ad5539jq Analog Devices, Inc., ad5539jq Datasheet - Page 7

• Manufacturer Part Number: ad5539jq
• Description: Ultrahigh Frequency Operational Amplifier
• Manufacturer: Analog Devices, Inc.

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.
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
Figure 13. Small Signal Open-Loop Gain and
Phase vs. Frequency
–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
junction and ground, C
sociated with the particular type of op amp package used plus
the stray wiring capacitance at the summing junction.
Evaluating the lead capacitance first (ignoring R
for now): the feedback network, consisting of R2 and C
a pole frequency equal to:
and a zero frequency equal to:
Usually, frequency F
(R2 C
an attenuator or scope probe. However, if the pole frequency,
F
then the optimum location of F
Figure 15. Inverting Amplifier Model Showing Both Lead
and Lag Compensation
A
, will lie above the unity gain crossover frequency (440 MHz),
Figure 16. A Model of the Feedback Network of the
Inverting Amplifier
LEAD
), in a manner similar to the compensation used for
F
A
=
2
F
B
A
C
=
is made equal to F
X
LEAD
, represents whatever capacitance is as-
2
R1 C
1
C
B
X
will be near the crossover
1
R1 || R2
LEAD
B
; that is, (R1C
AD5539
LAG
and C
LEAD
X
) =
LAG
, has
(1)
(2)