8

®

OPA404

APPLICATIONS INFORMATION

OFFSET VOLTAGE ADJUSTMENT

The OPA404 offset voltage is laser-trimmed and will require

no further trim for most applications. If desired, offset volt-

age can be trimmed by summing (see Figure 1). With this

trim method there will be no degradation of input offset drift.

GUARDING AND SHIELDING

As in any situation where high impedances are involved,

careful shielding is required to reduce “hum” pickup in input

leads. If large feedback resistors are used, they should also

be shielded along with the external input circuitry.

Leakage currents across printed circuit boards can easily

exceed the bias current of the OPA404. To avoid leakage,

utmost care must be used in planning the board layout. A

“guard” pattern should completely surround the high imped-

ance input leads and should be connected to a low-impedance

point which is at the signal input potential. (See Figure 3).

FIGURE 2. Input Current vs Input Voltage with

±V

Grounded.

–15

–10

–5

0

+5

+10

+15

Input Voltage (V)

–2

+2

+1

–1

INPUT CURRENT vs INPUT VOLTAGE

WITH ±V

0

Maximum Safe Current

Maximum Safe Current

I

IN

V

FIGURE 3. Connection of Input Guard.

HANDLING AND TESTING

Measuring the unusually low bias current of the OPA404 is

difficult without specialized test equipment; most commer-

cial benchtop testers cannot accurately measure the OPA404

bias current. Low-leakage test sockets and special test

fixtures are recommended if incoming inspection of bias

current is to be performed.

To prevent surface leakage between pins, the DIP package

should not be handled by bare fingers. Oils and salts from

fingerprints or careless handling can create leakage currents

that exceed the specified OPA404 bias currents.

If necessary, DIP packages and PC board assemblies can be

°C,

rinsed with de-ionized water, and baked again for 30 min-

utes at 85

°C. Surface contamination can be prevented by the

application of a high-quality conformal coating to the cleaned

PC board assembly.

–15V

+15V

±2mV

Offset

Trim

150k

Ω

100k

Ω

In

Out

20

Ω

1/4

OPA404

FIGURE 1. Offset Voltage Trim.

INPUT PROTECTION

Conventional monolithic FET operational amplifiers require

external current-limiting resistors to protect their inputs against

destructive currents that can flow when input FET gate-to-

substrate isolation diodes are forward-biased. Most BIFET

Unlike BIFET amplifiers, the Difet OPA404 requires input

current limiting resistors only if its input voltage is greater

will limit the input current to a safe value with up to

±15V

input levels even if both supply voltages are lost. (See Figure

2 and Absolute Maximum Ratings).

Static damage can cause subtle changes in amplifier input

characteristics without necessarily destroying the device.

In precision operational amplifiers (both bipolar and FET

types), this may cause a noticeable degradation of offset

voltage and drift.

Static protection is recommended when handling any preci-

sion IC operational amplifier.

In

Out

Non-Inverting

In

Out

Buffer

In

Out

Inverting

For input guarding,

guard top and bottom of board.