Fluke 43B - Page 47

Circuit Descriptions
3.3 Detailed Circuit Descriptions
3
3-23
Reference Voltage Circuit
This circuit derives several reference voltages from the 1.23 V main reference source.
73
6
2
5
6
5
5
5
7
6
3
6
4
5
4
5
3
5
2
51
8
7
2
71
+
-
+
-
+
-
+
-
+
-
R307
V301
1.23V
REFPWM2
REF
P
R309
R311
R312
R310
R303
R306
R308
R302
R301
R305
GAINPW
M
REFPWM1
GNDRE
F
GAINREF
N
REF
N
GAINADCB
GAINADCT
REFADCB
REFADCT
REFATT
P-ASIC
OQ0256
T-ASIC
OQ0257
REFP
+1.23V
-1.23V
+3.3V
+0.1V
+1.6V
1
2
3
4
+3.3V
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.