OPA129

6

SBOS026A

www.ti.com

0

Supply Voltage (

±V

COMMON-MODE INPUT RANGE vs SUPPLY VOLTAGE

510

020

15

10

5

15

BIAS CURRENT vs ADDITIONAL POWER DISSIPATION

Additional Power Dissipation (mW)

100pA

10pA

1pA

100

10

1

0

200

350

50

100

150

250

300

T

APPLICATIONS INFORMATION

NON-STANDARD PINOUT

The OPA129 uses a non-standard pinout to achieve lowest

possible input bias current. The negative power supply is

connected to pin 5—see Figure 1. This is done to reduce the

leakage current from the V- supply (pin 4 on conventional

op amps) to the op amp input terminals. With this new

pinout, sensitive inputs are separated from both power

supply pins.

FIGURE 1. Offset Adjust Circuit.

OFFSET VOLTAGE TRIM

The OPA129 has no conventional offset trim connections.

Pin 1, next to the critical inverting input, has no internal

connection. This eliminates a source of leakage current and

allows guarding of the input terminals. Pin 1 and pin 4, next

to the two input pins, have no internal connection. This

allows an optimized circuit board layout with guarding—see

the Circuit Board Layout section.

Due to its laser-trimmed input stage, most applications do

not require external offset voltage trimming. If trimming is

required, the circuit shown in Figure 1 can be used. Power

supply voltages are divided down, filtered and applied to the

non-inverting input. The circuit shown is sensitive to varia-

tion in the supply voltages. Regulation can be added, if

needed.

GUARDING AND SHIELDING

Ultra-low input bias current op amps require precautions to

achieve best performance. Leakage current on the surface of

circuit board can exceed the input bias current of the ampli-

Ω from

a power supply pin to an input pin produces a current of

15pA—more than 100 times the input bias current of the op

amp.

To minimize surface leakage, a guard trace should com-

pletely surround the input terminals and other circuitry

connecting to the inputs of the op amp. The DIP package

should have a guard trace on both sides of the circuit board.

The guard ring should be driven by a circuit node equal in

potential to the op amp inputs—see Figure 2. The substrate,

pin 8, should also be connected to the circuit board guard to

assure that the amplifier is fully surrounded by the guard

potential. This minimizes leakage current and noise pick-up.

Careful shielding is required to reduce noise pickup. Shield-

ing near feedback components may also help reduce noise

pick-up.

Triboelectric effects (friction-generated charge) can be a

troublesome source of errors. Vibration of the circuit board,

input connectors and input cables can cause noise and drift.

Make the assembly as rigid as possible. Attach cables to

avoid motion and vibration. Special low noise or low leak-

age cables may help reduce noise and leakage current. Keep

all input connections as short possible. Surface-mount com-

ponents may reduce circuit board size and allow a more rigid

assembly.

OPA129

V

OUT

V

IN

R

F

R

IN

2

3

6

7

5

V+

V–

0.1

µF

220

Ω

470k

Ω

470k

Ω

V–

V+