Measurement Specialties user manual CIO-SSH16 vs. Programmable Gain A/D Boards

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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