867B,863
Service Manual
2-16
precharging the Slow A/D Converter through U30-67 (the analog processor). The output
also goes to an RC filter (R30 and C26) and then on to the Slow A/D Converter at U30-
66 (TP17). See Table 2-6.
Table 2-6. RMS Converter (U3)
Range
Stage
1 Gain
State 2
Gain
RC Net
Gain
Stage 3
Gain
Converter
Gain
DC
Divider
Gain
Total
Gain
300 mV 0.1 10 1 4 1 0.25 1
3V 0.1 1 1 4 1 0.25 0.1
30V 0.001 10 1 4 1 0.25 0.01
300V 0.001 1 1 4 1 0.25 0.001
1000V 0.001 1 0.1 4 1 0.25 0.0001
Table 2-7. Test Point Voltages
(Full scale input: 30000 counts, 10000 counts for 1000V range)
Range AC, TP25 AC, TP23 AC, TP21 DC, TP17 DC (Avg
VAC), U30-
75
300 mV 30 mV 300 mV 1200 mV 300 mV 300 mV
3V 300 mV 300 mV 1200 mV 300 mV 300 mV
30V 30 mV 300 mV 1200 mV 300 mV 300 mV
300V 300 mV 300 mV 1200 mV 300 mV 300 mV
1000V 1000 mV 1000 mV 400 mV 100 mV 100 mV
DC Volts 2-27.
DC Volts measurements are connected at the Ë and â inputs. Ë
connects directly to the 10 MΩ resistor of Z5, which is connected in series with the
101.01-kΩ (U30-17), 10.01-kΩ (U30-13), or 1.000-kΩ (U30-12) resistor. These total
series resistances divide down the input voltage by 0.01, 0.001 or 0.0001 for the 30V,
300V or 1000V ranges, respectively. Switches in U30 select the resistor and pass the
conditioned signal on to the combination circuit and the active filter. The input signal is
returned through selector contacts S1-4 and S1-7, through R109, and back to the â
input. All VDC ranges use the 300 mV Slow A/D Converter range.
mV DC 2-28.
Millivolt DC measurements are connected at the Ë and â inputs. The input
signal goes through input protection resistors R99 and R20 and into U30-21. U30 routes
the signal on to the combination circuit and the active filter.
For normal (low impedance) mode, the 10-MΩ resistor of Z5 is connected between the
Ë input and Ground 1 at U30-23. Ground 1 is connected to L8 and through R109
to â. For high impedance (HiZ) mode, the 10-MΩ resistor of Z5 is not connected to
ground.
The 300 mV range uses the 300 mV Slow A/D Converter range. The 3000 mV range
uses the 3V Slow A/D Converter range.
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