The FS9704B, a 50,000 count digital multi-function meter
front-end chip is the core of the GDM-8245. This versatile chip
contains a high resolution sigma-delta ADC, functional
network, operational amplifier, comparator, digital filter,
digital control logic and an embedded microprocessor.
Combined with a microprocessor, the FS9704B provides
auto-range capabilities to measure DC/AC voltage, current,
resistance, frequency and diode, etc.
Apart from the core chip, there is analog signal processing
circuitry to attenuate and process the DC/AC voltage and
current before feeding them into the ADC inside the FS9704B.
Major components of the analog signal processing circuitry
include the current shunt resistors that are used to render the
input AC/DC current into a measurable voltage (U403, 9.25X
voltage amplifier), and the U401, a true RMS-to-DC converter
(AD536).
The digital control circuitry displays the measurement reading,
reads the user’s input from the front panel and actuates the
relays used in the analog circuitry. The digital control circuitry
consists of U202 (W78E054C) and the other peripheral
controllers.
All the essential parts mentioned above need to be properly
regulated with DC power supply circuitry that provides six
different DC voltage outputs: ±15V, ±3.1V and two sets of +1.8V
outputs.
As shown in the figure below, the ac power sources on pin 8
and pin 14 of T101 are rectified by D105, D106, D107 and D108
to be the ±18Vdc unregulated power sources. The ±18Vdc
unregulated voltage sources are fed into the voltage regulators
U102 (AN7815) and U105 (AN7915) to obtain the regulated
±15Vdc power sources, and then the regulated ±15Vdc power
sources are fed into both the secondary voltage regulation
circuitry which are composed of Q101 (2SC1384) and Q102
(2SA684) to produce the ±3.1Vdc regulated power sources.
The secondary ac power source on pin 12 and pin 13 of T101
are rectified by D101, D102, D103, D104 to obtain an
unregulated +7.8Vdc power source which is then fed into two
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