OPA4820IPWR datasheet(13/38 Pages) TI2

13 / 38 page

OPA4820

SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND VOLTAGE-FEEDBACK

OPERATION

The combination of speed and dynamic range offered by

the OPA4820 is easily achieved in a wide variety of

application circuits, providing that simple principles of

good

design

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 printed circuit board (PCB) layout and careful

component selection will maximize the performance of the

OPA4820 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 50

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 804Ω

total feedback network load, presents the OPA4820 with

an effective load of approximately 90

Ω in Figure 1.

+5V

−5V

−V

Ω

2.2

µF

2.2

µF

0.1

µF

402

Ω

402

Ω

Ω Source

Ω Load

0.1

µF

1/4

OPA4820

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

and Characterization Circuit

WIDEBAND INVERTING OPERATION

Operating the OPA4820 as an inverting amplifier has

several benefits and is particularly useful when a matched

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.

+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

1/4

OPA4820

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 402

Ω 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 OPA4820 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 50Ω

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 OPA4820, an

inverting gain of −10 from a 50

Ω source to a 50Ω RG gives

42MHz bandwidth, whereas the noninverting gain of +10

gives 27MHz.

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