8

OPA640

®

Points to Remember

1) Making use of all four power supply pins will lower the

effective power supply impedance seen by the input and

output stages. This will improve the AC performance in-

cluding lower distortion. The lowest distortion is achieved

when running separate traces to V

bypassing with 0.01

µF and 2.2µF surface mount capacitors

on the topside of the PC board is recommended. It is

essential to keep the 0.01

µF capacitor very close to the

power supply pins. Refer to the DEM-OPA64X data sheet

for the recommended layout and component placements.

2) Whenever possible, use surface mount. Don’t use point-

to-point wiring as the increase in wiring inductance will be

detrimental to AC performance. However, if it must be used,

very short, direct signal paths are required. The input signal

ground return, the load ground return, and the power supply

common should all be connected to the same physical point

to eliminate ground loops, which can cause unwanted feed-

back.

3) Surface mount on backside of PC Board. Good compo-

nent selection is essential. Capacitors used in critical loca-

tions should be a low inductance type with a high quality

dielectric material. Likewise, diodes used in critical loca-

tions should be Schottky barrier types, such as HP5082-

2835 for fast recovery and minimum charge storage. Ordi-

nary diodes will not be suitable in RF circuits.

4) Whenever possible, solder the OPA640 directly into the

PC board without using a socket. Sockets add parasitic

capacitance and inductance, which can seriously degrade

AC performance or produce oscillations.

5) Use a small feedback resistor (usually 25

Ω) in unity-gain

voltage follower applications for the best performance. For

gain configurations, resistors used in feedback networks

should have values of a few hundred ohms for best perfor-

mance. Shunt capacitance problems limit the acceptable

resistance range to about 1k

Ω on the high end and to a value

that is within the amplifier’s output drive limits on the low

end. Metal film and carbon resistors will be satisfactory, but

wirewound resistors (even “non-inductive” types) are abso-

lutely unacceptable in high-frequency circuits. Feedback

resistors should be placed directly between the output and

the inverting input on the backside of the PC board. This

placement allows for the shortest feedback path and the

highest bandwidth. Refer to the demonstration board layout

at the end of the data sheet. A longer feedback path than

this will decrease the realized bandwidth substantially.

6) Due to the extremely high bandwidth of the OPA640, the

SO-8 package is strongly recommended due its low parasitic

impedance. The parasitic impedance in the DIP and package

causes the OPA640 to experience about 5dB of gain peaking

in unity-gain configurations. This is compared with virtually

no gain peaking in the SO-8 package in unity-gain. The gain

peaking in the DIP package is minimized in gains of 2 or

greater, however. Surface mount components (chip resistors,

capacitors, etc.) have low lead inductance and are also

strongly recommended.

7) Avoid overloading the output. Remember that output

current must be provided by the amplifier to drive its own

feedback network as well as to drive its load. Lowest

distortion is achieved with high impedance loads.

8) Don’t forget that these amplifiers use

±5V supplies.

Although they will operate perfectly well with +5V and

–5.2V, use of

±15V supplies will destroy the part.

9) Standard commercial test equipment has not been de-

signed to test devices in the OPA640’s speed range. Bench-

top op amp testers and ATE systems will require a special

test head to successfully test these amplifiers.

10) Terminate transmission line loads. Unterminated lines,

such as coaxial cable, can appear to the amplifier to be a

capacitive or inductive load. By terminating a transmission

line with its characteristic impedance, the amplifier’s load

then appears purely resistive.

11) Plug-in prototype boards and wire-wrap boards will not

be satisfactory. A clean layout using RF techniques is

essential; there are no shortcuts.

OFFSET VOLTAGE ADJUSTMENT

If additional offset adjustment is needed, the circuit in

Figure 1 can be used without degrading offset drift with

temperature. Avoid external adjustment whenever possible

since extraneous noise, such as power supply noise, can be

inadvertently coupled into the amplifier’s inverting input

terminal. Remember that additional offset errors can be

created by the amplifier’s input bias currents. Whenever

possible, match the impedance seen by both inputs as is

shown with R

the amplifier’s offset current.

FIGURE 1. Offset Voltage Trim.

NOTE: (1) R

bias currents.

INPUT PROTECTION

Static damage has been well recognized for MOSFET de-

vices, but any semiconductor device deserves protection

from this potentially damaging source. The OPA640 incor-

porates on-chip ESD protection diodes as shown in Figure 2.

R

2

OPA640

R

(1)

R

1

R

TRIM

47k

Ω

+V

CC

–V

CC

20k

Ω

V

Output Trim Range

+V

CC

to –V

CC

≅

R

Trim

R

2

R

2

R

Trim

10µF