THS4140, THS4141

HIGHSPEED FULLY DIFFERENTIAL I/O AMPLIFIERS

SLOS320E − MAY 2000 − REVISED JANUARY 2004

16

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

theory of operation (continued)

The following schematics depict the differences between the operation of the THS414x, fully differential

amplifier, in two different modes. Fully differential amplifiers can work with differential input or can be

implemented as single in/differential out.

−

Rf

R(g)

+

+

−

VCC−

VCC+

R(g)

Rf

Vs

VIN−

VIN+

VO+

VO−

VOCM

Note: For proper operation, maintain

symmetry by setting

Rf1 = Rf2 = Rf and R(g)1 = R(g)2 = R(g)

⇒ A = Rf/R(g)

Figure 29. Amplifying Differential Signals

−

Rf

R(g)

+

+

−

VCC−

VCC+

R(g)

Rf

Vs

VIN−

VIN+

VO+

VO−

VOCM

GAIN

R(g) Ω

Rf Ω

1

2

5

10

390

374

402

402

390

750

2010

4020

RECOMMENDED RESISTOR VALUES

Figure 30. Single In With Differential Out

If each output is measured independently, each output is one-half of the input signal when gain is 1. The

following equations express the transfer function for each output:

V

O +

1

2

V

I

The second output is equal and opposite in sign:

V

O +

– 1

2

V

I

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 31 illustrates the increase in dynamic range. The

gain factor should be considered in this scenario. The THS414x 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.