Datasheet

TL03x, TL03xA
ENHANCED-JFET LOW-POWER LOW-OFFSET
OPERATIONAL AMPLIFIERS
SLOS180C − FEBRUARY 1997 − REVISED DECEMBER 2001
45
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
4-mA to 20-mA current loops
Often, information from an analog sensor must be sent over a distance to the receiving circuitry. For many
applications, the most feasible method involves converting voltage information to a current before transmission.
The following circuits give two variations of low-power current loops. The circuit in Figure 66 requires three wires
from the transmitting to receiving circuitry, while the second variation in Figure 67 requires only two wires, but
includes an extra integrated circuit. Both circuits benefit from the high input impedance of the TL03xA because
many inexpensive sensors do not have low output impedance.
Assuming that the voltage at the noninverting input of the TL03xA is zero, the following equation determines
the output current:
I
O
+ V
I
ǒ
R3
R1 R
S
Ǔ) 5Vǒ
R3
R2 R
S
Ǔ+ 0.16 V
I
) 4mA
The circuits presently provide 4-mA to 20-mA output current for an input voltage of 0 to 100 mV. By modifying
R1, R2, and R3, the input voltage range or the output current range can be adjusted.
Including the offset voltage of the operational amplifier in the above equation clearly illustrates why the low offset
TL03xA was chosen:
I
O
+ V
I
ǒ
R3
R1 R
S
Ǔ) 5Vǒ
R3
R2 R
S
Ǔ *V
I
ǒ
R3
R1 R
S
)
R3
R2 R
S
)
R1
R
S
Ǔ
+ 0.16 V
I
) 4mA – 0.17 V
I
For example, an offset voltage of 1 mV decreases the output current by 0.17 mA.
Due to the low power consumption of the TL03xA, both circuits have at least 2 mA available to drive the actual
sensor from the 5-V reference node.