Page 465 - Instrumentation Reference Book 3E

448 Electrical measurements
Reset Scaling 20.3.2 Elements in DVMs and DMMs
I 1
The ADC is the central element of a DVM or
Voltage to DMM. The ADC is, however. a limited input
range device operating usually on unipolar d.c.
Serial
output signals. Figure 20.30 shows the elements of a com-
plete DVM or DMM.
Parallel
mt a 'in output
20.3.2.1 D.c. input stage and guarding
- ance together with attenuationlamplification and
CL The d.c. input stage provides high input imped-
D W polarity sensing of the signal to ensure that the
Number of countsn a Vi" voltage applied to the ADC is of the correct
magnitude and polarity.
Figure 20.27 Pulse-width ADC.
DVMs and DMMs are often used to measure
small d.c. or a.c. signals superimposed on much
Table 20.6 (taken from Spreadbury 1981) com- larger common-mode signals. For example, in
pares the characteristics of these two types of measuring the output signal from a d.c. Wheat-
Zener devices with bandgap devices and the stone bridge, as shown in Figure 20.31(a), the
Weston standard cell. common-mode voltage is half the bridge supply.

tv

Up gate

Counter

Down gate
Display

Integration time
-V
-
r Reference

/'
n
UP
- 1 UP UP
Count

down (b)
Figure 20.28 Precision pulse-width ADC. (a) Circuit; (b) timing. Courtesy Solartron Instruments Ltd.