4 Theory of Operation
106 Keysight E3633A and E3634A Service Guide
A-to-D Converter
The analog-to-digital converter (ADC) is used to change dc voltages into digital
information. The circuitry consists of an integrator amplifier (U25 and U28),
current steering switch U32, resistors (R72, R73, and R98), voltage reference U31,
ADC controller U20, and residue ADC in U19.
The ADC method used by the power supply is called multislope III. Multislope III is
a charge balancing continuously integrating analog-to-digital converter. The
input voltage continuously forces charge onto the integrator capacitors C50 and
C52 through R73.
Switch U32 steers fixed positive or negative reference currents onto the integrator
capacitors to cancel, or balance the accumulated input charge. The level shifted
(R99 and R100) output of the integrator is checked every 2.66 msec by the U20
COMP input. Logic state machines in U20 control the U32 current steering to
continuously seek an approximate 2.5 V level on the integrator amplifier output,
FLASH. If the ADC input voltage is between ±15 V, the integrator output (FLASH)
will remain within the 0 V to 5 V range of the U19 on-chip ADC. The U19 ADC
input (FLASH) is clamped to 0 V or 5 V by R47 and CR14 to protect U19.
The integrator amplifier is formed by U25 and U28. Resistors R60 and R61 affect
the amplifier stability. Amplifier oscillation may occur if their values are incorrect.
Amplifier U28 improves the offset voltage characteristics of integrator amplifier
U25.
Each analog-to-digital conversion occurs continuously. The ADC starts by
clearing the integrator slope count in U20. At the end of the integration period,
the slope count is latched. The slope count provides the most significant bits of
the input voltage conversion. The least significant bits are converted by the
on-chip ADC of U19.
U40 provides a stable +5 V reference voltage for ADC. U31A amplifies the voltage
reference to +10 V while amplifier U31B inverts the +10 V reference to -10 V. The
reference voltage forces precision slope currents for the integrating ADC through
R72 and R98.
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