Theory of Operation
Functional Description
4
4-9
The dc input voltages for all ranges are divided by the appropriate factor of
10 to produce a proportional dc signal which is then filtered and applied to
the input to the a/d converter. The dc and ac voltage ranges and division
factors are listed in Table 4-1 along with the corresponding range of inputs to
the a/d converter. Notice in Table 4-1 that the 2V dc voltage range is divided
by 1 (not 10). The microcomputer compensates by decreasing the integrator
gain in the a/d converter by a factor of 10 (refer to Figure 4-2). The
integrator gain is also reduced by a factor of 10 in the 1000V dc voltage
range, which uses the same divider arrangement as the 200V dc voltage
range.
The ac input voltages are divided with the same divider arrangement as the
dc input voltages, with the exception that the 2V ac voltage range is divided
by 10. The divider output signals for ac voltages are ac-coupled to the input
of a true rms ac converter which produces a current output. This negative dc
representation is applied through a calibrated scaling resistor. The resultant
negative voltage is filtered and applied to the input of the a/d converter.
V/Ω/S
Voltage
Divider
÷
1DC
AC
True RMS
AC
Converter
Inputs
to A/D
Converter
Common
LO
HI
÷
10
÷
100
÷
1000
÷
100
÷
10
÷
1000
dx28f.eps
Figure 4-4. Voltage Measurement
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