Average Converter - Fluke 867B Service Manual

resistor of Z1, which is connected to a virtual ground created by U6. C31 blocks DC
voltages. CR3 clamps any over voltages to a safe level for U18. The signal is returned
through Ground 3, to TP6, Ground 5, U26, L8, the low leg of R109, and then to the
â input.
The following three amplifier stages condition the ac signal for the average and rms
converters, the Fast A/D Converter, and the frequency comparators:
•
The first amplifier (stage 1) uses op amp U6 in an inverting configuration. The 1.111
MΩ resistor of Z1 is the input. The 111.1-kΩ (0.1 gain) and 1.111-kΩ (0.001 gain)
resistors of Z1 are feedback resistors selected by U18 switches. TP25 is the first
stage output.
•
The second amplifier (stage 2) uses op amp U5 in an inverting configuration. TP25 is
the second stage input. U17 selects resistors of Z1 for a gain 1 or 10. TP23 is the
second stage output. From TP23 the signal goes through an RC network (C25, R53,
and R104) with a gain of either 1 (all ranges except 1000V) or 0.1 (1000V range).
The gain is selected by U17.
•
The third amplifier (stage 3) uses op amp U8 in a non-inverting configuration. U17-4
is the third stage input. The 3.30-kΩ and 1.18-kΩ resistors of Z3 provide two output
gains. The output to the rms converter (TP21) is provided at a gain of 4. The output
to the Average converter (U30), the Fast A/D Converter (U10), and the frequency
comparators (U30) is provided at a gain of 1.
Starting with the conditioned ac signal, the average converter produces a dc voltage
equal to the average value of the full wave rectified ac signal multiplied by 1.111. The
average converter uses an op amp in U30 (the analog processor) in an inverting
configuration. Direct output from U8 provides the converter input and is routed through
dc blocking capacitor C8 and the 20-kΩ resistor of Z3 to U30-97, which is a virtual
ground. The 22.2-kΩ resistors of Z3 provide the feedback. The 22.2-kΩ resistor
connected to U30-95 provides feedback for the positive portion of the signal and the
22.2-kΩ resistor at U30-94 provides feedback for the negative portion. The dc voltage
(the differential across U30-94 and U30-95) is routed to the active filter and the Slow
A/D Converter in U30. See Table 2-5.
All VAC ranges use the 300 mV Slow A/D Converter ranges.
Range Stage 1 Stage 2
Gain Gain
300 0.1 10
mV
3V 0.1 1
30V 0.001 10
300V 0.001 1
1000V 0.001 1
The rms converter (U3) provides a dc voltage equal to the rms value of the conditioned
ac signal. Input to the rms converter is from TP21 through dc blocking capacitors C69
and C68. Output of the rms converter goes through a dc divider gain of 0.25 created by
the 4.95-kΩ and 1.65-kΩ resistors of Z3. The output from this divider is used for
Table 2-5. Average Converter (part of U30)
RC Net Stage 3
Gain Gain
1 1
1 1
1 1
1 1
0.1 1
Converter DC
Gain Divider
Gain
1.111 1
1.111 1
1.111 1
1.111 1
1.111 1
Theory of Operation
Input Signal Conditioning
Total Gain
1.111
0.1111
0.01111
0.001111
0.0001111
2
2-15