Agilent Technologies 34972A User Manual page 276

Chapter 7 Tutorial
Measurement Fundamentals
DC Voltage Measurements
To make a useful DC meter, a "front-end" is required to condition the
input before the analog-to-digital conversion. Signal conditioning
increases the input resistance, amplifies small signals, and attenuates
large signals to produce a selection of measuring ranges.
Signal Conditioning for DC Measurements Input signal
conditioning for DC voltage measurements includes both amplification
and attenuation. A simplified input to the internal DMM is shown below.
Low V
HI
High V
100:1
LO
For input voltages less than 12 VDC, the Low V switch is closed and
applies the input signal directly to the input amplifier. For higher
voltages, the High V switch is closed and the signal is attenuated 100:1
before being applied to the input amplifier. The input amplifier gain is
set to one of three values (x1, x10, or x100) to yield a signal in the range
of ±12 VDC for the analog-to-digital converter.
For the lower voltage ranges, the internal DMM's input resistance is
essentially that of the input amplifier. The input amplifier uses a low-
bias current (less than 50 pA) FET input stage yielding an input
resistance greater than 10 G. On the 100V and 300V input ranges, the
input resistance is determined by the total resistance of the 100:1
divider. You can also set the input resistance to 10 M by continuously
closing the High V switch (for more information on DC input resistance,
see page 130).
276
+/- 12 Vcd to
Analog-to-Digital
Converter
X1
X10
Ranging
X100