ADA4857-1YCPZ-RL Просмотр технического описания (PDF) - Analog Devices
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ADA4857-1YCPZ-RL Datasheet PDF : 21 Pages
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ADA4857-1/ADA4857-2
Data Sheet
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
ADA4857-2 is an ideal candidate for active filters. Figure 55
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 2800 V/μs.
The circuit shown in Figure 56 is a 4-pole, Sallen-Key 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 55 is 12 dB.
This does not take into account the output voltage being divided in
half by the series matching termination resistor, RT, 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 must 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.
R
+IN1
RT
49.9Ω
C1
3.9pF
+5V 10µF
R
C2
5.6pF
U1
0.1µF
10µF
–5V
R2
348Ω
0.1µF
R1
348Ω
R
OUT1
Figure 55 shows the output of each stage is of the filter and the
two different filters corresponding to R = 182 Ω and R = 365 Ω.
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, fc of the filter. The capacitor values shown
in Figure 56 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
12
9
6
3
0
–3
–6
–9
–12
–15
–18
–21
–24
–27
–30
–33
–36
–39
RL = 100Ω
VS = ±5V
–42
0.1
1
OUT1, f = 90MHz
OUT1, f = 45MHz
OUT2, f = 90MHz
OUT2, f = 45MHz
10
100
500
FREQUENCY (MHz)
Figure 55. Low-Pass Filter Response
C3
3.9pF
+5V 10µF
R
C4
5.6pF
U2
0.1µF
10µF
–5V
R4
348Ω
0.1µF
R3
348Ω
RT
49.9Ω
OUT2
Figure 56. 4-Pole, Sallen-Key Low-Pass Filter (ADA4857-2)
Rev. D | Page 18 of 21
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