OPA2690 datasheet(14/33 Pages) TI

14 / 33 page

OPA2822

SBOS188E

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND NONINVERTING OPERATION

The OPA2822 provides a unique combination of features in

a wideband dual, unity-gain stable, voltage-feedback ampli-

fier to support the extremely high dynamic range require-

ments of emerging communications technologies. Combin-

ing low 2nV/

√Hz input voltage noise with harmonic distortion

performance that can exceed 100dBc SFDR through 2MHz,

the OPA2822 provides the highest dynamic range input

interface for emerging high speed 14-bit (and higher) con-

verters. To achieve this level of performance, careful atten-

tion to circuit design and board layout is required.

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

for the Electrical Characteristics table and most of the Typical

Characteristics at

±6V operation. While the characteristics are

given using split

±6V supplies, most of the electrical and typical

characteristics also apply to a single-supply +12V design where

the input and output operating voltages are centered at the

midpoint of the +12V supply. Operation at

±5V will very nearly

match that shown for the

±6V operating point. Most of the

reference curves were characterized using signal sources with

Ω driving impedance, and with measurement equipment

presenting a 50

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

resistor at the VI terminal matches the source impedance of the

test signal generator, while the 50

Ω series resistor at the V

terminal provides a matching resistor for the measurement

equipment load. Generally, data sheet voltage swing specifica-

tions are at the output pin (VO in Figure 1), while output power

(dBm) specifications are at the matched 50

Ω load. The total

100

Ω load at the output, combined with the total 804Ω total

feedback network load for the noninverting configuration of

Figure 1, presents the OPA2822 with an effective output load of

Ω. While this is a good load value for frequency response

measurements, distortion will improve rapidly with lighter output

loads. Keeping the same feedback network and increasing the

load to 200

Ω will result in a total load of 160Ω for the distortion

performance reported in the Electrical Characteristics table.

For higher gains, the feedback resistor (RF) was held at 402Ω

and the gain resistor (RG) adjusted to develop the Typical

Characteristics.

Voltage-feedback op amps, unlike current-feedback designs,

can use a wide range of resistor values to set their gains. A low-

noise part like the OPA2822 will deliver low total output noise

only if the resistor values are kept relatively low. For the circuit

of Figure 1, the resistors contribute an input-referred voltage

noise component of 1.8nV/

√Hz, which is approaching the value

of the amplifier’s intrinsic 2nV/

√Hz. For a more complete

description of the feedback network’s impact on noise, see the

Setting Resistor Values to Minimize Noise section later in this

data sheet. In general, the parallel combination of RF and RG

should be < 300

Ω to retain the low-noise performance of the

OPA2822. However, setting these values too low can impair

distortion performance due to output loading, as shown in the

distortion versus load data in the Typical Characteristics.

WIDEBAND INVERTING OPERATION

Operating the OPA2822 as an inverting amplifier has several

benefits and is particularly appropriate as part of the hybrid

design in an xDSL receiver application. Figure 2 shows the

inverting gain of –1 circuit used as the basis of the inverting

mode Typical Characteristics.

In the inverting case, only the RF element of the feedback

network 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 an effective load of 86

Ω in this

inverting configuration. Gain resistor RG is set to achieve the

desired inverting gain (in this case 604

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 = 54.9Ω in parallel with the 604Ω

gain setting resistor yields a matched input impedance of

Ω. R

M is needed only when the input must be matched to

a source impedance, as in the characterization testing done

using the circuit of Figure 2.

FIGURE 1. Noninverting G = +2 Specification and Test

Circuit.

FIGURE 2. Inverting G = –1 Specification and Test

Circuit.

1/2

OPA2822

+5V

–5V

–V

Ω

0.1

µF

6.8

µF

6.8

µF

402

Ω

402

Ω

Ω Source

Ω Load

0.1

µF

1/2

OPA2822

+5V

–5V

–V

Ω

0.1

µF

6.8

µF

6.8

µF

54.9

Ω

309

Ω

604

Ω

Ω Source

Ω Load

0.1

µF

0.1

µF

604

Ω

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