AMPS input
The ACA or DCA input signal is applied to the Current Shunt circuit, which is made up of
K103, Q165, Q164, R205, R450, R386, and R158. U178 controls if either Q164 or Q165 is
turned on. For the 3A and 1A DCA ranges and all ACA ranges, 0.1Ω (R158) is shunted across
the input (K103 off). For the 100mA DCA range, K103 is energized (on). Q164 (both) are turned
on with the LOAMP control line set high. 1.01Ω (R386 + R158) is shunted across the input. For
the 10mA DCA range, K103 is energized (on). Q165 (both) are turned on with the LOAMP
control line set low. 10.1Ω (R205//R450 + R386 + R158) is shunted across the input.
The ACA signal is then sent to the AC Switching & Gain circuit, while the DCA signal is
routed directly to the A/D MUX & Gain circuit.
Signal switching
Signal switching for DCV and OHMS is done by the DCV & Ohms Switching circuit. FETs
Q113, Q105, Q104, and Q108 connect the DCV or ohms signal to the X1 buffer (Q137, U167,
and U166). (See Analog signal switching states at the end of this section for a summary of
switching states of these FETs for the various DCV and OHMS ranges.)
Note that the reference current for OHMS is generated by the Ohms I-Source circuit. For
4-wire ohms measurements, SENSE LO is connected to the circuit by controlling U163.
Signal switching and gain for ACV, FREQ, and ACA is done by the AC Switching & Gain
circuit, which is primarily made up of K102, U102, U103, U105, U112, U118, U111, and U110.
(See Analog signal switching states.) Note that U111 is used for frequency adjustment. The
states of these analog switches vary from range to range.
Multiplexer and A/D converter
All input signals, except FREQ, are routed to the A/D MUX & Gain circuit. The multiplexer
(U163) switches the various signals for measurement. In addition to the input signal, the
multiplexer also switches among reference and zero signals at various phases of the
measurement cycle.
When the input signal is selected by the MUX, it is amplified by a composite operational
amplifier made up of Q156 and U177. Tables in Analog signal switching states at the end of this
section provide the switch states of U176, which determine the gain for Q156 and U177.
The multiplexed signals of the measurement cycle are routed to the A/D Converter (U165),
where it converts the analog signals to digital form. The digital signals are then routed through
an opto-isolator to the MPU to calculate a reading.
Troubleshooting 4-9
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