–11–
REV. F
AD8009
10F
AD8009
HP8753D
49.9
301
49.9
+5V
–5V
301
3
10F
+
Z
OUT
= 50
Z
IN
= 50
+
0.001F
0.1F
0.001F
0.1F
2
7
4
6
WAVETEK 5201
BPF
TPC 37. AD8009 Driving a Band-Pass RF Filter
CENTER 50.000 MHz SPAN 80.000 MHz
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
REJECTION (dB)
AD8009
G = 2
R
F
= R
G
= 301
DRIVING
WAVETEK 5201
TUNABLE BPF
f
C
= 50MHz
TPC 38. Frequency Response of Band-Pass Filter Circuit
APPLICATIONS
All current feedback op amps are affected by stray capacitance
on their –INPUT. TPCs 35 and 36 illustrate the AD8009’s
response to such capacitance.
TPC 35 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/band-
width is shown in TPC 36.
As a practical consideration, the higher the capacitance on the
–INPUT to GND, the higher R
F
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.
TPC 37 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 measurement. The analyzer has
a 50 Ω source impedance that drives a cable that is terminated in
50 Ω at the high impedance noninverting input of the AD8009.
The AD8009 is set at a gain of +2. 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 TPC 38 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.
