ADA4857-1/ADA4857-2

Data Sheet

Rev. D | Page 18 of 21

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.

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.

Figure 55. Low-Pass Filter Response

Figure 56. 4-Pole, Sallen-Key Low-Pass Filter (ADA4857-2)

–42

–39

–36

–33

–30

–27

–24

–21

–18

–15

–12

–9

–6

–3

0

3

6

9

12

15

0.1

1

10

100

500

FREQUENCY (MHz)

OUT1, f = 90MHz

OUT2, f = 90MHz

OUT1, f = 45MHz

OUT2, f = 45MHz

RL = 100Ω

VS = ±5V

U1

C1

3.9pF

C2

5.6pF

R

RT

49.9Ω

R

R1

348Ω

R

R2

348Ω

RT

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

OUT1