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.
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 GΩ. 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 MΩ by continuously
closing the High V switch (for more information on dc input resistance,
see page 113).
HI
LO
Low V
High V
100:1
X1
X10
X100
+/- 12 Vdc to
Analog-to-Digital
Converter
Ranging
Chapter 8 Tutorial
Measurement Fundamentals
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