Fluke NetDAQ 2640A, Service Manual

Page: 80 / 260

NetDAQ
Service Manual
2-38
Voltage Reference Circuit 2-66.
The voltage reference circuit creates a well-reg3.45/-3.45V dc source for use by
the A/D converter, and as a source for ohms and current measurements. The circuit is
formed around two dual op-amps A3U12 and A3U20. A3U12 controls balance between
+3.45V dc and -3.45V dc by adjusting the +3.45V dc through A3Q2 as the divider
between these voltages in zener diode A3Z1 reads above or below zero. The other half of
A3U12 adjusts the absolute voltage difference between the two outputs by regulating the
-3.45V dc so as to produce zero collector-base volts on A3Q5. If the collector voltage
rises, then A3Q5 needs more current, which is produced by lowering the -3.45V dc
through A3Q3. Resistor A3R101 and capacitor A3C48 stabilize the loop.
HI SENSE
A3R11
1K
2W FUS
A3K25
S
R
A3R127
1K
A3C80
A3K25
S
R
A3Z6
1.111M
A3Q15 A3Q12 A3U29
+
_
A3CR2
A3Z6
12.25K
A3Z6
111.1K
A3Z6
115.7
A3Z6
2.776K
A3C72
A3C71
LO
A3L52
A3R146
270
R
A3K27
A3R119
1K
S33
AD LOW
VIN
RMSOUT
A3Z2
4.95K A3R103
100K
A3C57
A3Z2
3.3K
A3C58
A3R102
100K
A3R104
100K
A3C59
+
_
To Pin 31 of A3U30
S41 S44
TO
BUFFER
AMP
A3U26
Figure 2-10. AC Volts 3V Range Simplified Schematic