9

®

OPA4658

The feedback resistor value acts as the frequency response

compensation element for a current feedback type amplifier.

The 402

Ω used in setting the specification achieves a nomi-

nal maximally flat butterworth response while assuming a

2pF output pin parasitic. Increasing the feedback resistor

will over compensate the amplifier, rolling off the frequency

response, while decreasing it will decrease phase margin,

peaking up the frequency response.

d) Connections to other wideband devices on the board

may be made with short direct traces or through on-board

transmission lines. For short connections, consider the trace

and the input to the next device as a lumped capacitive load.

Relatively wide traces (50 to 100 mils) should be used,

preferably with ground and power planes opened up around

nominally compensated to operate with a 2pF parasitic load.

If a long trace is required and the 6dB signal loss intrinsic to

doubly terminated transmission lines is acceptable, imple-

ment a matched impedance transmission line using microstrip

or stripline techniques (consult an ECL design handbook for

microstrip and stripline layout techniques). A 50

Ω environ-

ment is not necessary on board, and in fact a higher imped-

ance environment will improve distortion as shown in the

distortion vs load plot. With a characteristic impedance

defined based on board material and desired trace dimen-

sions, a matching series resistor into the trace from the

output of the amplifier is used as well as a terminating shunt

resistor at the input of the destination device. Remember

also that the terminating impedance will be the parallel

combination of the shunt resistor and the input impedance of

the destination device; the total effective impedance should

match the trace impedance. Multiple destination devices are

best handled as separate transmission lines, each with their

own series and shunt terminations.

If the 6dB attenuation loss of a doubly terminated line is

unacceptable, a long trace can be series-terminated at the

source end only. This will help isolate the line capacitance

from the op amp output, but will not preserve signal integrity

as well as a doubly terminated line. If the shunt impedance

at the destination end is finite, there will be some signal

attenuation due to the voltage divider formed by the series

and shunt impedances.

e) Socketing a high speed part like the OPA4658 is not

recommended. The additional lead length and pin-to-pin

capacitance introduced by the socket creates an extremely

troublesome parasitic network which can make it almost

impossible to achieve a smooth, stable response. Best results

are obtained by soldering the part onto the board. If socket-

ing for the DIP package is desired, high frequency flush

mount pins (e.g., McKenzie Technology #710C) can give

good results.

the root sum of the squares of equation (4) and applying the

spectral noise values found in the Typical Performance Curve

graph section. This applies to noise from the op amp only.

Note that both the noise figure (NF) and the equivalent input

offset voltages improve as the closed loop gain increases (by

INCREASING BANDWIDTH AT HIGH GAINS

The closed-loop bandwidth can be extended at high gains by

width reduction is caused by the feedback current being split

(for a fixed R

CIRCUIT LAYOUT AND BASIC OPERATION

Achieving optimum performance with a high frequency am-

plifier like the OPA4658 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

OPA4658. 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.