HP 3455A Operating And Service Manual page 109

Section VII
are below. Both the positive and negative references have a
"fast" discharge reference and a "'slow"' discharge reference.
The fast discharge.references are used during the td cycles
of the integration period to discharge the integrator and are
also used for the "fast run-down" period (time tf). The
"slow" discharge references are used during the "slow run-
down" period (time ts} only. Diodes are used to switch the
teferences because of their high speed switching ability.
The following description uses the positive "fast-discharge"
reference, consisting of Ula, R4, CR2 and Uld, to explain
the reference switching operation. Except for different
input levels to the negative reference switches, operation of
all reference switching is identical. :
8-95. During the time the switch is turned "off", diode
CR2 is forward biased by approximately - 2 V de on the
cathode. Current flows from the + 20 volt supply through
R4 and CR2. Under this condition the voltage at the anode
of Uld is negative (approximately - 1.5 V dc) which reverse
biases Uld, holding it off. (The cathode of Uld is held at
virtual ground by the integrator.) During the "on" condi-
tion, CR2 is reverse biased by applying approximately
+ 3 V de to the cathode. Diode Uld becomes forward >
biased and allows the current to flow through R4 to the
integrator input. The purpose of diode Ula is to compen-
sate for the voltage drop across switching diode Uld by
raising the reference voltage by one diode drop. The refer-
ence current is determined by the voltage across R4 (10 V
dc/10 kilotims = | mA).
8-96. Integrator. The voltmeter uses a conventional inte-
grator circuit with a dual FET input stage for isolation.
Operational amplifier U3 provides the gain necessary to
keep the input voltage at 0 V (see Figure 8-29).
8-97. Slope Amplifier. The purpose of the slope amplifier
is to increase the speed of the "'auto-zero" function and
+10V
R4
OK
3455-B-4473
THEORY OF OPERATION
Model 3455A
reduce sensitivity to offsets in the zero detect comparator.
The slope amplifier is a conventional non-inverting operat-
tional amplifier with a gain of 100 and is used to drive the
auto-zero circuitry and zero detect comparator. Sensitivity
at the output of the integrator is approximately .5 milli-
volts per count of output reading.
8-98. Auto-Zero. The purpose of auto-zero is to reset the
integrator to a known level. During this mode of operation
FET switch Q4 is closed, completing the auto-zero loop
through slope amplifier U4. The integrator capacitor (C2)
is used as the auto-zero capacitor and stores a charge
equal in amplitude and opposite in polarity to any offsets
in the integrator and slope amplifier circuits. This charge
effectively cancels the offset errors generated by these
circuits.
8-99. Zero Detect Comparator. The output signal of the
zero-detect comparator is used to determine the polarity of
the output reading and which integrator discharge refer-
ences to apply. The output of this circuit is approximately
+ § volts for negative inputs and near 0 volts for positive
inputs to the A/D Converter.
8-100. Absolute Value Amplifier. As the name implies, the
absolute value amplifier is a unity gain circuit which pro-
duces a positive output for either a positive or negative
input. During positive inputs, the negative output of US
forward biases transistor Q6 allowing it to conduct. For
negative inputs transistor Qé6 is biased off and amplifier US
conducts through diode CR12.
8-101. 10 volt Detect Amplifier. The purpose of the 10
volt Detect Amplifier is to detect when the charge on the
integrator has reached plus or minus 10 volts. This informa-
tion is used by the inguard controller in determining when
to apply the discharge references during the integration per-
POLARITY
COMPARATOR
lOV DETECT
COMPARATOR
OUTPUTS
TO
SNTROLLER
ABSOLUTE gonrnpee
VALUE
AMPLIFIER
+2V DETECT
COMPARATOR:
Figure 8-29. Simplified A/D Converter Diagram.
8-18