OPA656UB datasheet(10/25 Pages) NSC

10 / 25 page

OPA656

SBOS196G

www.ti.com

APPLICATIONS INFORMATION

WIDEBAND, NONINVERTING OPERATION

The OPA656 provides a unique combination of a broadband,

unity gain stable, voltage-feedback amplifier with the DC

precision of a trimmed JFET-input stage. Its very high Gain

Bandwidth Product (GBP) of 230MHz can be used to either

deliver high signal bandwidths for low-gain buffers, or to

deliver broadband, low-noise transimpedance bandwidth to

photodiode-detector applications. To achieve the full perfor-

mance of the OPA656, 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 +2 circuit used as the

basis for most of the Typical Characteristics. Most of the curves

were characterized using signal sources with 50

Ω driving im-

pedance, and with measurement equipment presenting a 50

Ω

load impedance. In Figure 1, the 50

Ω shunt resistor at the V

terminal matches the source impedance of the test generator,

while the 50

Ω series resistor at the V

O terminal provides a

matching resistor for the measurement equipment load. Gener-

ally, data sheet voltage swing specifications are at the output pin

(VO in Figure 1) while output power specifications are at the

matched 50

Ω load. The total 100Ω load at the output combined

with the 500

Ω total feedback network load, presents the OPA656

with an effective output load of 83

Ω for the circuit of Figure 1.

WIDEBAND, INVERTING GAIN OPERATION

The circuit of Figure 2 shows the inverting gain of –1 test

circuit used for most of the inverting Typical Characteristics.

In this case, an additional resistor RM is used to achieve the

Ω input impedance required by the test equipment using in

characterization. This input impedance matching is optional

in a circuit board environment where the OPA656 is used as

an inverting amplifier at the output of a prior stage.

FIGURE 1. Noninverting G = +2 Specifications and Test

Circuit.

Voltage-feedback op amps, unlike current feedback prod-

ucts, can use a wide range of resistor values to set their gain.

To retain a controlled frequency response for the noninverting

voltage amplifier of Figure 1, the parallel combination of

RF || RG should always < 200Ω. In the noninverting configu-

ration, the parallel combination of RF || RG will form a pole

with the parasitic input capacitance at the inverting node of

the OPA656 (including layout parasitics). For best perfor-

mance, this pole should be at a frequency greater than the

closed loop bandwidth for the OPA656. For this reason, a

direct short from output to inverting input is recommended for

the unity gain follower application.

In this configuration, the output sees the feedback resistor as

an additional load in parallel with the 100

Ω load used for test.

It is often useful to increase the RF value to decrease the

loading on the output (improving harmonic distortion) with the

constraint that the parallel combination of RF || RG < 200Ω.

For higher inverting gains with the DC precision provided by

the FET input OPA656, consider the higher gain bandwidth

product OPA657.

Figure 2 also shows the noninverting input tied directly to

ground. Often, a bias current canceling resistor to ground is

included here to null out the DC errors caused by input bias

current effects. This is only useful when the input bias

currents are matched. For a JFET part like the OPA656, the

input bias currents do not match but are so low to begin with

(< 5pA) that DC errors due to input bias currents are

negligible. Hence, no resistor is recommended at the

noninverting inputs for the inverting signal path condition.

WIDEBAND, HIGH SENSITIVITY, TRANSIMPEDANCE

DESIGN

The high GBP and low input voltage and current noise for the

OPA656 make it an ideal wideband transimpedance ampli-

fier for low to moderate transimpedance gains. Higher

transimpedance gains (> 100k

Ω) will benefit from the low

input noise current of a FET input op amp such as the

OPA656. One transimpedance design example is shown on

the front page of the data sheet. Designs that require high

bandwidth from a large area detector will benefit from the low

input voltage noise for the OPA656. This input voltage noise

FIGURE 2. Inverting G = –1 Specifications and Test Circuit.

OPA656

+5V

–5V

–V

Ω

0.1

µF

6.8

µF

6.8

µF

250

Ω

250

Ω

Ω Source

Ω Load

0.1

µF

OPA656

+5V

–5V

–V

57.6

Ω

6.8

µF

0.1

µF

6.8

µF

0.1

µF

402

Ω

402

Ω

Ω Source

Ω Load

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