OPA643PB datasheet(16/18 Pages) TI1

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OPA643

capacitive loads without an RS are allowed as the signal

gain increases (increasing the unloaded phase margin). 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 50

Ω environment is normally not necessary on board,

and in fact a higher impedance environment will improve

distortion as shown in the distortion vs 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 OPA643 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. Multiple destination devices are best

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

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 OPA643 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

OPA643 onto the board. If socketing for the DIP package

is desired, high frequency flush mount pins (e.g.,

McKenzie Technology #710C) can give good results.

INPUT AND ESD PROTECTION

The OPA643 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 are protected with internal

ESD protection diodes to the power supplies as shown in

Figure 11

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 OPA643), 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.

High input overdrive signals can also cause significant

differential voltage between the + and – inputs. Where this

voltage can exceed the maximum rated voltage of

±1.2V,

external Schottky protection diodes should be added across

the two inputs. Again, the capacitance added by these diodes

can degrade the noise and AC performance and should be

used only where necessary. Figure 12 shows a fully featured

input protection circuit for the OPA643. This is the circuit of

Figure 1 with additional limiting resistors into the inputs and

Schottky clamp diodes across the inputs. These resistor

values have been selected to limit the degradation in noise

and frequency response, achieve DC bias current cancellation,

and limit the current that will flow under overdrive conditions.

External

–V

Internal

Circuitry

DESIGN-IN TOOLS

DEMONSTRATION BOARDS

Several PC boards are available in the initial evaluation of

circuit performance using the OPA643 in its three package

styles. Two partially assembled boards are available for sale

to support the DIP (P suffix) and SO-8 (U-suffix) packages.

These boards come partially assembled with power supply

and I/O connectors but do not have the amplifier or resistor

networks loaded. Both boards are configured for low

505

Ω

Power Supply

Decoupling Not Shown

D1, D2

→ IN5911 (or equivalent)

126

Ω

Ω Source

125

Ω

OPA643

+5V

–5V

FIGURE 12. OPA643 Gain of +5 with Input Protection.

FIGURE 11. Internal ESD Protection.

OPA643PB Datasheet(PDF) 16 Page - Texas Instruments

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