AD5539

–7–

REV. B

APPLYING THE AD5539

The AD5539 is stable for closed-loop gains of 4 or more as an

inverter and at (noise) gains of 5 or greater as a voltage follower.

This means that whenever the AD5539 is operated at noise

gains below 5, external frequency compensation must be used to

insure stable operation.

The following sections outline specific compensation circuits

which permit stable operation of the AD5539 down to follower

(noise) gains of 3 (inverting gains of 2) with corresponding

–3 dB bandwidths up to 390 MHz. External compensation is

achieved by modifying the frequency response to the AD5539’s

external feedback network (i.e., by adding lead-lag compensa-

tion) so that the amplifier operates at a noise gain of 5 (or more)

at frequencies over 44 MHz, independent of signal gain.

Figure 13. Small Signal Open-Loop Gain and

Phase vs. Frequency

GENERAL PRINCIPLES OF LEAD AND LAG

COMPENSATION

The AD5539 has its first pole or breakpoint in its open-loop fre-

quency response at about 10 MHz (see Figure 13). At frequen-

cies beyond 100 MHz, phase shift increases such that the output

lags the input by 180

°—well before the unity gain crossover fre-

quency. Therefore, severe peaking (and possible oscillation) will

result if the AD5539 is operated at noise gains below 5, unless

external compensation is employed. Figure 14 shows the un-

compensated closed-loop frequency response of the AD5539

Figure 14. AD5539 Uncompensated Response, Closed-

Loop Gain = 7

when operating at a noise gain of 7. Under these conditions, ex-

cess phase shift causes nearly 10 dB of peaking at 150 MHz.

Figure 15 illustrates the use of both lead and lag compensation

to permit stable low-gain operation. The AD5539 is shown con-

nected as an inverting amplifier with the required external com-

ponents added to provide stability and improve high frequency

response. The stray capacitance between the amplifier summing

sociated with the particular type of op amp package used plus

the stray wiring capacitance at the summing junction.

a pole frequency equal to:

1

2

π C

()

(1)

and a zero frequency equal to:

1

2

π R1 × C

LEAD

()

(2)

an attenuator or scope probe. However, if the pole frequency,

Figure 15. Inverting Amplifier Model Showing Both Lead

and Lag Compensation

Figure 16. A Model of the Feedback Network of the

Inverting Amplifier