AD652 datasheet(8/29 Pages) AD

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AD652

Rev. C | Page 7 of 28

COMPARATOR

AND

G Q

D FLOP

LATCH

CLOCK IN

ONE

SHOT

COS

CINT

RIN

INTEGRATOR

1mA

–VS

VIN

tOS

INTEGRATOR

OUTPUT

THRESHOLD

CLOCK

COMPARATOR

OUT

AND

OUT

D FLOP

OUT

LATCH

OUT

FREQ

OUT

Figure 4. Block Diagram and System Waveforms

Figure 4 shows that the period between output pulses is

constrained to be an exact multiple of the clock period.

Consider an input current of exactly one quarter the value of

the reference current. In order to achieve a charge balance, the

output frequency equals the clock frequency divided by four:

one clock period for reset and three clock periods of integrate.

This is shown in Figure 5. If the input current is increased by a

very small amount, the output frequency should also increase

by a very small amount. Initially, however, no output change is

observed for a very small increase in the input current. The

output frequency continues to run at one quarter of the clock,

delivering an average of 250 µA to the summing junction. Since

the input current is slightly larger than this, charge accumulates

in the integrator and the sawtooth signal starts to drift down-

ward. As the integrator sawtooth drifts down, the comparator

threshold is crossed earlier and earlier in each successive cycle,

until finally, a whole cycle is lost. When the cycle is lost, the

integrate phase lasts for two periods of the clock instead of the

usual three periods. Thus, among a long string of divide-by-

fours, an occasional divide-by-three occurs; the average of the

output frequency is very close to one quarter of the clock, but

the instantaneous frequency can be very different.

INTEGRATOR

OUT

CLOCK

THRESHOLD

Figure 5. Integrator Output for IIN = 250 µA

Because of this, it is very difficult to observe the waveform on

an oscilloscope. During all of this time, the signal at the output

of the integrator is a sawtooth wave with an envelope that is also

a sawtooth. See Figure 6.

200

µs/BOX

100

µs/BOX

CINT

FREQ OUT

µs/BOX

CLOCK IN

µs/BOX

Figure 6. Integrator Output for IIN Slightly Greater than 250 µs

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