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AD5520

Rev. B | Page 18 of 24

SETTLING TIME CONSIDERATIONS

Fast throughput is a key requirement in automatic test

equipment because it relates directly to the cost of manufac-

turing the DUT; thus reducing the time required to make a

measurement is of greatest importance. When taking

measurements using a PMU, the limiting factor is usually the

time it takes the output to settle to the required accuracy so a

measurement can be taken. DUT capacitance, measurement

accuracy, and the design of the PMU are the major contributors

to this time.

Figure 26 shows a simplified block diagram of the AD5520

PMU. In brief, the device consists of a force control amplifier,

access to a number of selectable sense resistors, a voltage

measure instrumentation amplifier, and a current measure

instrumentation amplifier. To optimize the performance of the

device, there are also nodes provided where external compensa-

tion capacitors are added. As mentioned, making an accurate

measurement in the fastest time while avoiding overshoots and

ringing is the key requirement in any automatic test equipment

(ATE) system. Doing so provides challenges, however. The

external compensation capacitors set up different settling times

or bandwidths on the force control amplifier, and while one

compensation capacitor value may suit one range, it may not

suit other ranges. To optimize measurement performance and

speed, differences in signal behavior on each range and

frequency of use of each range need to be taken into account.

When selecting a faster settling time, there is a trade-off.

A small compensation value results in faster settling, but

may incur penalties in overshoots or ringing at the DUT.

Compensation capacitor selection should be optimized to

ensure minimum overshoots while still giving decent settling

time performance.

While careful selection of the compensation capacitor is

required to minimize the settling time, another factor can

greatly contribute to the overall settling of the loop if the

feedback loop is broken in some manner, and the force control

amplifier goes to either the positive or negative rails. There is a

finite amount of time required for the amplifier to recover from

this condition, typically 85 μs, which adds to the settling of the

loop. Ensuring that the force control amplifier never goes into

saturation is the best solution. This solution can be helped by

putting the device into standby mode any time the operating

mode or range selection is changed. In addition, ensure that the

selected output range can supply the required current needed by

the DUT.

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