Chapter 4 Theory of Operation
A-to-D Converter
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4
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 (U6 and U8), current
steering switch U5, resistors (R5, R4, R15 and R16), voltage reference (U3A and
U3B), ADC controller U16, and residue ADC U17 on the top board.
The ADC method used by the Agilent E3631A 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 C18 and C19
through R15 on the top board.
Switch U5 steers fixed positive or negative reference currents onto the integrator
capacitors to cancel, or balance the accumulated input charge. The level shifted (R19
and R20 on the top board) output of the integrator is checked every 2.66 μA by the
U16 COMP input. Logic state machines in U16 control the U5 current steering to
continuously seek an approximate 2.5 V level on the integrator amplifier output,
FLASH. If the ADC input voltage ADC REF is between ±15 V, the integrator output
(FLASH) will remain within the 0 V to 5 V range of the U17 on-chip ADC. An input
greater than +15 V may cause the integrator output in U8 at pin 6 (U8-6) to saturate
at about -15 V. An input less than -15 V may cause U8 to saturate with an output of
about +15 V. The U17 ADC input (FLASH) is clamped to 0 V or 5 V by R21 and
CR3 to protect U17.
The integrator amplifier is formed by U6 and U8. Resistors R10 and R11 affect the
amplifier stability. Amplifier oscillation may occur if their values are incorrect.
Amplifier U6 improves the offset voltage characteristics of integrator amplifier U8
on the top board.
Each analog-to-digital conversion occurs continuously. The ADC starts by clearing
the integrator slope count in U16. 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.
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