ADA4891-2ARZ-R7 datasheet(12/20 Pages) AD

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ADA4891-1/ADA4891-2

Rev. A | Page 12 of 20

APPLICATIONS INFORMATION

USING THE ADA4891

Understanding the subtleties of the ADA4891 family gives users

insight into how to exact its peak performance. In this section, how

the gain, component values, and parasitics affect the performance

of the ADA4891 are discussed. The wideband, noninverting

configuration of the ADA4891 is shown in Figure 41, while the

wideband, inverting configuration of the ADA4891 is shown

in Figure 42.

WIDEBAND, NONINVERTING OPERATION

ADA4891-1

Ω

SOURCE

+VS

10µF

0.1µF

–VS

10µF

0.1µF

Figure 41. Noninverting Configuration

In Figure 41, RF and RG denote the feedback and the gain resistor,

respectively. Together, RF and RG determine the noise gain of the

amplifier, and the value of RF defines the 0.1 dB bandwidth. The

effect of RF on the 0.1 dB gain flatness is discussed in the Effect

of RF on 0.1 dB Gain Flatness section. Typical RF values range

from 549 Ω to 698 Ω.

In a controlled impedance signal path, RT is used as the input

termination resistor designed to match that of the input source

impedance. Note that it is not required for normal operation.

RT is generally set to match the input source impedance.

WIDEBAND, INVERTING GAIN OPERATION

ADA4891-1

Ω

SOURCE

+VS

–VS

10µF

0.1µF

10µF

0.1µF

Figure 42. Inverting Configuration

Figure 42 shows the inverting gain configuration. To match the

input source impedance for the inverting gain configuration, set

the parallel combination of RT//RG to match that of the input

source impedance.

Note that a bias current cancellation resistor is not required in

the noninverting input of the amplifier because the input bias

current of the ADA4891 is very low (less than 10 pA). Therefore,

the dc errors caused by the bias current are negligible.

For both configurations, it is often useful to increase the RF value

to decrease the loading on the output. Increasing the RF value

improves the harmonic distortion at the expense of reducing

the 0.1 dB bandwidth of the amplifier. This effect is discussed

further in the Effect of RF on 0.1 dB Gain Flatness section.

RECOMMENDED VALUES

Table 4 provides a handy reference for various configurations

and shows the effect of gain on the −3 dB small signal bandwidth,

slew rate, and peaking of the ADA4891-1/ADA4891-2. Note

that as the gain increases, the small signal bandwidth decreases

as is expected from the gain bandwidth product relationship. In

addition, the phase margin improves with higher gains, and the

amplifier becomes more stable. As a result, the peaking in the

frequency response is reduced (see Figure 6).

Table 4. Recommended Values for the ADA4891-1/ADA4891-2 Performance

Feedback Network Values

−3 dB Small Signal Bandwidth (MHz)

Slew Rate (V/μs)

Peaking (dB)

Gain

VOUT = 200 mV p-p

−1

604

118

188

192

1.3

236

154

263

2.6

604

120

178

204

1.4

604

151

32.5

149

154

+10

604

67.1

12.7

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