OPA4820ID Datasheet, PDF(15/34 Page) Texas Instruments – Quad, Unity-Gain Stable, Low-Noise, Voltage-Feedback Operational Amplifier

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OPA4820ID Datasheet, PDF (15/34 Pages) Texas Instruments – Quad, Unity-Gain Stable, Low-Noise, Voltage-Feedback Operational Amplifier

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OPA4820

www.ti.com

DIFFERENTIAL INTERFACE APPLICATIONS

Dual and quad op amps are particularly suitable to

differential input to differential output applications.

Typically, these fall into either ADC input interface or line

driver applications. Two basic approaches to differential

I/O are noninverting or inverting configurations. Since the

output is differential, the signal polarity is somewhat

meaninglessâthe noninverting and inverting terminology

applies here to where the input is brought into the

OPA4820. Each has its advantages and disadvantages.

Figure 5 shows a basic starting point for noninverting

differential I/O applications.

+VCC

SBOS317D â SEPTEMBER 2004 â REVISED AUGUST 2008

Figure 6 shows a differential I/O stage configured as an

inverting amplifier. In this case, the gain resistors (RG)

become the input resistance for the source. This provides

a better noise performance than the noninverting

configuration, but does limit the flexibility in setting the

input impedance separately from the gain.

+VCC

VCM

1/4

OPA4820

1/4

OPA4820

402â¦

1/4

OPA4820

402â¦

âVCC

Figure 5. Noninverting Differential I/O Amplifier

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 5 is:

)

(1)

Figure 5 shows the recommended value of 402â¦.

However, the gain may be adjusted using just the RG

resistor.

Various combinations of single-supply or AC-coupled

gains can also be delivered using the basic circuit of

Figure 5. Common-mode bias voltages on the two

noninverting inputs pass on to the output with a gain of 1

since an equal DC voltage at each inverting node creates

no current through RG, giving that voltage a common-

mode gain of 1 to the output.

1/4

OPA4820

VCM

â VCC

Figure 6. Inverting Differential I/O Amplifier

The two noninverting inputs provide an easy

common-mode control input. This is particularly useful if

the source is AC-coupled through either blocking caps or

a transformer. In either case, the common-mode input

voltages on 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 will also be

changing the input impedance as well. The differential gain

for this circuit is:

VO + * RF

(2)

▷