Chapter 8 Tutorial

Measurement Fundamentals

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The main purpose of an ac “front end” is to change an ac voltage input into a dc voltage which can be measured by the IML.

j j tInput signal conditioning for ac voltage measurements includes both attenuation and amplification. An input coupling capacitor (C) blocks the dc portion of the input signal so that only the ac component is measured. Ranging is accomplished by combining signal attenuation from the first-stage amplifier and gain from the second-stage amplifier.

X 0.01

X1

C 1 MΩ

HI A1

LO

A2

 

 

 

 

 

AC to DC

 

 

 

 

 

 

 

 

 

 

Converter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

X1

X10

X100

The first stage implements a high input impedance (1 MΩ) switchable compensated attenuator. The second stage provides variable-gain signal amplification to scale the input to the ac converter to the full-scale level. Any residual dc offset from the attenuator and amplifier stages is blocked by a capacitor.

An ac voltage front end similar to the one discussed above is also used

to measure ac current. Shunt resistors convert the ac current into an 8 ac voltage which can then be measured. Current shunts are switched to provide selectable ac current ranges.

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HP 34970A manual HI A1 AC to DC Converter X10 X100