OPA656 datasheet(15/25 Pages) TI

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OPA656

SBOS196G

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c) Careful selection and placement of external components

will preserve the high frequency performance of the OPA656.

Resistors should be a very low reactance type. Surface-mount

resistors work best and allow a tighter overall layout. Metal film

and carbon composition axially leaded resistors can also pro-

vide good high frequency performance. Again, keep their leads

and PCB trace length as short as possible. Never use wirewound

type resistors in a high frequency application. Since the output

pin and inverting input pin are the most sensitive to parasitic

capacitance, always position the feedback and series output

resistor, if any, as close as possible to the output pin. Other

network components, such as noninverting input termination

resistors, should also be placed close to the package. Where

double side component mounting is allowed, place the feed-

back resistor directly under the package on the other side of the

board between the output and inverting input pins. Even with a

low parasitic capacitance shunting the external resistors, exces-

sively high resistor values can create significant time constants

that can degrade performance. Good axial metal film or surface-

mount resistors have approximately 0.2pF in shunt with the

resistor. For resistor values > 1.5k

Ω, this parasitic capacitance

can add a pole and/or zero below 500MHz that can effect circuit

operation. Keep resistor values as low as possible consistent

with load driving considerations. It has been suggested here

that a good starting point for design would be to keep RF || RG

< 250

Ω for voltage amplifier applications. Doing this will auto-

matically keep the resistor noise terms low, and minimize the

effect of their parasitic capacitance. Transimpedance applica-

tions (see Figure 3) can use whatever feedback resistor is

required by the application as long as the feedback compensa-

tion capacitor is set considering all parasitic capacitance terms

on the inverting node.

d) Connections to other wideband devices on the board

may be made with short direct traces or through onboard

transmission lines. For short connections, consider the trace

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

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

preferably with ground and power planes opened up around

them. Estimate the total capacitive load and set RS from the

plot of

Recommended RS vs Capacitive Load. Low parasitic

capacitive loads (< 5pF) may not need an RS since the

OPA656 is nominally compensated to operate with a 2pF

parasitic load. Higher parasitic capacitive loads without an RS

are allowed as the signal gain increases (increasing the

unloaded phase margin) If a long trace is required, and the

6dB signal loss intrinsic to a doubly-terminated transmission

line is acceptable, implement a matched impedance trans-

mission line using microstrip or stripline techniques (consult

an ECL design handbook for microstrip and stripline layout

techniques). A 50

Ω environment is normally not necessary

onboard, and in fact a higher impedance environment will

improve distortion as shown in the distortion versus load

plots. With a characteristic board trace impedance defined

based on board material and trace dimensions, a matching

series resistor into the trace from the output of the OPA656

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—

this total effective impedance should be set to match the

trace impedance. If the 6dB attenuation of a doubly-termi-

nated transmission line is unacceptable, a long trace can be

series-terminated at the source end only. Treat the trace as

a capacitive load in this case and set the series resistor value

as shown in the plot of

Recommended RS vs Capacitive

Load. This will not preserve signal integrity as well as a

doubly-terminated line. If the input impedance of the destina-

tion device is low, there will be some signal attenuation due

to the voltage divider formed by the series output into the

terminating impedance.

e) Socketing a high speed part like the OPA656 is not

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

pacitance introduced by the socket can create an extremely

troublesome parasitic network which can make it almost impos-

sible to achieve a smooth, stable frequency response. Best

results are obtained by soldering the OPA656 onto the board.

INPUT AND ESD PROTECTION

The OPA656 is built using a very high speed complementary

bipolar process. The internal junction breakdown voltages are

relatively low for these very small geometry devices. These

breakdowns are reflected in the Absolute Maximum Ratings

table. All device pins are protected with internal ESD protec-

tion diodes to the power supplies as shown in Figure 5.

These diodes provide moderate protection to input overdrive

voltages above the supplies as well. The protection diodes

can typically support 30mA continuous current. Where higher

currents are possible (for example, in systems with

±12V

supply parts driving into the OPA656), current limiting series

resistors should be added into the two inputs. Keep these

resistor values as low as possible since high values degrade

both noise performance and frequency response.

FIGURE 5. Internal ESD Protection.

External

–V

Internal

Circuitry

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