ADA4817-2ACPZ-R2 datasheet(22/25 Pages) AD

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ADA4817-1/ADA4817-2

Data Sheet

Rev. C | Page 22 of 25

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, 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,

set the R value to 182 Ω. However, if the R value is doubled, the

corner frequency is cut in half to 45 MHz, which is an easy way

to tune the filter by simply multiplying the R value (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, fc, 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.

–42

100k

FREQUENCY (Hz)

–3

–6

–9

–12

–15

–18

–21

–24

–27

–30

–33

–36

–39

10M

100M

OUT2, f = 90MHz

OUT1, f = 90MHz

OUT1, f = 45MHz

OUT2, f = 45MHz

Figure 54. Low-Pass Filter Response

3.9pF

5.6pF

49.9Ω

348Ω

49.9Ω

+IN1

–5V

+5V

0.1µF

10µF

3.9pF

5.6pF

348Ω

–5V

+5V

0.1µF

10µF

OUT2

OUT1

Figure 55. 4-Pole Sallen-Key Low-Pass Filter (ADA4817-2)

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