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Model 3465B
Section IV
The reference supply is + 10 V for all ranges except the
20 M range. For this range the reference supply is + 1 V.
The full-scale output of the ohms converter is 2 V dc. On
the 20 M range with a Rx of 20 MFi (full-scale), an output
of 2 V dc is needed. From the formula for the ohms
output, it can be seen that 10" would have to equal
100 Mf2. Since the range of 10" is 10 kfi to 10 Mf2, a 10"
of 10Mf2 combined with a reference supply of IV
provides the desired 1 V dc full-scale ohms converter
output.
4-15. AC—DC Converter. The ac—dc converter is an average
responding ac converter. It measures the average value of a
sine wave and multiplies this by a fixed scale factor to
convert it to an rms value. The output of the converter is a
dc voltage equal to the rms value of the sine wave.
4-16. Figure 4-3 is a block diagram of the ac -dc converter.
The blocks consist of an impedance converter, an ac
converter and a filter. The impedance converter has a high
input impedance to prevent loading of the input signal. It
also provides the gain necessary to drive the ac converter.
An impedance converter gain of unity, 9.964 or 10 is
selected by the function and range switching. The gain of
9.964 is used with the ac current function and the gain of
10 is used with the 200 mV, .2 mA, 200
and 20 V,
20 mA, 20 kfl ranges.
4-17. The ac converter amplifies the signal from the
impedance converter by the scale factor. This converts the
average value of the sine wave to the rms value. Half-wave
rectification of the sine wave is also performed by the ac
converter. This rectified signal is filtered to provide the
proportional dc output which is applied to the analog—to-
digital converter.
4-18. Analog-to-Digital (A-D) Converter.
4-19. The A—D converter block is comprised of an input
amplifier, reference supply, integrator, slope amplifier,
comparator and auto-zero circuit. It makes an analog—to-
digital conversion using the dual-slope integrating tech
nique. Four control state signals from the logic section (10,
IZ, II and 12) regulate the measurement sequence. 10 and
IZ regulate the input amplifier and auto-zero switching
respectively while 11 and 12 select the reference supply
required during the run-down interval.
4-20. Input Amplifier. The first stage of the A—D con
verter is the input amplifier. During the run-up interval of
the measurement sequence, control state signal 10 switches
the output of the signal conditioning block to the input
amplifier. The output of the signal conditioning block is a
dc voltage which varies between 20 mV and 2 V for
full-scale inputs, depending on the function and range
selected. The gain of the input amplifier is adjusted by the
function and range switching to provide an output of 2 V
dc for any full-scale input signal. See Input Amplifier Gain
Table on Figure 7-3.
4-21. Reference Supply. The A—D converter uses a mono-
polar reference supply of -tlOV. A reference voltage is
applied to the integrator during the run-down interval to
discharge the integrating capacitor. Since the discharge rate
is constant, the time required for the integrator to reach a
zero reference is proportional to the input signal. This time
period is the run-down interval and is processed to
determine the display. A positive and negative reference
voltage is required since the input signal can be either
polarity. A detailed discussion of the operation of the
monopolar reference supply can be found in the detailed
theory.
4-22. Integrator. The integrator output is a result of a
current summation at the integrator summing junction
(inverting input). A positive current summation (current
flowing into the integrator input) will cause the integrator
to ramp negative. A negative current summation (current
flowing out of the integrator input) will cause the integra
tor to ramp positive. The integrator sums currents from the
input amplifier, reference supply, - 7 V supply and the
auto-zero loop during designated times.
4-23. Slope Amplifier. Following the integrator is a X4000
amplifier. This amplifier is divided into two stages; the first
with a gain of 40 and the second with a gain of 100. The
slope amplifier amplifies the integrator output to provide a
more vertical crossing of this output with the reference
level. This provides greater accuracy of the voltage—to-
time conversion during the run-down interval.
4-24. Comparator. The comparator provides two logic
outputs; a high output of 0 V or a low output of - 7 V. The
comparator output is high when the integrator output is
greater than the reference level. The comparator is low
when the integrator output is less than the reference level.
AC
INPUT-
SIGNAL
IMPEDANCE
CONVERTER
X!
X 9.964
orXIO
AC
CONVER
TER
FILTER
DC VOLTAGE TO
►ANAUDG-TO-
DIGITAL CONVERTER
Figure 4-3. Block Diagram, AC—to—DC Converter.
4-3
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