OPA830 datasheet(26/45 Pages) TI

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DIFFERENTIAL INTERFACE APPLICATIONS

=A =1+2 ´

(6)

750W

1/4

OPA4830

-VCC

1/4

OPA4830

750W

1/4

OPA4830

-VCC

1/4

OPA4830

= -

(7)

OPA4830

SBOS350A – DECEMBER 2006 – REVISED MAY 2008.................................................................................................................................................... www.ti.com

Various combinations of single-supply or ac-coupled

gains can also be delivered using the basic circuit of

Dual and quad op amps are particularly suitable to

Figure 80. Common-mode bias voltages on the two

differential input to differential output applications.

noninverting inputs pass on to the output with a gain

Typically, these op amps fall into either ADC input

of 1V/V because an equal dc voltage at each

interface or line driver applications. Two basic

inverting node creates no current through RG, giving

approaches to differential I/O are noninverting or

that voltage a common-mode gain of 1 to the output.

inverting

configurations.

Because

the

output

differential,

the

signal

polarity

somewhat

Figure 81 shows a differential I/O stage configured as

meaningless—the

noninverting

and

inverting

an inverting amplifier. In this case, the gain resistors

terminology applies here to where the input is brought

(RG) become the input resistance for the source. This

into the OPA4830. Each has its advantages and

configuration provides a better noise performance

disadvantages. Figure 80 shows a basic starting point

than the noninverting configuration, but does limit the

for noninverting differential I/O applications.

flexibility in setting the input impedance separately

from the gain.

This approach provides for a source termination

impedance that is independent of the signal gain. For

instance, simple differential filters may be included in

the signal path right up to the noninverting inputs

without interacting with the amplifier gain. The

differential signal gain for the circuit of Figure 80 is

shown in Equation 6:

Figure 80 shows the recommended value of 750

Ω.

However, the gain may be adjusted using just the RG

resistor.

Figure 81. Inverting Differential I/O Amplifier

The

two

noninverting

inputs

provide

easy

common-mode

control

input.

This

control

particularly useful if the source is ac-coupled through

either blocking caps or a transformer. In either case,

the

common-mode

input

voltages

the

two

noninverting inputs again have a gain of 1 to the

output pins, giving an easy common-mode control for

single-supply operation. The input resistors may be

adjusted to the desired gain but also change the input

impedance as well. The differential gain for this circuit

is shown in Equation 7:

Figure 80. Noninverting Differential I/O Amplifier

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