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Introduction to the 34980A
26
Keysight 34980A Mainframe User’s Guide
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 decides
which 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 34980A 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 34980A's 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 3,000 readings per second. The
integration time, resolution, and number of digits are all interrelated (see
on page 127 for more details).
