Measurement Computing WBK Options - Manual (Page 42)

WBK16, pg. 2 949794 WBK16, Strain-Gage Module
Channel Selection
The eight independent channels are routed to the Channel Selection MUX (multiplexer) for output to the
WaveBook through the Analog Interface. The Digital Interface controls the channel-scanning process and
allows digital configuration of all channels through the WaveBook's Serial Control Bus.
Excitation Source
Excitation power is programmable from a dual source—channels 1 to 4 from one source and channels 5 to
8 from another source. Each channel has a separate regulator with a fold-back current limiter. Up to 85 mA
is provided at 10 V out, decreasing to 30 mA when shorted. This is sufficient current to operate 120
Ω
gages at any voltage. Programmable output voltages of 0, 0.5, 1, 2, 5, and 10 volts are available. Remote-
sense inputs are provided and should be connected at the strain gage for best accuracy. If they are not used,
they need to be jumpered to the excitation output at the connector. The remote-sense inputs are fully
differential, and may even be connected across the completion resistor to form a constant-current linearized
quarter-bridge configuration.
Bridge Configuration
The strain gage is connected to the amplifiers through the 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 box. 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.
Amplifiers
Each channel has an 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 WBK16 will complete the task automatically.
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.