Fluke 43B Service Manual page 47

Reference Voltage Circuit
This circuit derives several reference voltages from the 1.23 V main reference source.
+3.3V
+1.23V
R309
R311
+3.3V R312
R308
R306
-1.23V
R310
R303
+0.1V
R302
R301
+1.6V
R305
Figure 3-12. Reference Voltage Section
The output of an amplifier in the P-ASIC supplies a current to the +1.23 V reference
source V301 via R307. The +3.3 V REFPWM2 voltage is used as reference for the
PWMB outputs of the D-ASIC on the Digital part.
The +1.23 V REFP voltage is used as main reference source for the reference circuit.
This circuit consists of four amplifiers in the T-ASIC, external gain resistors, and filter
capacitors.
Amplifier 1 and connected resistors supply the REFPWM1 reference voltage. This
voltage is a reference for the PWMA outputs of the D-ASIC on the Digital section. It is
also used as reference voltage for the LCD supply on the LCD unit.
Amplifier 2 and connected resistors supply the -1.23 V REFN reference voltage, used for
the trigger level voltages TRIGLEV1&2, the C-ASIC POS-A and POS-B voltages, and
the C-ASIC OFFSET-A and OFFSET-B voltages. REFN is also the input reference for
amplifiers 3 and 4.
REFPWM2 73
R307
REFP 72
V301
1.23V
71
REFP 62
GAINPWM 56
REFPWM1 55
GNDREF 57
GAINREFN 63
REFN 64
GAINADCB 54
REFADCB 53
GAINADCT 52
REFADCT 51
REFATT 8
3.3 Detailed Circuit Descriptions
P-ASIC
OQ0256
+
-
T-ASIC
OQ0257
+
1
-
+
2
-
+
3
-
+
4
-
Circuit Descriptions
3-23
3