ADA4891-2ARMZ-R7 datasheet(16/24 Pages) AD

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

Rev. B | Page 16 of

Table 6. Recommended Component Values and Effect of Gain on ADA4891-3/ADA4891-4 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

453

186

194

0.9

Open

220

151

262

4.1

453

181

223

0.9

453

90.6

112

120

+10

453

45.3

EFFECT OF RF ON 0.1 dB GAIN FLATNESS

Gain flatness is an important specification in video applications.

It represents the maximum allowable deviation in the signal

amplitude within the pass band. Tests have revealed that the

human eye is unable to distinguish brightness variations of

less than 1%, which translates into a 0.1 dB signal drop within

the pass band or, put simply, 0.1 dB gain flatness.

The PCB layout configuration and bond pads of the chip often

contribute to stray capacitance. The stray capacitance at the

inverting input forms a pole with the feedback and gain resistors.

This additional pole adds phase shift and reduces phase margin

in the closed-loop phase response, causing instability in the

amplifier and peaking in the frequency response.

Figure 52 and Figure 53 show the effect of using various values

for Feedback Resistor RF on the 0.1 dB gain flatness of the parts.

Figure 52 shows the effect for the ADA4891-1/ADA4891-2.

Figure 53 show the effect for the ADA4891-3/ADA4891-4.

Note that a larger RF value causes more peaking because the

additional pole formed by RF and the input stray capacitance

shifts down in frequency and interacts significantly with the

internal poles of the amplifier.

–0.4

–0.3

–0.2

–0.1

0.1

0.2

0.1

100

FREQUENCY (MHz)

VS = 5V

G = +2

VOUT = 2V p-p

RL = 150Ω

RG = RF = 604Ω

RG = RF = 549Ω

RG = RF = 649Ω

RG = RF = 698Ω

Figure 52. 0.1 dB Gain Flatness, Noninverting Gain Configuration,

ADA4891-1/ADA4891-2

–0.4

–0.5

–0.3

–0.2

–0.1

0.1

0.2

0.3

0.1

100

FREQUENCY (MHz)

VS = 5V

G = +2

VOUT = 2V p-p

RL = 150Ω

RG = RF = 453Ω

RG = RF = 402Ω

RG = RF = 357Ω

RG = RF = 301Ω

Figure 53. 0.1 dB Gain Flatness, Noninverting Gain Configuration,

ADA4891-3/ADA4891-4

To obtain the desired 0.1 dB bandwidth, adjust the feedback

resistor, RF, as shown in Figure 52 and Figure 53. If RF cannot

be adjusted, a small capacitor can be placed in parallel with RF

to reduce peaking.

The feedback capacitor, CF, forms a zero with the feedback

resistor, which cancels out the pole formed by the input stray

capacitance and the gain and feedback resistors. For a first pass

in determining the CF value, use the following equation:

RG × CS = RF × CF

where:

RG is the gain resistor.

CS is the input stray capacitance.

RF is the feedback resistor.

CF is the feedback capacitor.

Using this equation, the original closed-loop frequency response of

the amplifier is restored, as if there is no stray input capacitance.

Most often, however, the value of CF is determined empirically.

Figure 54 shows the effect of using various values for the feedback

capacitor to reduce peaking. In this case, the ADA4891-1/

ADA4891-2 are used for demonstration purposes and RF = RG =

604 Ω. The input stray capacitance, together with the board

parasitics, is approximately 2 pF.

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