+

−

+

−

THS1050

THS413x

C3

C3

R4

R2

R4

+

C1

+

C1

R2

R3

R3

C2

R1

R1

Vs

H

d

(f) +

K

–

f

FSF x fc

2

)

1

Q

jf

FSF x fc )

1

x

Rt

2R4 ) Rt

1 )

j2

πfR4RtC3

2R4 ) Rt

Where K +

R2

R1

FSF x fc +

1

2

π 2 x R2R3C1C2

and Q +

2 x R2R3C1C2

R3C1 ) R2C1 ) KR3C1

FSF +

Re

2

and Q +

2

2Re

FSF x fc +

1

2

πRC 2 x mn

and Q +

2 x mn

1 ) m(1 ) K)

THS4130

THS4131

SLOS318H

– MAY 2000 – REVISED MAY 2011

www.ti.com

ACTIVE ANTIALIAS FILTERING

For signal conditioning in ADC applications, it is important to limit the input frequency to the ADC. Low-pass

filters can prevent the aliasing of the high-frequency noise with the frequency of operation. Figure 34 presents a

method by which the noise may be filtered in the THS413x.

Figure 34. Antialias Filtering

The transfer function for this filter circuit is:

(3)

(4)

K sets the pass band gain, fc is the cutoff frequency for the filter, FSF is a frequency scaling factor, and Q is the

quality factor.

(5)

where Re is the real part, and Im is the imaginary part of the complex pole pair. Setting R2 = R, R3 = mR, C1 =

C, and C2 = nC results in:

(6)

Start by determining the ratios, m and n, required for the gain and Q of the filter type being designed, then select

C and calculate R for the desired fc.

16

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© 2000–2011, Texas Instruments Incorporated