Measurement Computing CIO-EXP-RTD16 - Manual (Page 11)

4 VOLTAGE MEASUREMENT CONFIGURATION
The CIO-EXP-RTD16 is an amplification, signal conditioning and multiplexing accessory for DAS
boards. The inputs are suitable for connecting a voltage to the DAS board so it may be measured. The
CIO-EXP-RTD16 is a “one of sixteen” multiplexer which means that for every channel in your DAS
board, you can multiplex sixteen different signals into it. You can expand the number of inputs of your
DAS board by sixteen for every CIO-EXP-RTD16 board, up to the number of inputs on the DAS board.
For example, a DAS08 has eight inputs. Eight times sixteen is one hundred and twenty eight. Using
CIO-EXP-RTD16 boards you can bring 128 inputs into the PC with one DAS08 in one slot.
It is unlikely that you purchased a CIO-EXP-RTD16 to measure voltages. The CIO-EXP-RTD16 has
quite a bit of special circuitry designed for RTD sensors. For only voltage measurements, a CIO-EXP16
or CIO-EXP32 would be less expensive and do the same job.
However, you may have one or two voltages to measure in addition to RTD sensors and would like to
connect those signals to the CIO-EXP-RTD16. This chapter explains how to make those connections.
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-RTD16 with your data acquisition board. Configure your
boards as described in that section before continuing with this section.
4.2 POWER SOURCE SWITCH
The General Configuration section describes the power selection options, setting the power select switch
and verifying the installation and operation of the CIO-EXP-RTD16 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, a sine wave which varies between 0 and 1 volt would only be using 1/10th of the ±5V A/D
converter's resolution. By switching the input signal of the DAS board to unipolar (no negative voltage)
and amplifying the sign wave signal by 5, the entire range of the A/D converter is used and a higher
resolution measurement may be made.
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-RTD16 for the correct gain.
Gain, or amplification, allows you to boost your signal to take full advantage of the resolution of the A/D
converter. While amplifying the signal, any noise is amplified as well. If your signal ranges greater than
±5V, please go to section on attenuation.
The first stage has gain options of 1, 2, 4 or 8 and the second stage gain options are 1, 7 or CUSTOM.
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