AD8009

–9–

REV. B

FREQUENCY – MHz

10

1000

100

8

7

6

5

4

–1

3

2

1

0

12

9

6

3

0

–15

–12

–9

–6

–3

50

V

IN

499

V

V

OUT

499

100

1 dB/div

1 dB/div

3 dB/div

1

Figure 34. Small Signal Frequency Response vs. Parasitic

Capacitance

1.5ns

40mV

5V

499

100

50

V

OUT

V

IN

499

Figure 35. Small Signal Pulse Response vs. Parasitic

Capacitance

10 F

AD8009

HP8753D

49.9

301

49.9

+5V

–5V

301

2

10 F

+

+

0.001 F

0.1 F

0.001 F

0.1 F

3

7

4

6

WAVETEK 5201

BPF

Figure 36. AD8009 Driving a Bandpass RF Filter

CENTER 50.000 MHz

SPAN 80.000 MHz

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

AD8009

G = 2

DRIVING

WAVETEK 5201

TUNABLE BPF

Figure 37. Frequency Response of Bandpass Filter Circuit

APPLICATIONS

All current feedback op amps are affected by stray capacitance

on their –INPUT. Figures 34 and 35 illustrate the AD8009’s

response to such capacitance.

Figure 34 shows the bandwidth can be extended by placing a

capacitor in parallel with the gain resistor. The small signal pulse

response corresponding to such an increase in capacitance/

bandwidth is shown in Figure 35.

As a practical consideration, the higher the capacitance on the

peaking/ringing.

RF Filter Driver

The output drive capability, wide bandwidth and low distortion

of the AD8009 are well suited for creating gain blocks that can

drive RF filters. Many of these filters require that the input be

driven by a 50

Ω source, while the output must be terminated in

50

Ω for the filters to exhibit their specified frequency response.

Figure 36 shows a circuit for driving and measuring the

frequency response of a filter, a Wavetek 5201 Tunable Band

Pass Filter that is tuned to a 50 MHz center frequency. The

HP8753D network provides a stimulus signal for the measure-

ment. The analyzer has a 50

Ω source impedance that drives a

cable that is terminated in 50

Ω at the high impedance nonin-

verting input of the AD8009.

The AD8009 is set at a gain of two. The series 50

Ω resistor at

the output, along with the 50

Ω termination provided by the

filter and its termination, yield an overall unity gain for the

measured path. The frequency response plot of Figure 37

shows the circuit to have an insertion loss of 1.3 dB in the pass

band and about 75 dB rejection in the stop band.