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7

OPA128

APPLICATIONS INFORMATION

OFFSET VOLTAGE ADJUSTMENT

The OPA128 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 effort. Note that the trim (Figure 1) is similar to

operational amplifiers such as HA-5180 and AD515. The

OPA128 can replace many other amplifiers by leaving the

external null circuit unconnected.

The amplifier case should be connected to any input shield or

guard via pin 8. This insures that the amplifier itself is fully

surrounded by guard potential, minimizing both leakage and

noise pickup (see Figure 2).

Triboelectric charge (static electricity generated by friction)

can be a troublesome noise source from cables connected to

the input of an electrometer amplifier. Special low-noise cable

will minimize this effect but the optimum solution is to mount

the signal source directly at the electrometer input with short,

rigid, wiring to preclude microphonic noise generation.

TESTING

Accurately testing the OPA128 is extremely difficult due to its

high level of performance. Ordinary test equipment may not

be able to resolve the amplifier’s extremely low bias current.

Inaccurate bias current measurements can be due to:

1. Test socket leakage

2. Unclean package

3. Humidity or dew point condensation

4. Circuit contamination from fingerprints or anti-static

treatment chemicals

5. Test ambient temperature

6. Load power dissipation

FIGURE 2. Connection of Input Guard.

TO-99 Bottom View

3

OPA128

2

8

6

In

Out

Non-Inverting

3

OPA128

2

8

6

In

Out

Buffer

3

OPA128

2

8

6

In

Out

Inverting

3

2

4 5

6

7

8

1

BOARD LAYOUT

FOR INPUT GUARDING

Guard top and bottom of board.

Alternate: use Teflon

for sensitive input pins.

Teflon

FIGURE 1. Offset Voltage Trim.

7

6

2

3

4

5

1

±10mV Typical

Trim Range

NOTE: (1) 10k

Ω to 1MΩ

Trim Potentiometer

(100k

Ω Recommended)

+V

CC

–V

CC

OPA128

(1)

INPUT PROTECTION

Conventional monolithic FET operational amplifiers’ inputs

must be protected against destructive currents that can flow

when input FET gate-to-substrate isolation diodes are for-

the loss of –V

Because of its dielectric isolation, no special protection is

needed on the OPA128. Of course, the differential and

common-mode voltage limits should be observed.

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.

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 OPA128. To avoid leakage problems, it is

recommended that the signal input lead of the OPA128 be

wired to a Teflon standoff. If the input is to be soldered

directly into a printed circuit board, utmost care must be used

in planning the board layout. A “guard” pattern should

completely surround the high impedance input leads and

should be connected to a low impedance point which is at the

signal input potential.