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ADA4891-1/ADA4891-2/ADA4891-3/ADA4891-4

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APPLICATIONS INFORMATION

USING THE ADA4891

Understanding the subtleties of the ADA4891 family of amplifiers

provides insight into how to extract the peak performance from

the device. The following sections describe the effect of gain,

component values, and parasitics on the performance of the

ADA4891. The wideband, noninverting gain configuration of

the ADA4891 is shown in Figure 50; the wideband, inverting

gain configuration of the ADA4891 is shown in Figure 51.

WIDEBAND, NONINVERTING GAIN OPERATION

ADA4891

50Ω

SOURCE

+VS

–VS

10µF

0.1µF

10µF

0.1µF

Figure 50. Noninverting Gain Configuration

In Figure 50, RF and RG denote the feedback and gain resistors,

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

amplifier. The value of RF defines the 0.1 dB bandwidth (for

more information, see the Effect of RF on 0.1 dB Gain Flatness

section). Typical RF values range from 549 Ω to 698 Ω for the

ADA4891-1/ADA4891-2. Typical RF values range from 301 Ω

to 453 Ω for the ADA4891-3/ADA4891-4.

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

termination resistor designed to match the input source imped-

ance. Note that RT is not required for normal operation. RT is

generally set to match the input source impedance.

WIDEBAND, INVERTING GAIN OPERATION

ADA4891

50Ω

SOURCE

+VS

–VS

10µF

0.1µF

10µF

0.1µF

Figure 51. Inverting Gain Configuration

Figure 51 shows the inverting gain configuration. For the

inverting gain configuration, set the parallel combination of

RT and RG to match 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 2 pA). Therefore,

the dc errors caused by the bias current are negligible.

For both noninverting and inverting gain configurations, it is

often useful to increase the RF value to decrease the load on the

output. Increasing the RF value improves 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 5 and Table 6 provide a quick reference for various configu-

rations and show the effect of gain on the −3 dB small-signal

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

ADA4891-3/ADA4891-4. 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 7 and Figure 10).

Table 5. Recommended Component Values and Effect of Gain on ADA4891-1/ADA4891-2 Performance (RL = 1 kΩ)

Feedback Network Values

−3 dB Small-Signal Bandwidth (MHz)

Slew Rate (V/μs)

Peaking (dB)

Gain

RF (Ω)

RG (Ω)

VOUT = 200 mV p-p

−1

604

118

188

192

1.3

Open

240

154

263

2.6

604

120

170

210

1.4

604

151

32.5

149

154

+10

604

67.1

12.7

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