Datasheet

TL03x, TL03xA
ENHANCED-JFET LOW-POWER LOW-OFFSET
OPERATIONAL AMPLIFIERS
SLOS180C − FEBRUARY 1997 − REVISED DECEMBER 2001
44
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
transimpedance amplifier
The low-power precision TL03x allows accurate measurement of low currents. The high input impedance and
low offset voltage of the TL03xA greatly simplify the design of a transimpedance amplifier. At room temperature,
this design achieves 10-bit accuracy with an error of less than 1/2
LSB.
Assuming that R2 is much less than R1 and ignoring error terms, the output voltage can be expressed as:
V
O
+ –I
IN
R
F
ǒ
R1 ) R2
R2
Ǔ
Using the resistor values shown in the schematic for a 1-nA input current, the output voltage equals −0.1 V. If
the V
O
limit for the TL03xA is measured at ±12 V, the maximum input current for these resistor values is ±120 nA.
Similarly, one LSB on a 10-bit scale corresponds to 12 mV of output voltage, or 120 pA of input current.
The following equation shows the effect of input offset voltage and input bias current on the output voltage:
V
O
+ –
ƪ
V
IO
) R
F
ǒ
I
IO
) I
IB
Ǔ
ƫ
ǒ
R1 ) R2
R2
Ǔ
If the application requires input protection for the transimpedance amplifier, do not use standard PN diodes.
Instead, use low-leakage Siliconix SN4117 JFETs (or equivalent) connected as diodes across the TL03xA
inputs (see Figure 65).
As with all precision applications, special care must be taken to eliminate external sources of leakage and
interference. Other precautions include using high-quality insulation, cleaning insulating surfaces to remove
fluxes and other residue, and enclosing the application within a protective box.
−
+
15 V
−15 V
TL03xA
R
F
10 MΩ
90 kΩ
V
O
10 kΩR2
SN4117
R1
Input Current
Figure 65. Transimpedance Amplifier