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level signal to the analog input boards single ended inputs, reducing the impact of
EMI or RFI along the signal cable between the CIO-SSH16 and the analog input
board.
4.3 CIO-SSH16 vs. PROGRAMMABLE GAIN A/D BOARDS
Having a dedicated differential amplifier for every channel produces flexible and
powerful data acquisition capability. Data throughput of independent amplifiers
exceeds that of A/D boards with single programmable gain amplifiers, even those
equipped with a channel/gain queue.
For example, a programmable gain amplifier is often desired so that signals with
different ranges can be measured using the A/D converter's full resolution.
There are two ways to implement programmable gain; the least expensive is to place a
programmable gain amplifier on the analog input and locate it's control register at one
of the A/D board's I/O addresses. The gain is changed by writing to the address. This
method works like switch-selectable gain except that the switch is in software. There
is one gain for all the analog inputs and changing it is done by writing to a register.
High speed acquisition using DMA rules out gain changes made during a sample run.
This scheme is inadequate if you want to change gains 'on the fly' and take samples at
high speed.
An on-board channel/gain queue is required to enable assigning a different gain to
each channel while data is being acquired at DMA speed. This adds considerable cost
and programming complexity to the A/D board.
Having independent gain amplifiers on each channel permits selecting optimum gain
without any degradation in speed.
Another useful application of programmable gain is to 'tune' the range of the A/D
board for the current value of a signal so that optimum resolution possible is obtained.
This is done by reading the signal, testing the level and adjusting the gain until A/D
board range and the optimum range is achieved.
An alternative and possibly better method of matching the A/D converter to the signal
is shown in Figure 4-5. Simply connect the signal to several of the CIO-SSH16 inputs.
Set each amplifier to a different gain. Data is collected at full speed!
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