XRD87L99 Datasheet, PDF(9/23 Page) Exar Corporation – LOW POWER, 2 MSPS, 10-BIT, A/D CONVERTER WITH 8-CHANNEL MUX

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XRD87L99 Datasheet, PDF (9/23 Pages) Exar Corporation – LOW POWER, 2 MSPS, 10-BIT, A/D CONVERTER WITH 8-CHANNEL MUX

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XRD87L99

LOW POWER, 2 MSPS, 10-BIT, A/D CONVERTER WITH 8-CHANNEL MUX

REV. 1.0.0

FIGURE 3. XRD87L99 TIMING DIAGRAM

tAP

CLOCK

Sample

N-1

Analog

Input

Data

Auto

Balance

Sample

Auto

Balance

VIH

VIL

Sample

N+1

VOH

VOL

N-1

tDL

tHLD

THEORY OF OPERATION

1.0 ANALOG-TO-DIGITAL CONVERSION

The XRD87L99 converts analog voltages into 1024

digital codes by encoding the outputs of coarse and

fine comparators. Digital logic is used to generate the

overflow bit. The conversion is synchronous with the

clock and it is accomplished in 2 clock periods.

The reference resistance ladder is a series of resis-

tors. The fine comparators use a patented interpola-

tion circuit to generate the equivalent of 1024 evenly

spaced reference voltages between VREF(-) and

VREF(+).

The clock signal generates the two internal phases,

ÏB (CLK high) and ÏS (CLK low = sample) (See Fig-

ure 1). The rising edge of the CLK input marks the

end of the sampling phase (ÏS). Internal delay of the

clock circuitry will delay the actual instant when ÏS

disconnects the latches from the comparators. This

delay is called aperture delay (tAP).

The coarse comparators make the first pass conver-

sion and selects a ladder range for the fine compara-

tors. The fine comparators are connected to the se-

lected range during the next ÏB phase.

FIGURE 4. XRD87L99 COMPARATORS

ÏS

VIN

VTAP

Ref

Ladder

ÏB

ÏS

VIN

VTAP

Selected

ÏB

Range

ÏB

ÏS

Latch

COARSE COMPARATOR

ÏS

ÏB

Latch

FINE COMPARATOR

AIN Sampling, Ladder Sampling, and Conversion

Timing

Figure 3 shows this relationship as a timing chart. AIN

sampling, ladder sampling and output data relation-

ships are shown for the general case where the levels

which drive the ladder need to change for each sam-

pled AIN time point. The ladder is referenced for both

last AIN sample and next AIN sample at the same

time. If the ladder's levels change by more than 1

LSB, one of the samples must be discarded. Also

note that the clock low period for the discarded AIN

can be reduced to the minimum tS time.

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