Chapter 8 Tutorial

Measurement Fundamentals

DC Voltage Measurements

To make a useful dc meter, a “front-end” is required to condition the input before the analog-to-digital conversion. Signal conditioning increases the input resistance, amplifies small signals, and attenuates large signals to produce a selection of measuring ranges.

Signal Conditioning for DC Measurements Input signal conditioning for dc voltage measurements includes both amplification and attenuation. A simplified input to the internal DMM is shown below.

HI

 

 

 

 

 

Low V

 

 

 

 

 

 

 

+/- 12 Vdc to

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analog-to-Digital

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Converter

 

 

 

 

 

 

High V

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

X1

100:1

X10

Ranging

X100

LO

For input voltages less than 12 Vdc, the Low V switch is closed and applies the input signal directly to the input amplifier. For higher voltages, the High V switch is closed and the signal is attenuated 100:1 before being applied to the input amplifier. The input amplifier gain is set to one of three values (x1, x10, or x100) to yield a signal in the range of ±12 Vdc for the analog-to-digital converter.

For the lower voltage ranges, the internal DMM’s input resistance is essentially that of the input amplifier. The input amplifier uses a low-bias current (less than 50 pA) FET input stage yielding an input resistance greater than 10 GΩ. On the 100V and 300V input ranges, the input resistance is determined by the total resistance of the 100:1 divider. You can also set the input resistance to 10 MΩ by continuously closing the High V switch (for more information on dc input resistance, see page 113).

354