OPA333 Datasheet, PDF (17/42 Pages) Burr-Brown (TI) – 1.8V, microPOWER CMOS OPERATIONAL AMPLIFIERS Zer-PI Drift Series

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OPA333, OPA2333
SBOS351E – MARCH 2006 – REVISED DECEMBER 2015
Typical Applications (continued)
8.2.2 Precision, Low-Level Voltage-to-Current (V-I) Converter
The circuit shown in Figure 22 is a precision, low-level voltage-to-current (V-I) converter. The converter translates
in input voltage of 0 V to 5 V and output current of 0 µA to 5 µA. Figure 23 shows the measured transfer function
for this circuit. The low offset voltage and offset drift of the OPA333 facilitate excellent dc accuracy for the circuit.
Figure 24 shows the calibrated error for the entire range of the circuit.
R3 100 k
C1 10 nFR4 _{100 k}
5V
V_{OUT_OPA}
5V
OPA333
+
+
V_IN
Rset
100 k
R1
40.2 k
+
R1
U2
INA326
±^R1 _R2
V_{OUT_INA}
R_LOAD
I_OUT
+
A AM1
R2
200 k
C2 1 nF
Figure 22. Low-Level, Precision V-I Converter
8.2.2.1 Design Requirements
The design requirements are as follows:
• Supply Voltage: 5 V DC
• Input: 0 V to 5 V DC
• Output: 0 μA to 5 μA DC
8.2.2.2 Detailed Design Procedure
The V-I transfer function of the circuit is based on the relationship between the input voltage, V_IN, R_SET, and the
instrumentation amplifier (INA) gain. During operation, the input voltage divided by the INA gain appears across
the set resistor in Equation 1:
V_SET = V_IN/G_INA
(1)
The current through R_SET must flow through the load, so I_OUT is V_SET / R_SET. I_OUT remains a well-regulated current
as long as the total voltage across R_SET and R_LOAD does not violate the output limits of the operational amplifier
or the input common-mode limits of the INA. The voltage across the set resistor (V_SET) is the input voltage
divided by the INA gain (that is, V_SET = 1 V / 10 = 0.1 V). The current is determined by V_SET and R_SET shown in
Equation 2:
I_OUT = V_SET / R_SET = 0.1 V / 100 kΩ = 1 μA
(2)
A detailed error analysis, design procedure, and additional measured results are given in TIPD107.
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