OPA692 datasheet(26/31 Pages) BURR-BROWN

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INPUT AND ESD PROTECTION

External

-VCC

Internal

Circuitry

OPA3693

SBOS353 – DECEMBER 2006

noninverting input termination resistors, should also

e) Socketing a high-speed part such as the

be placed close to the package. Good axial metal

OPA3693 is not recommended. The additional lead

film or surface-mount resistors have approximately

length and pin-to-pin capacitance introduced by the

0.2pF in shunt with the resistor. For resistor values >

socket

can

create

extremely

troublesome

2.0k

Ω, this parasitic capacitance can add a pole

parasitic

network,

which

can

make

almost

and/or zero below 400MHz that can effect circuit

impossible to achieve a smooth, stable frequency

operation. Keep resistor values as low as possible

response. Best results are obtained by soldering the

consistent with load driving considerations.

OPA3693 directly onto the board.

d) Connections to other wideband devices on the

PCB may be made with short direct traces or through

onboard transmission lines. For short connections,

The OPA3693 is built using a very high-speed

consider the trace and the input to the next device as

complementary bipolar process. The internal junction

a lumped capacitive load. Relatively wide traces

breakdown voltages are relatively low for these very

(50mils to 100mils) should be used, preferably with

small geometry devices. These breakdowns are

ground and power planes opened up around them.

reflected in the Absolute Maximum Ratings table. All

Estimate the total capacitive load and set RS from

device

pins

are

protected

with

internal

ESD

the plot of Recommended RS vs Capacitive Load

protection diodes to the power supplies, as shown in

(Figure 15). Low parasitic capacitive loads (< 4pF)

Figure 59.

may not need an RS since the OPA3693 is nominally

compensated to operate with a 2pF parasitic load. If

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

intrinsic to a doubly-terminated transmission line is

acceptable,

implement

matched

impedance

transmission

line

using

microstrip

stripline

techniques (consult an ECL design handbook for

microstrip and stripline layout techniques). A 50

Ω

environment is normally not necessary on board, and

in fact, a higher impedance environment improves

Figure 59. Internal ESD Protection

distortion, as shown in the distortion versus load

plots. With a characteristic board trace impedance

defined

based

board

material

and

trace

These diodes provide moderate protection to input

dimensions, a matching series resistor into the trace

overdrive voltages above the supplies as well. The

from the output of the OPA3693 is used, as well as a

protection

diodes

can

typically

support

30mA

terminating shunt resistor at the input of

the

continuous

current.

Where

higher

currents

are

destination

device.

Remember

also

that

the

possible (for example, in systems with

±15V supply

terminating impedance is the parallel combination of

parts driving into the OPA3693), current limiting

the shunt resistor and the input impedance of the

series resistors may be added on the noninverting

destination device; this total effective impedance

input. Keep this resistor value as low as possible

should be set to match the trace impedance. If the

since high values degrade both noise performance

6dB attenuation of a doubly-terminated transmission

and frequency response. The inverting input already

line

unacceptable,

long

trace

can

has a 300

Ω resistor from the external pin to the

series-terminated at the source end only. Treat the

internal summing junction for the op amp. This

trace as a capacitive load in this case and set the

resistor provides considerable protection for that

series resistor value as illustrated in the plot of

node.

Figure 15. This configuration does not preserve

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

the input impedance of the destination device is low,

there will be some signal attenuation due to the

voltage divider formed by the series output into the

terminating impedance.

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