Fluke 5790B, Service Manual

Page: 35 / 172

Theory of Operation
Analog Section Detailed Circuit Description
2
2-9
OUTPUT
FROM
A16 DAC
2V DIVIDER
DC REFERENCE
SELECT
PRECISION
INVERTER
CHOPPER
OSCILLATOR
28 Hz
28 Hz
CHOPPER
SWITCHES
CHOPPER
ATTENUATORS
TO
A10
TRANSFER
ASSEMBLY
elu009.eps
Figure 2-4. Chopper Circuit Block Diagram
The null DAC is set by the CPU to equal the A/D measurement taken with the
instrumentation amplifier on the X1 setting (the first pass). This measurement
shows on the display as the reading with lower resolution and the U indicator lit.
This DAC has a 14-bit resolution, where 0 counts gives 0 V, 3fff hex counts gives
+2.2 V full scale.
The instrumentation amplifier amplifies the difference between the output of the
null DAC and the output of the FTS circuit on the transfer assembly. The output
of the instrumentation amplifier is fed directly into the A/D amplifier. The result of
this system is an improved resolution from the A/D IC. The instrumentation
amplifier is switched between a unity gain and an X10 configuration.
A five-pole filter attenuates low frequency ripple. See Figure 2-5.
CPU
CPU
NULL
DAC
SWITCHABLE
ACTIVE
LPF
FTS CIRCUIT
OUTPUT
FROM A10
TRANSFER
ASSEMBLY
FROM FTS
CIRCUIT
INPUT
CPU
x10 & x1
INSTRUMENTA-
TION AMPLIFIER
AND FILTERS
SWITCHABLE
LPF
CPU
CPU
A/D
CONVERTER
FREQUENCY
COUNTER
elu010.eps
Figure 2-5. A15 A/D Amplifier Block Diagram