Chapter 4 Theory of Operation
A-to-D Converter
91
4
A-to-D Converter
Referring to the schematic shown on page 131, the a nalog-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), volta ge 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 continuous ly integrating analog-to- di gital converter.
The input voltage continuousl y forces char ge onto the integrator capacitors
C50 and C52 through R73.
Switch U32 steers fixed positive or negative reference currents onto the
integrator capacitors to cance l, 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 -c hi p ADC . The U1 9 ADC i np ut ( FLA SH) is c lam pe d to 0 V o r 5 V by R 47 an d
CR14 to protect U19.
The integrator amplifi er is form ed by U25 and U28. Resis tors R 60 and R 61
affect the amplifie r stability . Amplifier oscillation may occur if their v alues are
incorrect. Amplifier U28 improves the offset voltage characteristics of
integrator ampli fier 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 significa nt bits are converted by the
on-chip A DC of U19.
U40 provides a stable +5 V reference voltage for ADC. U31A amplifies the
voltage reference to +10 V while amplifier U3 1B inverts the +10 V ref ere nce to
-10 V. The reference voltage f orces precision slope currents for the integrating
ADC through R 72 and R98.
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