OPA861IDBVT datasheet(24/28 Pages) TI

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INPUT AND ESD PROTECTION

External

−V

Internal

Circuitry

OPA861

SBOS338 – AUGUST 2005

a) Minimize parasitic capacitance to any AC ground

nections, consider the trace and the input to the next

for all of the signal I/O pins. Parasitic capacitance on

device as a lumped capacitive load. Relatively wide

the inverting input pin can cause instability: on the

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

noninverting input, it can react with the source im-

with ground and power planes opened up around

pedance to cause unintentional bandlimiting. To re-

them.

duce unwanted capacitance, a window around the

e) Socketing a high-speed part like the OPA861 is

signal I/O pins should be opened in all of the ground

not recommended. The additional lead length and

and power planes around those pins. Otherwise,

pin-to-pin capacitance introduced by the socket can

ground and power planes should be unbroken else-

create an extremely troublesome parasitic network

where on the board.

that makes it almost impossible to achieve a smooth,

b) Minimize the distance (< 0.25") from the

stable frequency response. Best results are obtained

power-supply pins to high-frequency 0.1µF decoup-

by soldering the OPA861 onto the board.

ling capacitors. At the device pins, the ground and

power-plane layout should not be in close proximity to

the signal I/O pins. Avoid narrow power and ground

The OPA861 is built using a very high-speed comp-

traces to minimize inductance between the pins and

lementary bipolar process. The internal junction

the decoupling capacitors. The power-supply connec-

breakdown voltages are relatively low for these very

tions should always be decoupled with these capaci-

small geometry devices. These breakdowns are re-

tors. An optional supply decoupling capacitor (0.1µF)

flected in the Absolute Maximum Ratings table. All

across the two power supplies (for bipolar operation)

device pins are protected with internal ESD protection

will improve 2nd-harmonic distortion performance.

diodes to the power supplies as shown in Figure 49.

Larger (2.2µF to 6.8µF) decoupling capacitors, effec-

tive at lower frequency, should also be used on the

main supply pins. These may be placed somewhat

farther from the device and may be shared among

several devices in the same area of the PC board.

c) Careful selection and placement of external

components will preserve the high-frequency per-

formance of the OPA861. Resistors should be a

very low reactance type. Surface-mount resistors

work best and allow a tighter overall layout. Metal film

or carbon composition, axially-leaded resistors can

Figure 49. Internal ESD Protection

also

provide

good

high-frequency

performance.

Again, keep their leads and PC board traces as short

These diodes provide moderate protection to input

as possible. Never use wirewound type resistors in a

overdrive voltages above the supplies as well. The

high-frequency application.

protection diodes can typically support 30mA continu-

ous current. Where higher currents are possible (for

d) Connections to other wideband devices on the

example, in systems with

±15V supply parts driving

board may be made with short, direct traces or

into the OPA861), current-limiting series resistors

through onboard transmission lines. For short con-

should be added into the two inputs. Keep these

resistor values as low as possible since high values

degrade both noise performance and frequency re-

sponse.

OPA861IDBVT Datasheet(PDF) 24 Page - Texas Instruments

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