OPA690 datasheet(21/23 Pages) TI

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OPA691

SBOS226A

www.ti.com

FIGURE 14. Internal ESD Protection.

External

–V

Internal

Circuitry

design handbook for microstrip and stripline layout tech-

niques). A 50

Ω environment is normally not necessary on

board, 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 OPA691

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 capa-

bility of the OPA691 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 versus 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 OPA691 is not

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

capacitance introduced by the socket can create an ex-

tremely troublesome parasitic network which can make it

almost impossible to achieve a smooth, stable frequency

response. Best results are obtained by soldering the OPA691

onto the board.

INPUT AND ESD PROTECTION

The OPA691 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 (e.g., in systems with

±15V supply parts

driving into the OPA691), 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.

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