Data Sheet

AD8057/AD8058

APPLICATIONS INFORMATION

DRIVING CAPACITIVE LOADS

When driving a capacitive load, most op amps exhibit overshoot in

their pulse response. Figure 43 shows the relationship between

the capacitive load that results in 30% overshoot and the closed-

loop gain of an AD8058. It can be seen that, under the gain = +2

condition, the device is stable with capacitive loads of up to 69 pF.

In general, to minimize peaking or to ensure device stability for

larger values of capacitive loads, a small series resistor (RS) can

be added between the op amp output and the load capacitor

(CL) as shown in Figure 44.

For the setup shown in Figure 44, the relationship between RS

and CL was empirically derived and is shown in Table 4.

CLOSED-LOOP GAIN

500

400

0

5

1

2

3

4

300

200

100

RS = 2.4Ω

RS = 0Ω

Figure 43. Capacitive Load Drive vs. Closed-Loop Gain

–2.5V

RG

RF

RS

CL

VIN = 200mV p-p

AD8058

0.1µF

10µF

0.1µF

10µF

+2.5V

VOUT

FET PROBE

Figure 44. Capacitive Load Drive Circuit

Table 4. Recommended Value for Resistors RS, RF, RG vs.

Capacitive Load, CL, Which Results in 30% Overshoot

Gain

RF

RG

CL (RS = 0 Ω)

CL (RS = 2.4 Ω)

1

100 Ω

11 pF

13 pF

2

100 Ω

100 Ω

51 pF

69 pF

3

100 Ω

50 Ω

104 pF

153 pF

4

100 Ω

33.2 Ω

186 pF

270 pF

5

100 Ω

25 Ω

245 pF

500 pF

10

100 Ω

11 Ω

870 pF

1580 pF

50ns/DIV

100mV

–100mV

200mV

–200mV

+OVERSHOOT

29.0%

100mV/DIV

Figure 45. Typical Pulse Response with CL = 65 pF, Gain = +2, and VS = ±2.5

VIDEO FILTER

Some composite video signals that are derived from a digital

source contain some clock feedthrough that can cause problems

with downstream circuitry. This clock feedthrough is usually at

27 MHz, which is a standard clock frequency for both NTSC

and PAL video systems. A filter that passes the video band and

rejects frequencies at 27 MHz can be used to remove these fre-

quencies from the video signal.

Figure 46 shows a circuit that uses an AD8057 to create a single

5 V supply, 3-pole Sallen-Key filter. This circuit uses a single RC

pole in front of a standard 2-pole active section. To shift the dc

operating point to midsupply, ac coupling is provided by R4, R5,

and C4.

2

3

0.1µF

+

10µF

AD8057

7

4

6

+5V

+5V

R4

10kΩ

R5

10kΩ

C4

0.1µF

R3

49.9Ω

R2

499Ω

C1

100pF

R1

200Ω

RF

1kΩ

C2

680pF

C3

36pF

Figure 46. Low-Pass Filter for Video

Rev. E | Page 13 of 16