Chapter 3 System Overview

Measurement Input

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 3 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 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 HP 34970A front panel allows you to control the integration period for precise rejection of noise signals.

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