User`s guide

281
Chapter 7 Tutorial
Measurement Fundamentals
4
7
AC Voltage Measurements
The main purpose of an AC “front end” is to change an AC voltage input
into a DC voltage which can be measured by the ADC.
Signal Conditioning for AC Measurements Input signal
conditioning for AC voltage measurements includes both attenuation
and amplification. An input coupling capacitor (C) blocks the DC portion
of the input signal so that only the AC component is measured. Ranging
is accomplished by combining signal attenuation from the first-stage
amplifier and gain from the second-stage amplifier.
The first stage implements a high input impedance (1 M) switchable
compensated attenuator. The second stage provides variable-gain signal
amplification to scale the input to the AC converter to the full-scale level.
Any residual DC offset from the attenuator and amplifier stages is
blocked by a capacitor.
An AC voltage front end similar to the one discussed above is also used to
measure AC current. Shunt resistors convert the AC current into an AC
voltage which can then be measured. Current shunts are switched to
provide selectable AC current ranges.
C1M
X 0.01
X1
A1
A2
X1
X10
X100
AC to DC
Converter
HI
LO
34970A Refresh UG.book Page 281 Wednesday, February 17, 2010 12:34 PM