+

R3 100 k

C1 10 nF

R4 100 k

VIN

+

OPA333-Q1

5 V

Rset

100 k

RLOAD

IOUT

+

AM1

A

±

R1

R1

R2

U2

INA326

R1

40.2 k

5 V

VOUT_OPA

VOUT_INA

R2

200 k

C2 1 nF

+

Copyright © 2017, Texas Instruments Incorporated

13

OPA333-Q1

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SBOS522A – JUNE 2010 – REVISED NOVEMBER 2019

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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-Q1 facilitate excellent dc accuracy for the

circuit. Figure 24 shows the calibrated error for the entire range of the circuit.

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, VIN, RSET, and the

instrumentation amplifier (INA) gain. During operation, the input voltage divided by the INA gain appears across

the set resistor in Equation 1:

VSET = VIN / GINA

(1)

The current through RSET must flow through the load, so IOUT is VSET / RSET × IOUT remains a well-regulated

current as long as the total voltage across RSET and RLOAD 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 (VSET) is the input

voltage divided by the INA gain (that is, VSET = 1 V / 10 = 0.1 V). The current is determined by VSET and RSET

shown in Equation 2:

IOUT = VSET / RSET = 0.1 V / 100 kΩ = 1 μA

(2)

A detailed error analysis, design procedure, and additional measured results are given in the Low-Level V-to-I

Converter reference design.