Digital Multimeter
Theory of Operation
15
routed within U1 to comparators that will produce a digital signal at COMP_O (U1 pin
24) ready for frequency or duty cycle measurement.
Ohms in the 6 M
Ω
& 50 M
Ω
Range and Siemens in the 60 nS Range
The U1 current source is enabled and routed to ISRC (U1 pin 36) and on to the V/Ohms
input jack, which is also connected to DIV_A (U1 pin 3) via the components indicated in
Table 11. DIV_A is routed by U1 internal switches to R0 (U1 pin 5) and on to Z1 (1.106
M
Ω
) to divide the sensed voltage by 10. The divided voltage is filtered, buffered with a
gain of two, and passed to FE_O (U1 pin19) ready for measurement. Only the 1
µ
A and
0.5
µ
A current sources are used in these ranges. If the operator selects the Hz function,
the signal is routed within U1 to comparators that will produce a digital signal at
COMP_O (U1 pin 24) ready for frequency or duty cycle measurement.
Continuity in all Ohms and the Siemens Ranges
The same measurement paths discussed above apply with the exception that the filtering
is turned off to speed up the circuit response and the signal at FE_O (U1 pin 19) is
routed to U1 comparators, which convert it to a digital signal at CONT_O (U1 pin 24)
ready for evaluation by the microprocessor.
Capacitance
The U1 current source is enabled and internally routed to COMMON. ISRC (U1 pin 36)
is routed to the V/Ohms input, which is also connected to SENSE_HI (U1 pin 1) via the
components indicated in Table 11. The voltage signal at SENSE_HI is internally routed
in U1 to the comparators converted to a digital signal at COMP_O (U1 pin 24) ready for
evaluation by the microprocessor, buffered and passed on to FE_O (U1 pin 19) ready for
measurement. The capacitor is discharged to below a value set on the comparator via
ISRC (U1 pin 36), which is connected to COMMON or the negative voltage rail
depending on how much voltage is on the capacitor being measured. Once the capacitor
is discharged sufficiently, current is applied to it in accurately measured packets until the
U1 comparator signals that the capacitor under test has been sufficiently charged. By
measuring the voltage at discharge and after charge the value of the capacitor is
calculated by the microprocessor.
Diode Test
The U1 1-mA current source is enabled and routed to ISRC (U1 pin 36) and on to the
V/Ohms input, which is also connected to SENSE_HI (U1 pin 1) via the components
indicated in Table 11. SENSE_HI is routed by U1 internal switches to R1 (U1 pin 7) and
on to Z1 (110.01 k
Ω
) & R8 which form 10-to-1 divider with Z2. The divided voltage
signal is filtered, buffered with a gain of two and passed to FE_O (U1 pin19) ready for
measurement.
mA,
µ
A and AAC
The mA/
µ
A input and the A input are connected to R4 (U1 pin 10) as described in Table
11. The voltage at R4 is proportional to the product of the current being measured and
the shunt resistance that is in use. The voltage at R4 is routed by U1 internal switches to
the inverting input of an internal operational amplifier. The overall gain to ACAMP_O
(U1 pin 22) is set by the total resistance from the shunt in use to the inverting input,
either resistor Z1(1.106 M
Ω
) or Z1(110.01 k
Ω
), R105, R12 & R8. The signal at
ACAMP_O is, therefore, a scaled representation of the AC current input signal. The
signal at ACAMP_O is routed to either the Low-Pass Filter or the RMS Converter or
both using multiplexer U12. The output of the Low-Pass Filter or the RMS Converter is
routed back into CONV_IN (U1 pin13), filtered, buffered with a gain of two and passed
to FE_O (U1 pin 19) ready for measurement. If the operator selects the Hz function, the
