OPA843 datasheet(12/32 Pages) TI2

12 / 32 page

OPA2846

SBOS274C −JUNE 2003 − REVISED AUGUST 2008

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND, NONINVERTING OPERATION

The OPA2846

provides

unique

combination

features—low input voltage noise along with a very low

distortion output stage—to give one of the highest dynamic

range dual op amps available. Its very high Gain

Bandwidth Product (GBP) can be used either to deliver

high signal bandwidths at high gains, or to deliver very low

distortion signals at moderate frequencies and lower

gains. To achieve the full performance of the OPA2846,

careful attention to printed circuit board (PCB) layout and

component selection is required, as discussed in the

remaining sections of this data sheet.

Figure 1 shows the noninverting gain of +10 circuit used as

the basis of the Electrical Characteristics and most of the

Typical Characteristics. Most of the curves were charac-

terized using signal sources with 50

Ω driving impedance,

and with measurement equipment presenting a 50

Ω load

impedance. In Figure 1, the 50

Ω 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 equip-

ment 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 503Ω

total feedback network load, presents the OPA2846 with

an effective output load of 83

Ω for the circuit of Figure 1.

1/2

O PA 2 846

+5V

−5V

−V

Ω

0.1

µF

6.8

µF

6.8

µF

Ω

453

Ω

Ω Source

Ω Load

0.1

µF

Figure 1. Noninverting, G = +10 Specification and

Test Circuit

Voltage-feedback op amps, unlike current-feedback

designs, can use a wide range of resistor values to set their

gains. The circuit of Figure 1, and the specifications at

other gains, uses the constraint that RG should always be

set to 50

Ω and RF adjusted to get the desired gain.

Observing this guideline will ensure that the thermal noise

contribution of the feedback network is insignificant

compared to the 1.2nV/

√Hz input voltage noise for the op

amp itself.

WIDEBAND, INVERTING GAIN OPERATION

Operating the OPA2846 as an inverting amplifier has

several benefits and is particularly appropriate when a

matched input impedance is required. Figure 2 shows the

inverting gain circuit used as the basis of the inverting

mode Typical Characteristics.

1/2

O P A 2846

+5V

−5V

−V

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

0.1

µF

Ω

Ω Source

Ω Load

Figure 2. Inverting, G = −20 Characterization

Circuit

Driving this circuit from a 50

Ω source, and constraining the

gain resistor (RG) to equal 50Ω, will give both a signal

bandwidth and noise advantage. RG acts as both the input

termination resistor and the gain setting resistor for the

circuit. Although the signal gain (VO/VI) for the circuit of

Figure 2 is double that for Figure 1, the noise gains are in

fact equal when the 50

Ω source resistor is included. This

has the interesting effect of doubling the equivalent GBP

of the amplifier. This can be seen in comparing the G = +10

and G = −20 small-signal frequency response curves. Both

show approximately 250MHz bandwidth, but the inverting

configuration of Figure 2 gives 6dB higher signal gain. If

the signal source is actually the low impedance output of

another amplifier, RG should be increased to the minimum

load resistance value allowed for that amplifier and RF

should be adjusted to achieve the desired gain. For stable

operation of the OPA2846, it is critical that this driving

amplifier show a very low output impedance at frequencies

beyond the expected closed-loop bandwidth for the

OPA2846.

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