43B
Service Manual
3-18
measurement ground. Any voltage across the PTC resistor will be added to the input
signal, and cause a mis-reading. This influences Input 2 in particular as this input
operates mostly in the lowest voltage ranges (see section 3.2.1). For this reason a hum
rejection circuit is added for Input 2. The voltage across the PTC is supplied to the
inverting X1 amplifier N202. Then the AC part of the N202 output signal is subtracted
from the input sigal on the C-ASIC LF input (pin 42).
Resistance Measurements (Input 1 only)
The unknown resistance Rx is connected to Input 1, and the black COM input. The
T-ASIC supplies a constant current to Rx via relay contacts K173, and the PTC resistor
R172. The voltage across Rx is supplied to a high impedance input buffer in the C-ASIC
via the LF input pin 42. The C-ASIC conditions the voltage across Rx to an output
voltage of 50 mV/d. This voltage is supplied to the ADC on the Digital part. The ADC
data is read and processed by the D-ASIC, and represented as a numerical reading, and a
bar graph.
Table 3-4 shows the relation between the reading range (Ω), the trace sensitivity (Ω/d.),
and the current in Rx,
Table 3-4. Ohms Ranges, Trace Sensitivity, and Current
Range 50Ω 500Ω 5kΩ 50 kΩ 500 kΩ 5 MΩ 30 MΩ
Sensitivity ../div 20Ω 200Ω 2 kΩ 20 kΩ 200 kΩ 2 MΩ 10 MΩ
Current in Rx 500 µA 500 µA 50 µA5 µA 500 nA 50 nA 50 nA
To protect the current source from being damaged by a voltage applied to the input, a
PTC resistor R172 and a protection circuit are provided (See Section 3.3.3 “Current
Source”).
During measuring, input voltage measurements, gain measurements, and zero
measurements are done. As a result, the voltage supplied to the ADC is a multiplexed
(zero, + reference, -reference, input voltage) signal.
Capacitance Measurements (Input 1 only)
The capacitance measurement is based on the equation: C x dV = I x dt. The unknown
capacitor Cx is charged with a constant known current. The voltage across Cx increases,
and the time lapse between two different known threshold crossings is measured. Thus
dV, I and dt are known and the capacitance can be calculated.
The unknown capacitance Cx is connected to the red Input 1 safety banana socket, and
the black COM input. The T-ASIC supplies a constant current to Cx via relay contacts
K173, and protection PTC resistor R172. The voltage on Cx is supplied to two
comparators in the C-ASIC via the LF input. The threshold levels th
1
and th
2
of the
comparators are fixed (see Figure 3-9). The time lapse between the first and the second
threshold crossing depends on the value of Cx. The resulting pulse is supplied to the
TRIGGER output pin 29, which is connected to the analog trigger input of the T-ASIC
(TRIG-A signal). The T-ASIC adjusts the pulse to an appropriate level, and supplies it
to the D-ASIC via its ALLTRIG output. The pulse width is measured and processed by
the D-ASIC, and represented on the LCD as numerical reading. There will be no trace
displayed.
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