Fluke 123B/S Service Manual - Page 52

123/124 Service Manual Reference Voltage Circuit This circuit derives several reference voltages from the 1.23V main reference source. +3.3V +1.23V REFPWM2 73 R307 V301 1.23V REFP 72 71 +3.3V -1.23V R309 R311 R312 R308 R306 REFP 62 GAINPWM 56 REFPWM1 55 GNDREF 57 GAINREFN 63 REFN 64 +0.1V R310 R303 GAINADCB 54 REFADCB 53 +1.6V R302 R301 R305 GAINADCT 52 REFADCT 51 REFATT 8 + - +1 + -2 + - 3 + -4 P-ASIC OQ0256 T-ASIC OQ0257 Figure 3-12. Reference Voltage Section 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