INPUT AND ESD PROTECTION

External

Pin

+V

CC

-VCC

Internal

Circuitry

OPA3875

www.ti.com ............................................................................................................................................ SBOS341D – DECEMBER 2006 – REVISED AUGUST 2008

With a characteristic board trace impedance defined

based on board material and trace dimensions, a

The OPA3875 is built using a very high-speed

matching series resistor into the trace from the output

complementary bipolar process. The internal junction

of the OPA3875 is used as well as a terminating

breakdown voltages are relatively low for these very

shunt resistor at the input of the destination device.

small geometry devices. These breakdowns are

Remember also that the terminating impedance will

reflected in the Absolute Maximum Ratings table. All

be the parallel combination of the shunt resistor and

device pins have limited ESD protection using internal

the input impedance of the destination device; this

diodes to the power supplies as shown in Figure 31.

total effective impedance should be set to match the

trace impedance. The high output voltage and current

capability of the OPA3875 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 Figure 5. This will

not

preserve

signal

integrity

as

well

as

a

Figure 31. Internal ESD Protection

doubly-terminated line. If the input impedance of the

destination device is low, there will be some signal

These diodes provide moderate protection to input

attenuation due to the voltage divider formed by the

overdrive voltages above the supplies as well. The

series output into the terminating impedance.

protection

diodes

can

typically

support

30mA

e) Socketing a high-speed part like the OPA3875

continuous

current.

Where

higher

currents

are

is not recommended. The additional lead length and

possible (for example, in systems with ±15V supply

pin-to-pin capacitance introduced by the socket can

parts driving into the OPA3875), current-limiting

create an extremely troublesome parasitic network

series resistors should be added into the two inputs.

which can make it almost impossible to achieve a

Keep these resistor values as low as possible

smooth, stable frequency response. Best results are

because high values degrade both noise performance

obtained by soldering the OPA3875 onto the board.

and frequency response.

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Product Folder Link(s): OPA3875