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OPA2650

APPLICATIONS INFORMATION

DISCUSSION OF PERFORMANCE

The OPA2650 is a dual low power, wideband voltage feed-

back operational amplifier. Each channel is internally com-

pensated to provide unity gain stability. The OPA2650’s

voltage feedback architecture features true differential and

fully symmetrical inputs. This minimizes offset errors, mak-

ing the OPA2650 well suited for implementing filter and

instrumentation designs. As a dual operational amplifier,

OPA2650 is an ideal choice for designs requiring multiple

channels where reduction of board space, power dissipation

and cost are critical. Its AC performance is optimized to

provide a gain bandwidth product of 160MHz and a fast 0.1%

settling time of 11ns, which is an important consideration in

high speed data conversion applications. Along with its

excellent settling characteristics, the low DC input offset of

±1mV and drift of ±3µV/°C support high accuracy require-

ments. In applications requiring a higher slew rate and wider

bandwidth, such as video and high bit rate digital communi-

cations, consider the dual current feedback OPA2658.

CIRCUIT LAYOUT AND BASIC OPERATION

Achieving optimum performance with a high frequency am-

plifier like the OPA2650 requires careful attention to layout

parasitics and selection of external components. Recommen-

dations for PC board layout and component selection include:

a) Minimize parasitic capacitance to any ac ground for all

of the signal I/O pins. Parasitic capacitance on the output

and inverting input pins can cause instability; on the non-

inverting input it can react with the source impedance to

cause unintentional bandlimiting. To reduce unwanted ca-

pacitance, a window around the signal I/O pins should be

opened in all of the ground and power planes. Otherwise,

ground and power planes should be unbroken elsewhere on

the board.

b) Minimize the distance (< 0.25") from the two power pins

to high frequency 0.1

µF decoupling capacitors. At the pins,

the ground and power plane layout should not be in close

proximity to the signal I/O pins. Avoid narrow power and

ground traces to minimize inductance between the pins and

the decoupling capacitors. Larger (2.2

µF to 6.8µF) decoupling

capacitors, effective at lower frequencies, should also be

used. These may be placed somewhat farther from the

device and may be shared among several devices in the same

area of the PC board.

c) Careful selection and placement of external compo-

nents will preserve the high frequency performance of the

OPA2650. Resistors should be a very low reactance type.

Surface mount resistors work best and allow a tighter overall

layout. Metal film or carbon composition axially-leaded

resistors can also provide good high frequency performance.

Again, keep their leads as short as possible. Never use

wirewound type resistors in a high frequency application.

Since the output pin and the inverting input pin are most

sensitive to parasitic capacitance, always position the feed-

back and series output resistor, if any, as close as possible to

the package pins. Other network components, such as non-

inverting input termination resistors, should also be placed

close to the package.

Even with a low parasitic capacitance shunting the resistor,

excessively high resistor values can create significant time

constants and degrade performance. Good metal film or

surface mount resistors have approximately 0.2pF in shunt

with the resistor. For resistor values > 1.5k

Ω, this adds a

pole and/or zero below 500MHz that can affect circuit

HARMONIC DISTORTION vs GAIN

(f = 5MHZ, V

–40

–50

–60

–70

–80

1

2

3

4

5

6

789

10

Non-Inverting Gain (V/V)

3f

O

2f

O