HP 3438A Operating And Service Manual page 59

Model 3438A
Section VlII
condition. The input voltage
@
must obviously be
amplified or attenuated t o keep the voltage a t
@)
within
these limits. This is accomplished by the combined gains
of the lnput and Post amplifiers. Figure 8-4 shows the
gain configuration for each of the five d c ranges.
8-31. T h e input voltage a t
@
is applied to the input
amplifier during integrator run-up only. Consequently,
the input voltage t o the l n p u t Amplifier is a square wave
as shown in Figure 8-4.
8-32. AC Voltmeter. The
A C Voltmeter circuit
configuration is shown in the Analog Simplified
Schematic (Figure 8-12). Figure 8-5 shows the gain
configuration for each of the five a c ranges.
8-33. In the A C Voltmeter configuration the output of
the Post Amplifier
@)
is the input t o the Ac t o D c
Converter. This signal will be ac in t h e ac volts o r a c
milliamps function.
8-34. U6 and its associated components comprise the Ac
t o Dc Converter. The output is a dc voltage equal t o the
rms value of the input. T h e output of the Ac t o Dc
Converter becomes the run-up voltage for the A to D
Converter.
8-35. Ohmmeter. Refer t o the Simplified Analog
Schematic for a simplification of the Ohmmeter circuit
configuration. Figure 8-6 is a block diagram of the
Ohmmeter circuit.
8-36. U8 functions as a low impedance voltage source t o
Rref.
It outputs
.5 V
in all ohmmeter ranges. This output
voltage is dropped across
Kref
t o a virtual ground
provided by the Input Amplifier (-). The resultant
current is the current thru the unknown resistance ( R r ) .
Figure 8-7 further simplifies the gain configuration
c o m b i n i n g t h e l n p u t A m p l i f i e r . a s s o c i a t e d
compensation, and protection circuitry as a n inverting
O p A m p with
Krer
as the input resistor and
Rx
as the
feedback resistor. I'he output of the Input Amplifier is
the run-up voltage to the A t o 1) Converter.
Flgure 8-4. DC Galn Conflguratlon.