Voltage Ranges And Trace Sensitivity - Fluke ScopeMeter 123 Service Manual

123
Service Manual
range
trace ../div
range
trace ../div
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. In ROLL mode however, no gain
and zero measurements are done. Now the ADC input voltage includes only the
conditioned input voltage.
The input voltage is connected to Input A. The shield of the input is connected to system
ground (⊥ ⊥ ⊥ ⊥ ) via a PTC ground protection resistor. If a voltage is applied between the
Input A and Input B ground shield, or between one of these ground shields and the black
COM input, the PTC resistor will limit the resulting current. The voltage across the PTC
resistor is supplied to the C-ASIC GPROT input, and causes a ground error warning
(high voltage level) on output pin 24 (DACTEST).
Resistance Measurements (Channel A)
The unknown resistance Rx is connected to Input A, 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/div. 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 graphical trace in a fixed time base.
Table 3-4 shows the relation between the reading range (Ω), the trace sensitivity
(Ω/div.), and the current in Rx. The selected trace sensitivity determines the C-ASIC
attenuation/gain factor. The reading range is only a readout function, it does not change
the hardware range or the wave form display.
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 (Channel A)
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,
3-18
Table 3-3. Voltage Ranges And Trace Sensitivity
50 mV 50 mV
5 mV 10 mV
5V 50V
2V 5V
Table 3-4. Ohms Ranges, Trace Sensitivity, and Current
Range 50Ω
Sensitivity ../div 20Ω
500 µA
Current in Rx
50 mV 500 mV 500 mV
20 mV 50 mV 100 mV
50V 50V 500V
10V 20V 50V
500Ω 5kΩ 50 kΩ
200Ω 2 kΩ 20 kΩ
500 µA 50 µA 5 µA
500 mV 5V 5V
200 mV 500 mV 1V
500V 500V 1250V
100V 200V 500V
500 kΩ 5 MΩ 30 MΩ
200 kΩ 2 MΩ 10 MΩ
500 nA 50 nA 50 nA