5-8
Range/function switching is performed by various relays and
associated components. These relay contacts control the cir-
cuit elements placed in the feedback path of the input pream-
plifier. For example, in the amp and ohm functions,
appropriate feedback resistors are placed in the feedback
path, while capacitors are used as the feedback element in
the coulombs function. In the volts function, the preamp is
configured as a unity-gain buffer by connecting its output
node to the inverting input.
Figure 5-5
Analog board block diagram
Input
HI
To
A/D
Converter
Multiplexer
U211, U212
U405
Output
Stage
Thermocouple
Output Stage
Q402 - Q413
Prescaler
U215
Serial/Parallel
Converter
U303 - U306
DAC
U308
Zero Check
K416, K417
Range/Function
Switching
K401, K402, K405
K406, K411, K412
Thermocouple
Input
U504
Offset
Compensation
Offset DAC
U407
+
-
LO
+220V
Preamp
Out
Input
Stage
U401
U402
Input Amplifier
Offset Data from MPU
Humidity
Humidity
Input
U502
U214
U209
2V
Analog
Output
Voltage Source
Data from MPU
U209
Ranging
V-Source
Output
+120V +1200V
Voltage Source
The input preamplifier is actually a compound operation
amplifier whose exact configuration depends on the selected
measuring function. As shown in Figure 5-6, the preamp is
configured as a unity-gain buffer in the volts function, as an
I-V converter in the amp and ohm functions, and as a Q-V
converter in the coulombs function.