AD652

REV. B

–5–

there are no problems with dielectric absorption causing the

duration of a reset pulse to be influenced by the length of time

since the last reset.

Figure 2. AD652 Block Diagram and System Waveforms

Referring to Figure 2, it can be seen 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 of the

value of the reference current. In order to achieve a charge bal-

ance, the output frequency will equal the clock frequency divided

by four; one clock period for reset and three clock periods of inte-

grate. This is shown in Figure 3. 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

µA

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

put 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 downward.

As the integrator sawtooth drifts down, the comparator thresh-

old 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 fre-

quency can be very different.

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 which is also

a sawtooth. This is shown in Figure 4.

than 250

µA

Another way to view this is that the output is a frequency of

approximately one quarter of the clock that has been phase

modulated. A constant frequency can be thought of as accumu-

lating phase linearly with time at a rate equal to 2

πf radians per

second. Hence, the average output frequency which is slightly in

excess of a quarter of the clock will require phase accumulation

at a certain rate. However, since the SVFC is running at exactly

one quarter of the clock, it will not accumulate enough phase

(see Figure 5). When the difference between the required phase

(average frequency) and the actual phase equals 2

π, a step in

phase is taken where the deficit is made up instantaneously. The

output frequency is then a steady carrier which has been phase

modulated by a sawtooth signal (see Figure 5). The period of

the sawtooth phase modulation is the time required to accumulate

a 2

π difference in phase between the required average frequency

and one quarter of the clock frequency. The amplitude of the

sawtooth phase modulation is 2

π.

Figure 5. Phase Modulation