THS4130, THS4131

HIGHSPEED, LOW NOISE, FULLY DIFFERENTIAL I/O AMPLIFIERS

SLOS318E − MAY 2000 − REVISED JANUARY 2004

20

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

theory of operation (continued)

Fully differential amplifiers may be viewed as two inverting amplifiers. In this case, the equation of an inverting

amplifier holds true for gain calculations. One advantage of fully differential amplifiers is that they offer twice as

with opposite signs and feed them into the differential input nodes of the ADC. In practice, the designer has been

able to feed a 2-V peak-to-peak signal into a 1-V differential ADC with the help of a fully differential amplifier.

The final result indicates twice as much dynamic range. Figure 39 illustrates the increase in dynamic range. The

gain factor should be considered in this scenario. The THS413x fully differential amplifier offers an improved

CMRR and PSRR due to its symmetrical input and output. Furthermore, second harmonic distortion is

improved. Second harmonics tend to cancel because of the symmetrical output.

VOCM

_

+

_

+

VCC+

VIN−

VIN+

VO+

VO−

VOD= 1−0 = 1

VOD = 0−1 = −1

a

b

+1

0

+1

0

VCC−

Similar to the standard inverting amplifier configuration, input impedance of a fully differential amplifier is

implement the differential amplifier as an instrumentation amplifier. This configuration improves the input

impedance of the fully differential amplifier. The following schematic depicts the general format of

instrumentation amplifiers.

The general transfer function for this circuit is:

V

OD

V

IN1

–V

IN2

+

R

f

R

(g)

1

) 2R2

R1

_

+

_

+

_

+

THS4012

THS4012

VIN1

VIN2

R2

R1

R2

R(g)

R(g)

Rf

Rf

THS413x

Figure 40. Instrumentation Amplifier