HP 34970A manual Nm w, 99A

Chapter 8 Tutorial

Measurement Fundamentals

nm w

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.

j tInput signal conditioning for dc voltage measurements includes both amplification and attenuation. A simplified input to the internal MWW is shown below.

HI

Low V

High V

100:1

X1

X10

+/- 12 Vdc to Analog-to-Digital Converter

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 MWW’s input resistance is essentially that of the input amplifier. The input amplifier uses a low-bias current (less than 50 pA) ONd 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 ( z ” z”4

z 99A ).

354