AD8045

Rev. A | Page 20 of 24

FREQUENCY (MHz)

0

5

10

15

20

25

30

0

–20

–40

–60

–80

–100

–120

SNR = 69.9dBc

SFDR = 65.3dBc

Shown in the First Nyquist Zone

90 MHZ ACTIVE LOW-PASS FILTER (LPF)

Active filters are used in many applications such as antialiasing

filters and high frequency communication IF strips.

With a 400 MHz gain bandwidth product and high slew rate,

the AD8045 is an ideal candidate for active filters. Figure 70

shows the frequency response of the 90 MHz LPF. 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

1200 V/µs. This performance is achievable only at 90 MHz

because of the AD8045’s wide bandwidth and high slew rate.

The circuit shown in Figure 73 is a 90 MHz, 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 70 is only 6 dB. This is due to the output voltage being

and the load resistor.

Setting the resistors and capacitors equal to each other greatly

simplifies the design equations for the Sallen-Key filter. The

corner frequency, or −3 dB frequency, can be described by the

equation

RC

2π

1

=

The quality factor, or Q, is shown in the equation

K

Q

−

=

3

1

The gain, or K, of the circuits are

1

1,

+

=

+

=

R7

R8

K

Stage

Second

R4

R3

K

Stage

First

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

Figure 73 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.

FREQUENCY (MHz)

0.1

1

10

100

1000

–40

–30

–20

0

–10

10

–50

–60

–70

–80

–90

20

Figure 70. 90 MHz Low-Pass Filter Response