National Instruments Isolated Analog Input Device NI PXI-4224 - Manual (Page 30)

Chapter 4 Theory of Operation
© National Instruments Corporation 4-3 NI PXI-4224 User Manual
Signal Conditioning Functional Overview
The NI PXI-4224 is part of the PXI-4200 series of DAQ devices with
integrated signal conditioning designed to provide application-specific
signal conditioning, DAQ, and integrated field wiring connectivity on the
same product. The NI PXI-4224 signal conditioning circuitry is designed
to provide attenuation, amplification, and filtering capabilities as described
in Table 4-1.
Measurement Considerations
This section provides more information about the type of signal connection
made to the NI PXI-4224 and important factors that can affect your
measurement.
Input Impedance
Figure 4-2 illustrates the input impedance of an NI PXI-4224 and its effect
on the measurement of a circuit under test. If you know the source
impedance of the circuit under test, you can correct for the attenuation
caused by the NI PXI-4224 in software. Since R
IN
is relatively large
(1 G
Ω), it requires a large source impedance, R
S
, to cause a significant
change in the measured voltage, V
MEAS
. In general, a source impedance of
less than 200 k
Ω does not interfere with the accuracy of the measurement.
For example, a 200 k
Ω source impedance results in a 0.02% gain error.
Table 4-1. Signal Conditioning Functional Blocks
Signal Conditioning Component Description
Input Protection Each NI PXI-4224 channel has overvoltage protection in the
event that a channel is improperly wired.
PGA Each channel has a programmable gain amplifier. The
available gains on the NI PXI-4224 are 1 and 10, which
covers the input range of ±1 V to ±10 V. The DAQ device can
provide a gain of up to 200 in order to maximize the ADC
resolution for signals below 1 V.
Isolation Amplifier Each channel has an isolation amplifier that creates true
channel-to-channel isolation.
Post Filter A post filter is provided to clean up noise spikes created by
the isolation amplifier.