THS4140, THS4141

HIGHSPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

SLOS320E − MAY 2000 − REVISED JANUARY 2004

14

POST OFFICE BOX 655303

APPLICATION INFORMATION

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. The following figure

presents a method by which the noise may be filtered in the THS414x.

VIN−

VIN+

+

−

+

−

VOCM

VOCM

VIN−

VIN+

VCC−

THS1050

THS414x

C3

C3

R4

R(t)

R2

R4

+

C1

+

VCC

C1

R2

R3

R3

C2

R1

R1

Vs

VIC

Figure 26. Antialias Filtering

The transfer function for this filter circuit is:

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

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.

FSF

+

) |Im|

2

and Q

+

) |Im|

2

2Re

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:

FSF x fc

+

1

2

πRC 2 x mn

and Q

+

2xmn

1

) m(1 ) K)

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.