ADA4817-1ACPZ-R7 datasheet(27/29 Pages) AD

27 / 29 page

Data Sheet

ADA4817-1/ADA4817-2

APPLICATIONS INFORMATION

analog.com

Rev. H | 27 of 29

ACTIVE LOW-PASS FILTER (LPF)

Active low-pass filters are used in many applications such as

antialiasing filters and high frequency communication intermediate

frequency (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 67 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 68 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 67 is 12 dB. This

gain 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 a straightforward

approach to tune the filter by multiplying the R value (182 Ω) by

the ratio of 90 MHz and the new corner frequency in megahertz.

Figure 67 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 capaci-

tance values used in the filter circuit, the PCB layout and minimiza-

tion of parasitics is critical. A few picofarads can detune the corner

frequency, fC, of the filter. The capacitor values shown in Figure 68

actually incorporate some stray PCB capacitance.

Capacitor selection is critical for optimal filter performance. Capaci-

tors with low temperature coefficients, such as NPO ceramic capac-

itors and silver mica, are good choices for filter elements.

Figure 67. Low-Pass Filter Response

Figure 68. 4-Pole, Sallen-Key LPF (ADA4817-2)

ADA4817-1ACPZ-R7 数据表(PDF) 27 Page - Analog Devices

ADA4817-1ACPZ-R7 数据表(HTML) 27 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

数据表下载