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Section IV
Model 3465B
4-47. In the event of open loop (R^ = °°), the ohms
amplifier output begins to drive negative. The input
(negative port), which is the auto-zero output, could exceed
± 1 mV under an open loop condition due to the lack of
negative feedback through an R^. This auto-zero output
must be maintained at < ± 1 mV for accurate operation of
the A-D converter. To satisfy this requirement, an over
load protection circuit consisting of CR23, CR24 and R86
is used. When the ohms amplifier output goes below
approximately -H.5 V, the zener diode (CR23) turns off.
The overload loop, CR24 and R86, is introduced by the
turn-on of CR24 when CR23 is off. This loop provides the
negative feedback required to maintain an auto-zero output
< ± 1 mV. When an R^ is introduced, CR23 turns-on,
CR24 turns-off, and the overload loop is inoperative.
4-48. A maximum output by the ohms converter of < 5 V
is guaranteed by a voltage divider composed of R93 and
R95. Additional protection components of the ohms
converter are: A) CR29 which prevents Q32 junction
breakdown due to fast transients, B) CR28 which blocks
negative transients that could come in via the LO terminal
point and C) R91 and C27 which suppress high voltage,
high frequency transients.
4M-9. Degradation of accuracy in the ohms function due to
changes in the ohms reference with respect to the A—D
reference is minimized since both reference voltages are
derived from the same -t 10 V reference supply. If the
reference supply voltage changes, both the ohms reference
and the A—D reference are affected alike and any change is
effectively cancelled.
4-50. AC-to-DC Converter.
4-51. The AC—to—DC converter is an average responding
ac converter. It has a bandwidth of 40 Hz to 20 kHz. The
converter is composed of two stages (see Figure 7-2). The
first stage, U19, is an impedance converter. The purpose of
this amplifier is to provide a high impedance to the input so
loading of the input signal does not occur. It also provides
high drive capability for the ac converter stage, U18. The
input of the impedance converter is protected against large
voltage swings by diodes CR35 and CR37. Voltages in
excess of -t 10 V or - 7 V peak ac will forward bias these
diodes, returning excess current to the power supply.
4-52. The impedance converter, U19, has a selection of
three gains; the 200 mV, .2 mA, 200
and 20 V, 20 mA,
20 kf2 ranges select a gain of 10. The ac current function
selects a gain of 9.964, while the remainder of the ranges
and functions select a gain of unity (see U19 Gain Table,
Figure 7-2).
4-53. The second stage of the AC—to—DC converter is the
ac converter, U18. A basic diagram of this stage is shown in
Figure 4-6. The amplifier has three feedback loops. These
loops are the ac negative feedback loop, the dc negative
feedback loop, and the positive feedback loop. The ac
negative feedback loop is divided into two branches; one
branch for the positive half cycle and the second branch for
the negative half cycle. Diodes CR33 and CR34 switch
,V(jc TO
INPUT AMP
-•—wv—4-
RI25
220K
CR33
AC
NEGATIVE
FEEDBACKt
LOOPS
10 K
CR34
RI27
220K
RI26
lOK
FROM
IMPEDANCE
CONVERTER
UI8
FILTER
POSITIVE
FEEDBACK
LOOP
DC
NEGATIVE
FEEDBACK
LOOP
Figure 4-6. Basic Diagram, AC Converter Amplifier.
between the positive and negative half-cycles to introduce
the correct loop for its respective half-cycle.
4-54. During switching of the diodes, little negative feed
back is present. During the switching transition, the positive
feedback loop (C45, R120 and R123) boosts the amplifier
gain. This boost in gain speeds the switching transition of
the diodes which gives a good frequency response at low
signal levels. Once the switching transition has occurred,
negative feedback is again present. The negative feedback
overrides the effects of the positive feedback loop at all
times other than the diode switching transition period.
4-55. The output of the AC-to-DC converter is derived
from the positive half-cycle, negative feedback loop. The
positive half-cycle developed across the load resistor R118
is the half-wave rectified signal of the ac converter amplifier
output. This rectified signal is filtered to provide the dc
output that is applied to the input amplifier during the
run-up interval of the measurement sequence. For full-scale
inputs, the AC-to-DC Converter output is 1.6 V dc. This
output is kept relatively free of the dc offset present on the
inverting input of U18 (pin 2) by the voltage divider R125
and R118. The portion of the offset appearing across the
load resistor R118 is attenuated by a factor of 23.
4-56. A-D Conversion Using a Monopolar Reference.
4-57. Before preceeding with this discussion, review the
4-6
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