Fluke ScopeMeter 123 Service Manual page 50

123
Service Manual
Reference Voltage Circuit
This circuit derives several reference voltages from the 1.23V main reference source.
The output of an amplifier in the P-ASIC supplies a current to the +1.23V reference
source V301 via R307. The +3.3V REFPWM2 voltage is used as reference for the
PWMB outputs of the D-ASIC on the Digital part.
The +1.23V 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 -1.23V 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.
3-24
REFPWM2
+3.3V
R307
+1.23V
V301
1.23V
REFP
R309
GAINPWM
R311
+3.3V R312
REFPWM1
GNDREF
R308
GAINREFN
R306
-1.23V
REFN
R310
GAINADCB
R303
+0.1V
REFADCB
R302
GAINADCT
R301
+1.6V
R305
REFADCT
REFATT
Figure 3-12. Reference Voltage Section
73
REFP 72
+
-
71
62
+
1
56
-
55
57
+
2
63
-
64
+
3
54
-
53
+
4
52
-
51
8
P-ASIC
OQ0256
T-ASIC
OQ0257