Datasheet

ADA4817-1/ADA4817-2 Data Sheet
Rev. B | Page 22 of 28
ACTIVE LOW-PASS FILTER (LPF)
Active filters are used in many applications such as antialiasing
filters and high frequency communication IF strips.
With a 410 MHz gain bandwidth product and high slew rate,
the ADA4817-1/ADA4817-2 is an ideal candidate for active
filters. Moreover, thanks to the low input bias current provided
by the FET stage, the ADA4817-1/ADA4817-2 eliminate any dc
errors. Figure 54 shows the frequency response of 90 MHz and
45 MHz LPFs. In addition to the bandwidth requirements, the slew
rate must be capable of supporting the full power bandwidth of the
filter. In this case, a 90 MHz bandwidth with a 2 V p-p output
swing requires at least 870 V/μs. This performance is achievable
at 90 MHz only because of the wide bandwidth and high slew
rate of the ADA4817-1/ADA4817-2.
The circuit shown in Figure 55 is a 4-pole, Sallen-Key, low-pass
filter (LPF). The filter comprises two identical cascaded Sallen-
Key LPF sections, each with a fixed gain of G = 2. The net gain
of the filter is equal to G = 4 or 12 dB. The actual gain shown in
Figure 54 is 12 dB. This does not take into account the output
voltage being divided in half by the series matching termination
resistor, R
T
, and the load resistor.
Setting the resistors equal to each other greatly simplifies the
design equations for the Sallen-Key filter. To achieve 90 MHz
the value of R should be set to 182 Ω. However, if the value of R
is doubled, the corner frequency is cut in half to 45 MHz. This
would be an easy way to tune the filter by simply multiplying
the value of R (182 Ω) by the ratio of 90 MHz and the new
corner frequency in megahertz. Figure 54 shows the output of
each stage of the filter and the two different filters corresponding
to R = 182 Ω and R = 365 Ω. It is not recommended to increase
the corner frequency beyond 90 MHz due to bandwidth and
slew rate limitations unless unity-gain stages are acceptable.
Resistor values are kept low for minimal noise contribution,
offset voltage, and optimal frequency response. Due to the low
capacitance values used in the filter circuit, the PCB layout and
minimization of parasitics is critical. A few picofarads can detune
the corner frequency, f
c
, of the filter. The capacitor values shown
in Figure 55 actually incorporate some stray PCB capacitance.
Capacitor selection is critical for optimal filter performance.
Capacitors with low temperature coefficients, such as NPO
ceramic capacitors and silver mica, are good choices for filter
elements.
15
–42
100k 1G
FREQUENCY (Hz)
MAGNITUDE (dB)
12
9
6
3
0
–3
–6
–9
–12
–15
–18
–21
–24
–27
–30
–33
–36
–39
1M 10M 100M
07756-062
OUT2, f = 90MHz
OUT1, f = 90MHz
OUT1, f = 45MHz
OUT2, f = 45MHz
Figure 54. Low-Pass Filter Response
U1
C1
3.9pF
C2
5.6pF
R
R
T
49.9Ω
R
R1
348Ω
R
R2
348Ω
R
T
49.9Ω
+IN1
–5V
+5V
0.1µF
0.1µF
10µF
10µF
U2
C3
3.9pF
C4
5.6pF
R
R3
348Ω
R4
348Ω
–5V
+5V
0.1µF
0.1µF
10µF
10µF
OUT2
07756-054
OUT1
Figure 55. 4-Pole Sallen-Key Low-Pass Filter (ADA4817-2)