OPA820ID datasheet(13/32 Pages) TI1

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OPA820

SBOS303C − JUNE 2004 − REVISED AUGUST 2008

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND VOLTAGE-FEEDBACK

OPERATION

The combination of speed and dynamic range offered by the

OPA820 is easily achieved in a wide variety of application

circuits, providing that simple principles of good design

practice are observed. For example, good power-supply

decoupling, as shown in Figure 1, is essential to achieve the

lowest possible harmonic distortion and smooth frequency

response.

Proper PC board layout and careful component selection will

maximize the performance of the OPA820 in all applications,

as discussed in the following sections of this data sheet.

Figure 1 shows the gain of +2 configuration used as the basis

for most of the typical characteristics. Most of the curves were

characterized using signal sources with 50

Ω driving

impedance and with measurement equipment presenting

Ω load impedance. In Figure 1, the 50Ω shunt resistor at the

VI terminal matches the source impedance of the test

generator while the 50

Ω series resistor at the VO terminal

provides a matching resistor for the measurement equipment

load. Generally, data sheet specifications refer to the voltage

swings at the output pin (VO in Figure 1). The 100Ω load,

combined with the 804

Ω total feedback network load,

presents the OPA820 with an effective load of approximately

Ω in Figure 1.

OPA820

+5V

−5V

−V

Ω

2.2

µF

2.2

µF

0.1

µF

402

Ω

402

Ω

Ω Source

Ω Load

0.1

µF

Figure 1. Gain of +2, High-Frequency Application

and Characterization Circuit

WIDEBAND INVERTING OPERATION

Operating the OPA820 as an inverting amplifier has several

benefits and is particularly useful when a matched 50

Ω source

and input impedance is required. Figure 2 shows the inverting

gain of −1 circuit used as the basis of the inverting mode

typical characteristics.

OPA820

+5V

−5V

Ω

0.1

µF

2.2

µF

0.1

µF

2.2

µF

57.6

Ω

205

Ω

402

Ω

Ω Source

Ω Load

0.01

µF

402

Ω

Figure 2. Inverting G = −1 Specifications and Test

Circuit

In the inverting case, just the feedback resistor appears as

part of the total output load in parallel with the actual load. For

the 100

Ω load used in the typical characteristics, this gives a

total load of 80

Ω in this inverting configuration. The gain

resistor is set to get the desired gain (in this case 402

Ω for a

gain of −1) while an additional input matching resistor (RM) can

be used to set the total input impedance equal to the source

if desired. In this case, RM = 57.6Ω in parallel with the 402Ω

gain setting resistor gives a matched input impedance of 50

Ω.

This matching is only needed when the input needs to be

matched to a source impedance, as in the characterization

testing done using the circuit of Figure 2.

The OPA820 offers extremely good DC accuracy as well as

low noise and distortion. To take full advantage of that DC

precision, the total DC impedance looking out of each of the

input nodes must be matched to get bias current cancellation.

For the circuit of Figure 2, this requires the 205

Ω resistor

shown to ground on the noninverting input. The calculation for

this resistor includes a DC-coupled 50

Ω source impedance

along with RG and RM. Although this resistor will provide

cancellation for the bias current, it must be well decoupled

(0.01

µF in Figure 2) to filter the noise contribution of the

resistor and the input current noise.

As the required RG resistor approaches 50Ω at higher gains,

the bandwidth for the circuit in Figure 2 will far exceed the

bandwidth at that same gain magnitude for the noninverting

circuit of Figure 1. This occurs due to the lower noise gain for

the circuit of Figure 2 when the 50

Ω source impedance is

included in the analysis. For instance, at a signal gain of −10

(RG = 50Ω, RM = open, RF = 499Ω) the noise gain for the

circuit of Figure 2 will be 1 + 499

Ω/(50Ω + 50Ω) = 6 as a result

of adding the 50

Ω source in the noise gain equation. This

gives considerable higher bandwidth than the noninverting

gain of +10. Using the 240MHz gain bandwidth product for the

OPA820, an inverting gain of −10 from a 50

Ω source to a 50Ω

RG gives 55MHz bandwidth, whereas the noninverting gain of

+10 gives 30MHz.

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