Measurement Computing CIO-EXP-GP - Manual (Page 12)

4 CONFIGURATION FOR VOLTAGE MEASUREMENT
The CIO-EXP-GP is an amplification, signal conditioning and multiplexing accessory for DAS boards.
The inputs are suitable for connecting a low frequency voltage to the DAS board so it can be measured.
The CIO-EXP-GP is a one-of-eight multiplexer which means that for every channel in your DAS board,
you can multiplex eight different signals to it. You can expand the number of inputs of your DAS board
by eight for every CIO-EXP-GP board, up to the number of inputs on the DAS board. For example, a
DAS08 has 8 inputs. Eight times eight is sixty four. Using CIO-EXP-GP boards you can bring 64 inputs
into the PC with one DAS08 in one slot.
It is unlikely that you purchase a CIO-EXP-GP to measure voltages. The CIO-EXP-GP has a 70Hz low
pass filter and quite a bit of elaborate circuitry designed for bridges, TCs, and RTD sensors. For
applications requiring only voltage measurements, a CIO-EXP16 or CIO-EXP32 would be less expensive
and do the same job.
Possibly you have one or two voltages to measure in addition to bridge or RTD sensors and would like to
connect those signals to the CIO-EXP-GP.
4.1 Channel Selection
The General Configuration section describes the channel selection, setting the jumper and verifying the
installation and operation of the CIO-EXP-GP with your data acquisition board. Configure your boards
as described in that section before continuing with this section.
4.2 Powering the CIO-EXP-GP
The General Configuration section describes the power selection options, setting the power select switch
and verifying the installation and operation of the CIO-EXP-GP with your data acquisition board.
Configure your boards as described in that section before continuing with this section.
4.3 Determining The Appropriate Gain
To accurately measure a voltage, the full scale of the signal should be matched to the full range of the
input circuit. (Most DAS boards have an input range of ±5V, which is the native range of the analog to
digital converter at the heart of the board. Some DAS boards include amplification on the input circuit to
allow the signal to be amplified to make better use of the resolution of the A/D.) For example, an input
signal which varies between 0 and 1 volt would only be using 1/10th of a ±5V A/D converter's
resolution. By switching the input signal of the DAS board to unipolar (no negative voltage) and
amplifying the input signal by 5, the entire range of the A/D converter is used and a higher resolution
measurement may be made. By adding this gain and selecting this range, the resolution on a 12-bit A/D
improves from 2.4 millivolts per bit to 0.24 millivolts per bit. If you needed to measure a change of 1
millivolt, you would need an amplification of 10.
In order to match your signals with the input range of the A/D board, you should do a similar calculation
and set switches on the CIO-EXP-GP for the required gain. Remember to make sure that the settings in
InstaCal match the switches on the DAS and CIO-EXP-GP boards.
If you are measuring signals greater than the maximum full scale range of the A/D, see the section on
attenuation.
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