www.ti.com

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

(3)

FSF x fc +

1

2

π 2 x R2R3C1C2

and Q +

2 x R2R3C1C2

R3C1 ) R2C1 ) KR3C1

(4)

FSF +

Re

2

and Q +

2

2Re

(5)

FSF x fc +

1

2

πRC 2 x mn

and Q +

2 x mn

1 ) m(1 ) K)

(6)

PRINCIPLES OF OPERATION

THEORY OF OPERATION

_

+

Differential Amplifier

_

+

_

+

THS412x

Fully Differential Amplifier

GND

Input voltage definition

V

ID +

V

I)

–

V

I–

V

IC +

V

I) )

V

I–

2

(7)

Output voltage definition

V

OD +

V

O)

–

V

O–

V

OC +

V

O) )

V

O–

2

(8)

Transfer function

V

OD +

V

ID

x A

f

(9)

Output common−mode voltage V

OC

(10)

THS4120

THS4121

SLOS319D – FEBRUARY 2001 – REVISED OCTOBER 2004

APPLICATION INFORMATION (continued)

The transfer function for this filter circuit is:

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.

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:

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.

The THS412x is a fully differential amplifier. Differential amplifiers are typically differential in/single out, whereas

fully differential amplifiers are differential in/differential out.

Figure 23. Differential Amplifier Versus a Fully Differential Amplifier

To understand the THS412x fully differential amplifiers, the definition for the pinouts of the amplifier are

provided.

13

Submit Documentation Feedback