Chapter 4 Analog InputNI 6238/6239 User Manual 4-2 ni.comfor all input ranges. The NI-PGIA can amplify or attenuate an AI signal toensure that you use the maximum resolution of the ADC.M Series devices use the NI-PGIA to deliver high accuracy even whensampling multiple channels with small input ranges at fast rates. M Seriesdevices can sample channels in any order at the maximum conversion rate,and you can individually program each channel in a sample with a differentinput range.A/D ConverterThe analog-to-digital converter (ADC) digitizes the AI signal by convertingthe analog voltage into a digital number.Isolation Barrier and Digital IsolatorsThe digital isolators across the isolation barrier provide a ground breakbetween the isolated analog front end and the earth/chassis/buildingground.AI FIFOM Series devices can perform both single and multiple A/D conversions ofa fixed or infinite number of samples. A large first-in-first-out (FIFO)buffer holds data during AI acquisitions to ensure that no data is lost.M Series devices can handle multiple A/D conversion operations withDMA, interrupts, or programmed I/O.Analog Input RangeInput range refers to the set of input voltages or currents that an analoginput channel can digitize with the specified accuracy. The NI-PGIAamplifies or attenuates the AI signal depending on the input range. You canindividually program the input range of each AI channel on your M Seriesdevice.The input range affects the resolution of the M Series device for an AIchannel. Resolution refers to the voltage or current of one ADC code.For example, a 16-bit ADC converts analog current inputs into one of65,536 (= 2 16 ) codes—that is, one of 65,536 possible digital values. Thesevalues are spread fairly evenly across the input range. So, for an input rangeof ±20 mA, the current of each code of a 16-bit ADC is:
