OPA684IDBVT datasheet(21/25 Pages) TI

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OPA684

SBOS219A

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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 ac-

ceptable, implement a matched impedance transmis-

sion 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 OPA684 is used, as well as

a terminating shunt resistor at the input of the destina-

tion device. Remember also that the terminating imped-

ance 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 OPA684 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 “RS vs CLOAD”. 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 imped-

ance.

Socketing a high-speed part like the OPA684 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 fre-

quency response. Best results are obtained by soldering

the OPA684 onto the board.

External

–V

Internal

Circuitry

FIGURE 15. Internal ESD Protection.

INPUT AND ESD PROTECTION

The OPA684 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 where an absolute maximum 13V across the

supply pins is reported. All device pins have limited ESD

protection using internal diodes to the power supplies, as

shown in Figure 15.

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 (e.g. in systems with

±15V supply parts

driving into the OPA684), 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.

OPA684IDBVT Datasheet(PDF) 21 Page - Texas Instruments

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