AD712JRZ datasheet(17/20 Pages) AD

17 / 20 page

AD712

Rev. G | Page 17 of 20

FILTERS

ACTIVE FILTER APPLICATIONS

In active filter applications using op amps, the dc accuracy of

the amplifier is critical to optimal filter performance. The

amplifier offset voltage and bias current contribute to output

error. Offset voltage is passed by the filter and can be amplified

to produce excessive output offset. For low frequency

applications requiring large value input resistors, bias currents

flowing through these resistors also generate an offset voltage.

In addition, at higher frequencies, the op amp dynamics must

be carefully considered. Here, slew rate, bandwidth, and open-

loop gain play a major role in op amp selection. The slew rate

must be fast as well as symmetrical to minimize distortion. The

amplifier bandwidth in conjunction with the filter gain dictates

the frequency response of the filter.

The use of a high performance amplifier such as the AD712

minimizes both dc and ac errors in all active filter applications.

SECOND-ORDER LOW-PASS FILTER

Figure 48 depicts the AD712 configured as a second-order,

Butterworth low-pass filter. With the values as shown, the

corner frequency is 20 kHz; however, the wide bandwidth of the

AD712 permits a corner frequency as high as several hundred

kilohertz. Equations for component selection are

as follows:

R1 = R2 = A user selected value (10 kΩ to 100 kΩ, typical)

C1 (in farads) = ()

()()1

414

cutoff

()

()()1

707

cutoff

+15V

1/2

AD712

0.1µF

–15V

VOUT

VIN

560pF

20kΩ

280pF

–

Figure 48. Second-Order Low-Pass Filter

An important property of filters is their out-of-band rejection.

The simple 20 kHz low-pass filter shown in Figure 48, can be

used to condition a signal contaminated with clock pulses or

sampling glitches that have considerable energy content at high

frequencies.

The low output impedance and high bandwidth of the AD712

minimize high frequency feedthrough as shown in Figure 49.

The upper trace is that of another low cost BiFET op amp

showing 17 dB more feedthrough at 5 MHz.

REF 20.0 dBm

10dB/DIV

RANGE 15.0dBm

OFFSET .0 Hz

0dB

CENTER 5 000 000.0Hz

RBW 30kHz

SPAN 10 000 000.0Hz

ST .8 SEC

VBW 30kHz

TYPICAL BIFET

AD712

Figure 49. High Frequency Feedthrough

AD712JRZ датащи(PDF) 17 Page - Analog Devices

AD712JRZ датащи(HTML) 17 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

датащи скачать

ссылки URL