When measuring resistance, the required resistance (Ω) power
source is provided by the fixed voltage generator inside
FS9704B. As shown below, the measuring current flows
through one of the five scale resistors in RN301 to pass the
unknown Rx and thus having a voltage drop Vx produced on
Rx. And then the Vx is measured by having it fed through R314
into the AX2 input terminal of the A/D converter inside
FS9704B. The unknown Rx can be calculated by having the Ω
power source’s voltage divided by Vx and then times the
selected scale resistor.
The ADC inside FS9704B is a high-resolution sigma-delta
converter; it includes a summing junction, an integrator, a
comparator, a one-bit DAC and a digital low-pass filter. The
input signal Vx comes into the integrator via a summing
junction. It then passes through the integrator which feeds a
comparator that acts as a one-bit quantizer. The comparator
output is fed back to the input summing junction via a one-bit
digital-to-analog converter (DAC), and it also passes through
the digital filter and emerges at the output of the converter. The
feedback loop forces the output of DAC to be equal to the input
signal Vx. The function of the digital filter is to provide a sharp
cutoff at the bandwidth of interest which essentially removes
out of band quantization noise and signals.
ADC’s reference voltage, which is approximately 1.024V,
is generated by dividing the Vz (6.2V) on the zener diode
(D316) with the ratio of 1/6. The Vz divider is composed of
R310 and R311, which are connected in series to provide the
dividing ratio of 1/6.
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