Datasheet
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
0.1µF
5 x 10
9
V/W
0.1µF
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.
FIGURE 5. High Impedance (10
15
Ω) Amplifier.
FIGURE 6. Piezoelectric Transducer Charge Amplifier.
1VDC
Output
2
3
6
8
Guard
500Ω 9.5kΩ
OPA129
pH Probe
R
S
≈ 500MΩ
50mV Out
7
5
V–
V+
FIGURE 4. Current-to-Voltage Converter.
2
3
6
7
5
8
OPA129
1000MΩ
R
F
Output
V
O
= –I
IN
• R
F
V
O
= –10V/nA
18kΩ
2kΩ
Current
Input
I
IN
V–
V+
V
OUT
2
3
6
8
C
F
OPA129
Low frequency cutoff =
1/(2πR
F
C
F
) = 0.16Hz
10pF
10
11
Ω
Output
V
OUT
= –∆Q/C
F
∆Q
R
F
7
5
V–
V+