Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-20
ni.com
The instrumentation amplifier stage presents a very high input impedance
to external signals and passes only the differential signal. The offset null
compensation circuitry adjusts the signal voltage by a specified offset after
an offset null compensation calibration is performed.
The signal from the instrumentation amplifier stage passes through a
lowpass filter stage, a variable gain stage, another lowpass filter stage,
and finally a simultaneous sample-and-hold stage before reaching the
multiplexing and analog-to-digital conversion stage.
The NI PXI-4220 includes a 4-pole Butterworth filter per channel with four
software-selectable cutoff frequencies to reduce signal noise and improve
accuracy. You can programmatically configure the filter bandwidths on a
per channel basis for cutoff frequencies of 10 Hz, 100 Hz, 1 kHz, 10 kHz,
or disable the filter. The variable gain stage allows you to set the gain at
many discrete settings between 1 and 50. These settings, along with the
1 or 20 gain setting of the instrumentation amplifier, permit the
NI PXI-4220 to have 49 gain settings between 1 and 1,000.
By default the NI PXI-4220 T/H circuitry is enabled to allow SS/H, which
allows you to acquire synchronized measurements from both channels.
You cannot enable or disable SS/H on a per channel basis. Disabling SS/H
results in higher maximum sample rates. Disabling SS/H introduces a small
offset voltage. You can compensate for this offset by performing offset null
calibration.
The NI PXI-4220 uses a multiplexed architecture that enables the
measurement of multiple channels using a single analog-to-digital
converter (ADC). With SS/H disabled, the multiplexing architecture of the
NI PXI-4220 results in measurements between channels that are separated
in time. The time delay between channels is determined by the sample rate
at which you acquire measurements. For most low-frequency measurement
applications, this time delay or phase delay is not significant.
Excitation circuitry and the shunt calibration switches are two circuitry
stages that are not directly in the signal path. The excitation stage is a stable
output with a controlled feedback loop called remote sense. The remote
sense signal is connected to the analog multiplexer. You can scan the
remote sense lines independently in your application.
The shunt calibration switches are controlled by the digital interface and
control circuitry. You must connect the shunt calibration pins to the bridge
for shunt calibration to function correctly. When the switch is closed, a
socketed shunt calibration resistor in the NI PXI-4220 connects across a leg
of a Wheatstone bridge.
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