®

OPA621

11

recommended. Circuits using all surface mount components

with the OPA621AU (SO-8 package) will offer the best AC

performance. The parasitic package inductance and capaci-

tance for the SO-8 is lower than the both the Cerdip and

8-lead Plastic DIP.

6) 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.

7) 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.

8) Standard commercial test equipment has not been

designed to test devices in the OPA621’s speed range.

Benchtop op amp testers and ATE systems will require a

special test head to successfully test these amplifiers.

9) 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.

10) 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

The OPA621’s input offset voltage is laser-trimmed and will

require no further adjustment for most applications. How-

ever, if additional adjustment is needed, the circuit in Figure

1 can be used without degrading offset drift with tempera-

ture. 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 R3. This will reduce input bias current errors to

the amplifier’s offset current, which is typically only 0.2

µA.

INPUT PROTECTION

Static damage has been well recognized for MOSFET

devices, but any semiconductor device deserves protection

from this potentially damaging source. The OPA621 incor-

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

This eliminates the need for the user to add external

protection diodes, which can add capacitance and degrade

AC performance.

All pins on the OPA621 are internally protected from ESD

by means of a pair of back-to-back reverse-biased diodes to

either power supply as shown. These diodes will begin to

conduct when the input voltage exceeds either power supply

by about 0.7V. This situation can occur with loss of the

amplifier’s power supplies while a signal source is still

present. The diodes can typically withstand a continuous

current of 30mA without destruction. To insure long term

reliability, however, diode current should be externally

limited to 10mA or so whenever possible.

The internal protection diodes are designed to withstand

2.5kV (using Human Body Model) and will provide

adequate ESD protection for most normal handling proce-

dures. However, static damage can cause subtle changes in

amplifier input characteristics without necessarily destroy-

ing the device. In precision operational amplifiers, this may

cause a noticeable degradation of offset voltage and drift.

Therefore, static protection is strongly recommended when

handling the OPA621.

OUTPUT DRIVE CAPABILITY

The OPA621’s design uses large output devices and has

been optimized to drive 50

Ω and 75Ω resistive loads. The

device can easily drive 6Vp-p into a 50

Ω load. This high-

output drive capability makes the OPA621 an ideal choice

for a wide range of RF, IF, and video applications. In many

cases, additional buffer amplifiers are unneeded.

Internal current-limiting circuitry limits output current to

about 150mA at 25

°C. This prevents destruction from

accidental shorts to common and eliminates the need for

external current-limiting circuitry. Although the device can

withstand momentary shorts to either power supply, it is not

recommended.

Many demanding high-speed applications such as ADC/

DAC buffers require op amps with low wideband output

impedance. For example, low output impedance is essential

* R

3

is optional and can be used to cancel offset errors due to input bias

currents.

FIGURE 1. Offset Voltage Trim.

ESD Protection diodes internally

connected to all pins.

External

Pin

+V

CC

–V

CC

Internal

Circuitry

FIGURE 2. Internal ESD Protection.

R

2

OPA621

*R = R || R

31

2

R

1

R

Trim

+V

CC

–V

CC

20k

V

or Ground

IN

Output Trim Range

+V

( R ) to –V

( R )

≅

CC

2

2

CC

R

Trim

R

Trim

47k

Ω

Ω