Keysight DAQ970A Guia Del Usuario página 230

4 Measurement Tutorials
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:
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.
230 Keysight DAQ970A/DAQ973A User's Guide