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Table 4-2. Converter
Attenuation
and Amplification
AC CONVERTER
Table
**x100
= a101 On
**x 1000 = KlOl On
4-3. DC Signal
Conditioning
Attenuation
Gain
Range
20mV
200mV
2v
2ov
2oov
1200v
Input
Attenuation
-
+ 10
t 100
i 1000
t
Chopper
Gain
x 100
x 10
Xl
Xl
Xl
Xl
1
and
4.4.1 Input Attenuation
On the 20, 200 and 1200V ranges, the input signal is atten-
uated by the resistive
divider to divide by 10, 100 and 1000,
respectively.
Depressing
one
of the
above
range
push-
buttons,
interrupts
the unattenuated
signal from the volts
switch
pin 17 and connects
the wiper
of the appropriate
range potentiometer
to the input of the chopper.
Division by
10, 100 and 1000 is accomplished
by connecting
the wiper of
R155, R156 or R157, respectively.
4.4.2 Chopper
The input signal to the chopper
is filtered
and applied
to a
modulator
circuit.
The
AC
signal
is then
amplified
and
demodulated.
A final high gain DC amplifier
provides
a DC
signal to the AID
converter
and the analog output.
1. Modular
Circuit-This
circuit converts
the input DC signal
to an AC signal with a fundamental
frequency
component
of approximately
390Hz.
This
frequency
was
chosen
because
it reduces
the amplitude
of higher harmonics
of
the 50Hz
and 60Hz
line frequencies,
which
minimizes
beating.
The circuit
utilizes a dual MOSFET
(Q104A
and
010413) connected
in a series-shunt
configuration
to main-
tain the required
high input impedance.
2. AC Amplifier-The
AC amplifier
uses a JFET-input
opera-
tional
amplifier
(U108).
The amplifier
feedback
network
provides a gain of approximately
133 at the modulating
fre-
quency
of 390Hz.
while
maintaining
a gain of one for
amplifier
(U108) DC offset voltages.
6
OHMS CONVERTER
Figure 4-2. Simplified
Signal Conditioning
3. Demodulator
Circuit-The
demodulator
is sychronized
with
the input
modulator.
A JFET (01051 with
low on-
resistance
is used to alternately
attenuate
and pass the
signal present
at the output
of AC amplifier
U108. This
produces
a "half-wave
rectified"
DC signal at the input of
the integrator
stage, that is inverted
with respect
to the
chopper
input signal.
4. DC Amplifier-Amplifier
lU109)
is connected
as an in-
tegrator
with an offset
adjustment.
U109 provides
suff-
cient gain to bring the total open-loop
DC gain to about
one million.
U109 drives the feedback
divider network
and
the A/D
converter,
and
provides
the low output
im-
pedance
necessary
to drive the analog output.
Feedback
capacitor
(Cl381 was selected
to provide
nominal
settling
time, while meeting
NMR requirements.
When the chop-
per is operating
as a unity-gain
follower,
the DC signal prey
sent at the output of U109 is the chopper
feedback.
This is
the condition
for the 2VDC
range
and above.
On the
200mV
and 20mV ranges, where chopper
gains of 10 and
100 are required,
the resistive divider feeds back a portion
of the output
of U109 to provide the required
gain.
5. Modulator-Demodulator
Drive
Circuit-CMOS
inverters
lUlllA
and UlllF)
provide
opposite
phase 0 to -15V
square waves used to drive MOSFET modulator
transistors
Q104A
and
Q1048.
lnverter
UlllF
also
drives
the
4-3
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