OPA129

7

SBOS026A

www.ti.com

CIRCUIT BOARD LAYOUT

The OPA129 uses a new pinout for ultra low input bias

current. Pin 1 and pin 4 have no internal connection. This

allows ample circuit board space for a guard ring surround-

ing the op amp input pins—even with the tiny SO-8 surface-

mount package. Figure 3 shows suggested circuit board

layouts. The guard ring should be connected to pin 8 (sub-

strate) as shown. It should be driven by a circuit node equal

in potential to the input terminals of the op amp—see Figure

2 for common circuit configurations.

TESTING

Accurately testing the OPA129 is extremely difficult due to

its high 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 condensations.

4. Circuit contamination from fingerprints or anti-static

treatment chemicals.

5. Test ambient temperature.

6. Load power dissipation.

7. Mechanical stress.

8. Electrostatic and electromagnetic interference.

18

5

4

18

5

4

(A) DIP package

(B) SOIC package

V–

V

0

V+

V–

V

0

V+

Connect to proper circuit

node, depending on circuit

configuration (see Figure 2).

Connect to proper circuit

node, depending on circuit

configuration (see Figure 2).

FIGURE 7. Sensitive Photodiode Amplifier.

2

3

6

8

OPA129

Output

Pin photodiode

HP 5082-4204

5

7

+15V

–15V

0.1

µF

5 x 10

0.1

µF

10

10

Ω

~1pF to prevent gain peaking

Guard

Circuit must be well shielded.

Out

In

2

3

6

8

(C) Inverting

Out

In

2

3

6

8

(A) Non-Inverting

Out

In

2

3

6

8

(B) Buffer

Guard top and bottom of board.

FIGURE 2. Connection of Input Guard.

FIGURE 3. Suggested Board Layout for Input Guard.

Ω) Amplifier.

FIGURE 6. Piezoelectric Transducer Charge Amplifier.

1VDC

Output

2

3

6

8

Guard

500

Ω

9.5k

Ω

OPA129

pH Probe

R

50mV Out

7

5

V–

V+

FIGURE 4. Current-to-Voltage Converter.

2

3

6

7

5

8

OPA129

1000M

Ω

R

F

Output

V

V

18k

Ω

2k

Ω

Current

Input

I

IN

V–

V+

V

OUT

2

3

6

8

C

F

OPA129

Low frequency cutoff =

1/(2

πR

10pF

Ω

Output

V

∆Q

R

F

7

5

V–

V+