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Each analog input channel can be configured as differential or single-ended inde-
pendently by software. When an even-number channel is configured as differential,
the corresponding odd-number channel will be fixed to differential automatically.
2.4.2
Analog Multiplexer
The analog multiplexer routes one of the analog input channels at a time (one for sin-
gle-ended configuration, and two for differential configuration) to the PGIA and ADC
to be measured. This mechanism is called channel scanning. It realizes multiple
channel measurement using only one ADC at a cost of sharing ADC sample rate
among all scanned channels.
User can select a range of channels to be scanned. The selected channels will be
scanned by the order of channel number. After the last channel is scanned, the next
channel will be the first channel. For example, if channels 3, 4, 5, 6, and 7 are
selected, the scanning sequence is channel 3, 4, 5, 6, 7, 3, 4, 5, 6, 7, 3, etc.
2.4.3
Programmable Gain Instrumentation Amplifier (PGIA)
Multiple input ranges are supported and can be selected by software for each chan-
nel independently. The input signal is amplified or attenuated by the PGIA before
being sampled by the ADC. The gain of the PGIA differs for different input ranges, so
that the signal range at input of the ADC is the same for every input range. Refer to
the device specifications for the supported input ranges.
2.4.4
Grounding Considerations
2.4.4.1
Signal Source Type
Signal sources can be categorized as grounded (ground-referenced) signal source or
ungrounded (floating) signal source. This is shown in Figure 2.4.
The voltage of a grounded signal source is referenced to a system ground, such as
earth or building ground. That is, the negative terminal of the signal source is con-
nected to the system ground. Examples of grounded signal source are devices that
are plugged into the building ground through a wall outlet. The grounds of two inde-
pendently grounded devices may not be at the same potential.
An ungrounded signal source is that in which the voltage is not referenced to a sys-
tem ground. Examples of grounded signal source are battery powered devices, ther-
mocouples, and isolated devices.
2.4.4.2
Measuring a Grounded Signal Source
For a grounded signal source, it is recommended to measure the signal using differ-
ential input configuration.
As described in the previous section, the grounds of two grounded devices may not
be at the same potential. If single-ended (grounded) input configuration is used to
measure a grounded signal source, a ground loop is formed and there will be current
PCI-1716_Series User Manual
Figure 2.4 Signal source type.
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