System Overview 3
Keysight 34970A/34972A User’s Guide 91
Signal conditioning, ranging, and amplification
Analog input signals are multiplexed into the internal DMM’s signal-conditioning
section – typically comprising switching, ranging, and amplification circuitry. If the
input signal is a DC voltage, the signal conditioner is composed of an attenuator
for the higher input voltages and a DC amplifier for the lower input voltages. If the
input signal is an AC voltage, a converter is used to convert the AC signal to its
equivalent DC value (true RMS value). Resistance measurements are performed
by supplying a known DC current to an unknown resistance and measuring the DC
voltage drop across the resistor. The input signal switching and ranging circuitry,
together with the amplifier circuitry, convert the input to a DC voltage which is
within the measuring range of the internal DMM’s analog-to-digital converter
(ADC).
You can allow the instrument to automatically select the measurement range
using autoranging or you can select a fixed measurement range using manual
ranging. Autoranging is convenient because the instrument automatically selects
the range to use for each measurement based on the input signal. For fastest
scanning operation, use manual ranging for each measurement (some additional
time is required for autoranging since the instrument has to make a range
selection).
Analog-to-Digital Conversion (ADC)
The ADC takes a prescaled DC voltage from the signal-conditioning circuitry and
converts it to digital data for output and display on the front panel. The ADC
governs some of the most basic measurement characteristics. These include
measurement resolution, reading speed, and the ability to reject spurious noise.
There are several analog-to-digital conversion techniques but they can be divided
into two types: integrating and non-integrating. The integrating techniques
measure the average input value over a defined time interval, thus rejecting many
noise sources. The non-integrating techniques sample the instantaneous value of
the input, plus noise, during a very short interval. The internal DMM uses an
integrating ADC technique.
You can select the resolution and reading speed from 6 digits (22 bits) at 3
readings per second to 4 digits (16 bits) at up to 600 readings per second. The
Advanced menu from the 34970A/34972A front panel allows you to control the
integration period for precise rejection of noise signals.
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