AD8067ART-R2 datasheet(13/24 Pages) AD

13 / 24 page

AD8067

Rev. A | Page 13 of 24

THEORY OF OPERATION

The AD8067 is a low noise, wideband, voltage feedback

operational amplifier that combines a precision JFET input

stage with Analog Devices’ dielectrically isolated eXtra Fast

Complementary Bipolar (XFCB) process BJTs. Operating

supply voltages range from 5 V to 24 V. The amplifier features a

patented rail-to-rail output stage capable of driving within

0.25 V of either power supply while sourcing or sinking 30 mA.

The JFET input, composed of N-channel devices, has a

common-mode input range that includes the negative supply

rail and extends to 3 V below the positive supply. In addition,

the potential for phase reversal behavior was eliminated for all

input voltages within the power supplies.

The combination of low noise, dc precision, and high

bandwidth makes the AD8067 uniquely suited for wideband,

very high input impedance, high gain buffer applications. It is

also useful in wideband transimpedance applications, such as a

photodiode interface, that require very low input currents and

dc precision.

BASIC FREQUENCY RESPONSE

The AD8067’s typical open-loop response (see Figure 41) shows

a phase margin of 60° at a gain of +10. Typical configurations

for noninverting and inverting voltage gain applications are

shown in Figure 40 and Figure 42.

The closed-loop frequency response of a basic noninverting

gain configuration can be approximated by:

() ()

GBP

Frequency

Loop–3

Closed

DC Gain = RF/RG + 1

GBP is the gain bandwidth product of the amplifier. Typical

GBP for the AD8067 is 300 MHz. See Table 5 for the

recommended values for RG and RF.

Gain

Noise

ion

Configurat

Noninverti

+VS

AD8067

–

RLOAD

VOUT

–

FOR BEST PERFORMANCE,

SET RS + RX = RG || RF

μF

0.1

μF

0.1

μF

SIGNAL

SOURCE

–VS

Figure 40. Noninverting Gain Configuration

FREQUENCY – MHz

–10

120

–150

–120

–90

–60

–30

–180

0.01

0.1

100

1000

PHASE

GAIN

Figure 41. Open-Loop Frequency Response

The bandwidth formula only holds true when the phase margin

of the application approaches 90°, which it will in high gain

configurations. The bandwidth of the AD8067 used in a

G = +10 buffer is 54 MHz, considerably faster than the 30 MHz

predicted by the closed loop –3 dB frequency equation. This

extended bandwidth is due to the phase margin being at 60°

instead of 90°. Gains lower than +10 show an increased amount

of peaking, as shown in Figure 4. For gains lower than +7, use

the AD8065, a unity gain stable JFET input op amp with a unity

gain bandwidth of 145 MHz, or refer to the Applications section

for using the AD8067 in a gain of 2 configuration.

Table 5. Recommended Values of RG and RF

Gain

RG (Ω)

RF (kΩ)

BW (MHz)

110

49.9

100

+VS

AD8067

–

RLOAD

VOUT

–

μF

0.1

μF

0.1

μF

–VS

SIGNAL

SOURCE

FOR BEST PERFORMANCE, SET RX = (RS + RG) || RF

Figure 42. Inverting Gain Configuration

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