ADA4891-2ARMZ-R7 datasheet(17/24 Pages) AD

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

Rev. B | Page 17 of

–0.3

–0.2

–0.1

0.1

0.2

0.1

100

FREQUENCY (MHz)

CF = 3.3pF

CF = 0pF

CF = 1pF

VS = 5V

G = +2

RF = 604Ω

RL = 150Ω

VOUT = 2V p-p

Figure 54. 0.1 dB Gain Flatness vs. CF, VS = 5 V,

ADA4891-1/ADA4891-2

DRIVING CAPACITIVE LOADS

A highly capacitive load reacts with the output impedance of

the amplifiers, causing a loss of phase margin and subsequent

peaking or even oscillation. The ADA4891-1/ADA4891-2 are

used to demonstrate this effect (see Figure 55 and Figure 56).

–10

–8

–6

–4

–2

0.1

100

FREQUENCY (MHz)

VS = 5V

VOUT = 200mV p-p

G = +1

RL = 1kΩ

CL = 6.8pF

Figure 55. Closed-Loop Frequency Response, CL = 6.8 pF,

ADA4891-1/ADA4891-2

50ns/DIV

50mV/DIV

VS = 5V

G = +1

RL = 1kΩ

CL = 6.8pF

100

–100

Figure 56. 200 mV Step Response, CL = 6.8 pF,

ADA4891-1/ADA4891-2

These four methods minimize the output capacitive loading effect.

•

Reducing the output resistive load. This pushes the pole

further away and, therefore, improves the phase margin.

•

Increasing the phase margin with higher noise gains. As

the closed-loop gain is increased, the larger phase margin

allows for large capacitive loads with less peaking.

•

Adding a parallel capacitor (CF) with RF, from −IN to the

output. This adds a zero in the closed-loop frequency

response, which tends to cancel out the pole formed by the

capacitive load and the output impedance of the amplifier.

See the Effect of RF on 0.1 dB Gain Flatness section for

more information.

•

Placing a small value resistor (RS) in series with the output

to isolate the load capacitor from the output stage of the

amplifier.

Figure 57 shows the effect of using a snub resistor (RS) on reducing

the peaking in the worst-case frequency response (gain of +1).

Using RS = 100 Ω reduces the peaking by 3 dB, with the trade-off

that the closed-loop gain is reduced by 0.9 dB due to attenuation

at the output. RS can be adjusted from 0 Ω to 100 Ω to maintain

an acceptable level of peaking and closed-loop gain, as shown in

Figure 57.

–10

–8

–6

–4

–2

0.1

100

FREQUENCY (MHz)

VS = 5V

VOUT = 200mV p-p

G = +1

RL = 1kΩ

CL = 6.8pF

RS = 0Ω

RS = 100Ω

50Ω

OUT

VIN

200mV

STEP

Figure 57. Closed-Loop Frequency Response with Snub Resistor, CL = 6.8 pF

Figure 58 shows that the transient response is also much improved

by the snub resistor (RS = 100 Ω) compared to that of Figure 56.

VS = 5V

G = +1

RL = 1kΩ

CL = 6.8pF

RS = 100Ω

50ns/DIV

50mV/DIV

100

–100

Figure 58. 200 mV Step Response, CL = 6.8 pF, RS = 100 Ω

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