Appendix B
5B Series Module Data Sheets
SC-2311 User Manual
B-38
ni.com
5B42 Isolated, Process Current Input
Functional Description
The 5B42 is a single-channel signal conditioning module
that interfaces with two-wire transmitters, providing an
isolated, regulated 20 V supply voltage. The module filters
and amplifies the 4-to-20 mA process-current input to
produce an accurately scaled, low-noise low-impedance
output of +1 to +5 V or +2 to +10 V.
True Three-Port Isolation
The floating, differential input circuit on the field side
eliminates the need for any input grounding. Signal and
power isolation by transformer coupling uses a proprietary
modulation technique for linear, stable and reliable
performance. A demodulator on the computer side of the signal transformer recovers the
original signal, which is then filtered and buffered to provide a low-noise, low-impedance
output signal. True three-port isolation (Input-Output-Power) includes common-mode
ratings of: 1500 V rms between input and output and input and power; 250 V rms between
power and output - no return path is required between the power and signal output
commons.
Accurate and Stable Performance
The 5B42 maintains accuracy over the wide operating temperature range of -40˚C to +85˚C
through design for low parameter drift. This enables the module to provide output offset
drift of only ±5 µV/˚C and gain drift of just ±25 ppm/˚C.
Why an Internal Current Sense Resistor
The 5B42 offers significant advantages over signal conditioners that require an external
current sense resistor. First, an external resistor is not protected from accidental connection
to a 240 V rms power line. Next, the external resistor tolerance must be added to the
conditioners specified errors. In contrast, the 5B42 is calibrated and specified with its
internal 25
sense resistor. Further, the 5B42 signal input, loop supply and the sense
resistor are all protected against accidental application of excess voltages up to 240 V rms.
Finally, there is no need to install an external sense resistor on the
backplane
, but if one is
installed it will not affect 5B42 performance.
Filtering and Protection
The 5B42 contains an optimized five-pole low-pass filter with a -3 dB bandwidth at 100-Hz
that provides a low-overshoot step response and exceptionally low noise of 200 µV pk-pk
in a 100 kHz bandwidth. Attenuation (normal mode rejection) increases at a 90 dB/decade
rate beyond 100 Hz.
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