8

OPA111

®

APPLICATIONS INFORMATION

OFFSET VOLTAGE ADJUSTMENT

The OPA111 offset voltage is laser-trimmed and will require

no further trim for most applications. As with most amplifi-

ers, externally trimming the remaining offset can change

drift performance by about 0.3

µV/°C for each 100µV of

adjusted offset. Note that the trim (Figure 1) is similar to

operational amplifiers such as 741 and AD547. The OPA111

can replace most other amplifiers by leaving the external

null circuit unconnected.

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

FIGURE 1. Offset Voltage Trim.

FIGURE 2. Input Current vs Input Voltage with

±V

Grounded.

Unlike BIFET amplifiers, The Difet OPA111 requires input

current limiting resistors only if its input voltage is greater

will limit input current to a safe level with up to

±15V input

levels, even if both supply voltages are lost.

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

precision IC operational amplifier.

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 OPA111. To avoid leakage

problems, it is recommended that the signal input lead of the

OPA111 be wired to a Teflon standoff. If the OPA111 is to

be soldered directly into a printed circuit board, utmost care

must be used in planning the board layout. A “guard” pattern

INPUT OFFSET VOLTAGE CHANGE

DUE TO THERMAL SHOCK

150

75

0

–75

–150

Time From Thermal Shock (Minutes)

–1

012345

25°C

85°C

BM

AM

T

Air Environment

OPA111

2

3

7

4

5

1

6

–V

CC

+V

CC

10k

Ω to 1M trim potentiometer

(100k

Ω recommended).

±10mV typical trim range.

V

I

IN

Maximum Safe Current

Maximum Safe Current

–15

Input Voltage (V)

–10

–5

0

5

10

15

2

1

0

–1

–2