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Input and ESD Protection

External

-VCC

Internal

Circuitry

OPA2652

SBOS125A – JUNE 2000 – REVISED MAY 2006

of the board between the output and inverting input

devices to be handled as separate transmission

pins. Even with a low parasitic capacitance shunting

lines,

each

with

respective

series

and

shunt

the external resistors, excessively high resistor

terminations.

the

6dB

attenuation

values can create significant time constants that can

doubly-terminated transmission line is unacceptable,

degrade performance. Good axial metal film or

a long trace can be series-terminated at the source

surface-mount resistors have approximately 0.2pF in

end only. Treat the trace as a capacitive load in this

shunt with the resistor. For resistor values >1.5k

Ω,

case and set the series resistor value as shown in

this parasitic capacitance can add a pole and/or zero

the plot of Recommended RS vs Capacitive Load

below 500MHz that can effect circuit operation. Keep

(Figure 17). This configuration will not preserve

resistor values as low as possible consistent with

signal integrity as well as a doubly-terminated line. If

load driving considerations. The 402

Ω feedback

the input impedance of the destination device is low,

used in the typical performance specifications is a

there will be some signal attenuation due to the

good starting point for design. Note that a 25

Ω

voltage divider formed by the series output into the

feedback resistor, rather than a direct short, is

terminating impedance.

suggested for the unity gain follower application. This

e) Socketing a high-speed part like the OPA2652

effectively isolates the inverting input capacitance

is not recommended. The additional lead length

from the output pin that would otherwise cause

and pin-to-pin capacitance introduced by the socket

additional peaking in the gain of +1 frequency

can

create

extremely

troublesome

parasitic

response.

network that can make it almost impossible to

d) Connections to other wideband devices on the

achieve a smooth, stable frequency response. Best

board may be made with short direct traces or

results are obtained by soldering the OPA2652

through

onboard

transmission

lines.

For

short

directly onto the board.

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,

The OPA2652 is built using a very high-speed

preferably with ground and power planes opened up

complementary bipolar process. The internal junction

around them. Estimate the total capacitive load and

breakdown voltages are relatively low for these very

set RS from the plot of Recommended RS vs

small geometry devices. These breakdowns are

Capacitive Load (Figure 17). Low parasitic capacitive

reflected in the Absolute Maximum Ratings table. All

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

device

pins

are

protected

with

internal

ESD

OPA2652 is nominally compensated to operate with

protection diodes to the power supplies as shown in

a 2pF parasitic load. Higher parasitic capacitive

Figure 37.

loads without an RS are allowed as the signal gain

increases (increasing the unloaded phase margin) If

These diodes provide moderate protection to input

a long trace is required, and the 6dB signal loss

overdrive voltages above the supplies as well. The

intrinsic to a doubly-terminated transmission line is

protection

diodes

can

typically

support

30mA

acceptable,

implement

matched

impedance

continuous

current.

Where

higher

currents

are

transmission

line

using

microstrip

stripline

possible (for example, in systems with

±15V supply

techniques (consult an ECL design handbook for

parts driving into the OPA2652), current-limiting

microstrip and stripline layout techniques). A 50

Ω

series resistors should be added into the two inputs.

environment is normally not necessary on board, and

Keep these resistor values as low as possible since

in fact, a higher impedance environment will improve

high values degrade both noise performance and

distortion as shown in the distortion versus load

frequency response.

plots. With a characteristic board trace impedance

defined

(based

board

material

and

trace

dimensions), a matching series resistor into the trace

from the output of the OPA2652 is used as well as a

terminating shunt resistor at the input of

the

destination

device.

Remember

also

that

the

terminating

impedance

will

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

Figure 37. Internal ESD Protection

impedance. The high output voltage and current

capability of the OPA2652 allows multiple destination

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