HP 3455A Operating And Service Manual page 112

Model 3455A
8-112. The "polarity detect" output of the converter is
also applied to the input of the "zero detect" circuit. The
zero detect circuit is used to detect the end of the "slow"
run-down period. At the beginning of the slow run-down
period, the "Q" output of U32B is set to the same state as
the polarity detect signal by a pulse from U14. The inter-
rupt enable signal from the processor is set high to enable
the zero detect circuit. As the charge on the A/D Integrator
passes through 0 volts, the polarity detect signal changes
state and causes the output of the zero detect circuit to go
low. The low output from the zero detect circuit sets the
processor's interrupt input to stop the A/D Conversion pro-
THEORY OF OPERATION
Section VIII
Table 8-2. A/D Converter Switch Control Signal
Descriptions.
True
Description State
A/D Converter input switch signal
{A14Q3)
Negative slow-discharge reference switch signal
(A14U2c)
Positive slow-discharge reference switch signal
Auto-zero switch signal (A1404)
Positive fast-discharge reference switch signal
{A14CR2)
Negative fast-discharge reference switch signal
{A14CR6)
cess. Upon completion of the A/D Conversion process, the
processor sets the interrupt enable signal low to disable the
zero detect circuit.
Figure 8-34. Simplified A/D Converter Control Circuit.
8-21