AD8009
8
7
6
5
12
9
CA = 2pF
3 dB/div
HP8753D
6
Z
= 50⍀
Z
= 50⍀
CA = 1pF
1 dB/div
IN
OUT
3
+5V
4
3
0
CA = 0pF
1 dB/div
+
0.1F
0.001F
10F
–3
7
2
–6
2
3
V
OUT = 200mV p–p
VOUT
100⍀
AD8009
49.9⍀
WAVETEK 5201
BPF
VIN
1
–9
50⍀
6
49.9⍀
4
499⍀
–12
0
301⍀
CA
499⍀
–15
–1
301⍀
–5V
10F
0.1F
0.001F
1000
+
1
10
100
FREQUENCY – MHz
Figure 34. Small Signal Frequency Response vs. Parasitic
Capacitance
Figure 36. AD8009 Driving a Bandpass RF Filter
0
V
AD8009
IN
V
OUT
C
= 2pF
A
G = 2
R = R = 301⍀
F
DRIVING
WAVETEK 5201
TUNABLE BPF
–10
–20
–30
–40
–50
–60
–70
–80
–90
50⍀
100⍀
499⍀
499⍀
G
C
A
C
A
= 1pF
V
V
= 200mV p–p
OUT
= ؎5V
S
f
= 50MHz
C
C
A
= 0pF
1.5ns
40mV
CENTER 50.000 MHz
SPAN 80.000 MHz
Figure 35. Small Signal Pulse Response vs. Parasitic
Capacitance
Figure 37. Frequency Response of Bandpass Filter Circuit
APPLICATIONS
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.
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.
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.
As a practical consideration, the higher the capacitance on the
–INPUT to GND, the higher RF needs to be to minimize
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.
REV. B
–9–
ADI [ ADI ]
