The internal DMM
A transducer or sensor converts a physical quantity being measured into an electrical signal which can be
measured by the internal DMM. To make these measurements, the internal DMM incorporates the fol-
lowing functions:
l Temperature (thermocouple, RTD, and thermistor)
l Voltage (DCand AC up to 300 V)
l Resistance (2-wire and 4-wire up to 1GΩ)
l Current (DC and AC up to 1 A)
l Frequency and Period (up to 300 kHz)
The internal DMM provides a universal input front-end for measuring a variety of transducer types without
the need for additional external signal conditioning. The internal DMM includes signal conditioning, amp-
lification (or attenuation), and a high resolution (up to 22 bits) analog-to-digital converter. A simplified dia-
gram of the internal DMM is shown below:
Signal conditioning, ranging, and amplification
Analog input signals are multiplexed into the internal DMM's signal-conditioning section - typically com-
prising switching, ranging, and amplification circuitry. If the input signal is a DC voltage, the signal con-
ditioner 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 DCvoltage 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 instru-
ment 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 autor-
anging 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
Keysight DAQ970A User's Guide 59
3 Features and Functions
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