Chapter 4
Connecting and Using Accessories and Transducers
©
National Instruments Corporation
4-25
The filtering consists of one of two lowpass filters, 4 kHz (–3 dB) or 4 Hz
(–10 dB), which you can select through software control. These cascaded
filters are located in two stages to help eliminate noise introduced by the
isolation amplifier.
The isolation amplifier fulfills two purposes on the SCXI-1122 module.
The isolation amplifier converts a small signal riding on a high
common-mode voltage into a single-ended signal with respect to the SCXI
chassis ground. With this conversion, you can extract the input analog
signal from a high common-mode voltage or noise before it is sampled and
digitized by the DAQ device. The isolation amplifier also amplifies the
input signal, which increases measurement resolution and accuracy. The
isolation amplifier drives the analog output stage, which contains circuitry
that permits several module output signals to multiplex into one DAQ
device channel.
The calibration hardware consists of a software-controlled shunt
calibration resistor for strain gauge calibration, an autozeroing calibration
for nulling the amplifier offsets, and an EEPROM that holds calibration
constants for software correction of gain and offset of the isolation
amplifier and of the current and voltage excitation channels.
Excitation Output Channels
In addition to the isolated input channels, the SCXI-1122 has two isolated
excitation output channels, one 3.333 V voltage output source and one
1 mA current output source. Both channels—like the input channels—have
450 V
rms
common-mode voltage with respect to earth ground and 250 V
rms
common-mode voltage between each other and any other channel. Both
channels are overvoltage-protected to 250 V
rms
and are current-limited.
The voltage excitation channel is for transducers, such as strain gauges,
which need voltage excitation to operate properly. The maximum current
that this channel can provide is 225 mA. Exceeding this limit will cause the
channel to lose regulation. This channel has four terminals, two sense
terminals (SENSE+ and SENSE–), and two excitation terminals (VEX+
and VEX–), which you can use for remote load regulation. For proper
operation, the SENSE+ terminal must always be connected to the VEX+,
and the SENSE– terminal to the VEX–. Refer to the SCXI-1322 Terminal
Block Installation Guide for further details on using the sense terminals for
remote load sensing.
The voltage excitation channel is primarily used to provide power to a
strain gauge configured in a Wheatstone bridge. To verify correct bridge
operation, you can use shunt calibration.
