AD8037ACHIPS datasheet(16/24 Pages) AD

16 / 24 page

AD8036/AD8037

REV. B

–15–

THEORY OF OPERATION

General

The AD8036 and AD8037 are wide bandwidth, voltage feedback

clamp amplifiers. Since their open-loop frequency response fol-

lows the conventional 6 dB/octave roll-off, their gain bandwidth

product is basically constant. Increasing their closed-loop gain

results in a corresponding decrease in small signal bandwidth. This

can be observed by noting the bandwidth specification, between

the AD8036 (gain of 1) and AD8037 (gain of 2). The AD8036/

AD8037 typically maintain 65 degrees of phase margin. This

high margin minimizes the effects of signal and noise peaking.

While the AD8036 and AD8037 can be used in either an invert-

ing or noninverting configuration, the clamp function will only

work in the noninverting mode. As such, this section shows con-

nections only in the noninverting configuration. Applications

that require an inverting configuration will be discussed in the

Applications section. In applications that do not require clamp-

ing, Pins 5 and 8 (respectively VL and VH) may be left floating.

See Input Clamp Amp Operation and Applications sections

otherwise.

Feedback Resistor Choice

The value of the feedback resistor is critical for optimum perfor-

mance on the AD8036 (gain +1) and less critical as the gain

increases. Therefore, this section is specifically targeted at

the AD8036.

At minimum stable gain (+1), the AD8036 provides optimum

dynamic performance with RF = 140

Ω. This resistor acts only

as a parasitic suppressor against damped RF oscillations that

can occur due to lead (input, feedback) inductance and parasitic

capacitance. This value of RF provides the best combination of

wide bandwidth, low parasitic peaking, and fast settling time.

In fact, for the same reasons, a 100–130

Ω resistor should be

placed in series with the positive input for other AD8036 non-

inverting configurations. The correct connection is shown in

Figure 3.

100 - 130

RTERM

G = 1+

+VS

–VS

VIN

VOUT

0.1 F

10 F

0.1 F

10 F

AD8036/

AD8037

Figure 3. Noninverting Operation

For general voltage gain applications, the amplifier bandwidth

can be closely estimated as:

3 dB ≅

π 1+





















This estimation loses accuracy for gains of +2/–1 or lower due

to the amplifier’s damping factor. For these “low gain” cases,

the bandwidth will actually extend beyond the calculated value

(see Closed-Loop BW plots, TPCs 13 and 25).

Pulse Response

Unlike a traditional voltage feedback amplifier, where the slew

speed is dictated by its front end dc quiescent current and gain

bandwidth product, the AD8036 and AD8037 provide “on

demand” current that increases proportionally to the input

“step” signal amplitude. This results in slew rates (1200 V/

µs)

comparable to wideband current feedback designs. This, com-

bined with relatively low input noise current (2.1 pA/

√Hz), gives

the AD8036 and AD8037 the best attributes of both voltage and

current feedback amplifiers.

Large Signal Performance

The outstanding large signal operation of the AD8036 and

AD8037 is due to a unique, proprietary design architecture.

In order to maintain this level of performance, the maximum

350 V-MHz product must be observed, (e.g., @ 100 MHz,

≤ 3.5 V p-p).

Power Supply and Input Clamp Bypassing

Adequate power supply bypassing can be critical when optimiz-

ing the performance of a high frequency circuit. Inductance in

the power supply leads can form resonant circuits that produce

peaking in the amplifier’s response. In addition, if large current

transients must be delivered to the load, then bypass capacitors

(typically greater than 1

µF) will be required to provide the best

settling time and lowest distortion. A parallel combination of at

least 4.7

µF, and between 0.1 µF and 0.01 µF, is recommended.

Some brands of electrolytic capacitors will require a small series

damping resistor

≈4.7 Ω for optimum results.

When the AD8036 and AD8037 are used in clamping mode,

and a dc voltage is connected to clamp inputs VH and VL, a 0.1

µF

bypassing capacitor is required between each input pin and

ground in order to maintain stability.

Driving Capacitive Loads

The AD8036 and AD8037 were designed primarily to drive

nonreactive loads. If driving loads with a capacitive compo-

nent is desired, the best frequency response is obtained by

the addition of a small series resistance as shown in Figure 4.

The accompanying graph shows the optimum value for RSERIES

vs. capacitive load. It is worth noting that the frequency response

of the circuit when driving large capacitive loads will be domi-

nated by the passive roll-off of RSERIES and CL. For capacitive

loads of 6 pF or less, no RSERIES is necessary.

RIN

RSERIES

AD8036/

AD8037

RIN

Figure 4. Driving Capacitive Loads

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