includes
discussion of the convenient CN-189 DB9 adapter option.
Amplifiers
Each channel has its own amplifier consisting of two series-connected stages. The instrumentation
amplifier (PGIA) has programmable gains of x1, x10, x100, and x1000. A programmable gain amplifier
(PGA) follows, with a gain range of 1 to 20 in 28% steps. This results in a combined programmable gain
range
of 1 to 20,000 in 28% steps. The optimal gain is automatically determined during the gage calibration
process.
Offset Source
A low-drift, programmable offset voltage source with a range of ±3.0 V is used to balance the bridge
during the gage calibration process. This offset source will correct for mismatched bridge resistors and
quiescent loads of the strain gage and still retain the full dynamic range.
Auto-zero removes the static portion of the strain load and zeros the input to compensate for any input
drift. Because this is done electronically, zeroing is independent of the user. Simply select the channels
that are to be auto-zeroed and the StrainBook will automatically complete the task.
Filters
Two different 4-pole Butterworth low-pass noise rejection filters are selectable through software by the
Output Selection MUX. The filters have a nominal cutoff frequency of 10 Hz and 1 kHz. Four SIP resistor
networks allow you to determine two cutoff frequencies. See the Hardware Configuration section for
details. If full bandwidth is required, a filter bypass mode is software selectable.
Output Selection
An AC coupling circuit with a 1-Hz cutoff frequency can be software selected by the MUX. This MUX
can also select an Inverting Amplifier for proper output signal polarity. The Inverter avoids having to
rewire the gage if the polarity is reversed. Note that the Inverter option is not available for AC-coupling
modes.
Digitizing Signals
One digital signal processor (DSP) is shared by all channels, including expansion channels. The DSP
processes digitized input signal values (directly from the StrainBook and from expansion modules) and
corrects the values for gain and offset errors. The DSP places the corrected result into the FIFO data buffer
that holds the samples until the PC reads the data. If the sample is used for triggering, the DSP determines
if a valid trigger event has occurred.
2-4 Operation Reference 967794 StrainBook/616