ADA4891-1ARJZ-R7 datasheet(14/20 Pages) AD

14 / 20 page

ADA4891-1/ADA4891-2

Rev. A | Page 14 of 20

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, as is shown in Figure 45 and

Figure 46. Four methods that minimize the output capacitive

loading effect include:

•

Reducing the output resistive load. This pushes the pole

further away and, hence, improves the phase margin.

•

Increase the phase margin with higher noise gains. As

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

allows for large capacitor 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 output impedance of the amplifier.

Refer to the Effect of RF on 0.1 dB Gain Flatness section for

more details.

•

Putting a small value resistor, RS, in series with the output

to isolate the load capacitor from the output stage of

the amplifier.

–10

–8

–6

–4

–2

0.1

100

FREQUENCY (MHz)

VS = 5V

VOUT = 200mV p-p

G = +1

RL = 1kΩ

CL = 6.8pF

Figure 45. Closed-Loop Frequency Response, CL = 6.8 pF

50ns/DIV

50mV/DIV

VS = 5V

G = +1

RL = 1kΩ

CL = 6.8pF

100

–100

Figure 46. 200 mV Step Response, CL = 6.8 pF

Figure 47 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 tradeoff

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 48.

Figure 48 shows that the transient response is also much improved

by the snub resistor RS = 100 Ω, compared to that of Figure 46.

–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

Ω

VOUT

VIN

200mV

STEP

Figure 47. Capacitive Load Drive vs. Closed-Loop Gain

VS = 5V

G = +1

RL = 1kΩ

CL = 6.8pF

RS = 100Ω

50ns/DIV

50mV/DIV

100

–100

Figure 48. 200 mV Step Response, CL = 50 pF

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