OPA694 datasheet(21/27 Pages) TI2

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OPA2694

SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013

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and set RS from the plot of Recommended RS vs

Capacitive Load. Low parasitic capacitive loads (< 5pF)

may not need an RS, since the OPA2694 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 a matched impedance transmission line using

microstrip or stripline techniques (consult an ECL design

handbook for microstrip and stripline layout techniques). A

50Ω 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

OPA2694 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. The high output

voltage and current capability of the OPA2694 allows

multiple destination devices to be handled as separate

transmission lines, each with their own series and shunt

terminations. If the 6dB attenuation of a doubly-terminated

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 destination 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 OPA2694 is not

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

capacitance introduced by the socket can create an

extremely troublesome parasitic network which can make

it almost impossible to achieve a smooth, stable frequency

response. Best results are obtained by soldering the

OPA2694 directly onto the board.

INPUT AND ESD PROTECTION

The OPA2694 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 have

limited ESD protection using internal diodes to the power

supplies, as shown in Figure 14.

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 ±15V supply parts driving into the

OPA2694), 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.

External

+VCC

−VCC

Internal

Circuitry

Figure 14. Internal ESD Protection

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