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Chapter 4 Theory of Operation
A-to-D Converter
85
4
A-to-D Converter
Referring to the schematic shown on page 123, the analog-to-digital converter
(ADC) is used to change dc voltages into digital information. The circuitry
consists of an integrator amplifier (U26 and U29), current steering switch U33,
resistors (R70, R71, and R96), voltage reference U32, ADC controller U18, and
residue ADC U17.
The ADC method used by the Agilent E3632A 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 C49 and C51 through R71.
Switch U33 steers fixed positive or negative reference currents onto the
integrator capacitors to cancel, or balance the accumulated input charge. The
level shifted (R97 and R98) output of the integrator is checked every 2.66
msec
by the U18 COMP input. Logic state machines in U18 control the U33 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 U17
on-chip ADC. The U17 ADC input (FLASH) is clamped to 0 V or 5 V by R53 and
CR16 to protect U17.
The integrator amplifier is formed by U26 and U29. Resistors R61 and R62
affect the amplifier stability. Amplifier oscillation may occur if their values are
incorrect. Amplifier U29 improves the offset voltage characteristics of
integrator amplifier U26.
Each analog-to-digital conversion occurs continuously. The ADC starts by
clearing the integrator slope count in U18. 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 U17.
U39 provides a stable +5 V reference voltage for ADC. U32A amplifies the
voltage reference to +10 V while amplifier U32B inverts the +10 V reference to
-10 V. The reference voltage forces precision slope currents for the integrating
ADC through R70 and R96.