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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
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.
34970A Refresh UG.book Page 76 Wednesday, February 17, 2010 12:34 PM
Summary of Contents for 34970A
Page 16: ...16 34970A Refresh UG book Page 16 Wednesday February 17 2010 12 34 PM ...
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Page 21: ...1 1 Quick Start 34970A Refresh UG book Page 21 Wednesday February 17 2010 12 34 PM ...
Page 39: ...2 2 Front Panel Overview 34970A Refresh UG book Page 39 Wednesday February 17 2010 12 34 PM ...
Page 59: ...3 3 System Overview 34970A Refresh UG book Page 59 Wednesday February 17 2010 12 34 PM ...
Page 87: ...4 4 Features and Functions 34970A Refresh UG book Page 87 Wednesday February 17 2010 12 34 PM ...
Page 217: ...5 5 Error Messages 34970A Refresh UG book Page 217 Wednesday February 17 2010 12 34 PM ...
Page 241: ...6 6 Application Programs 34970A Refresh UG book Page 241 Wednesday February 17 2010 12 34 PM ...
Page 254: ...254 34970A Refresh UG book Page 254 Wednesday February 17 2010 12 34 PM ...
Page 255: ...7 7 Tutorial 34970A Refresh UG book Page 255 Wednesday February 17 2010 12 34 PM ...
Page 324: ...324 34970A Refresh UG book Page 324 Wednesday February 17 2010 12 34 PM ...