CIRCUIT DESIGN

# Voltage-To-Current Converters

## Overview

Voltage-to-current converters are circuits which convert an input voltage into an output current. The output current is usually proportional to the input voltage, over some suitable range.

Many of these circuit designs use op-amps. Please see the Op-Amp page for more general information on operational amplifiers.

## Howland Current Source

The Howland current source is an op-amp based voltage-to-current converter.

For the circuit to work correctly, three pairs of resistors have to be equal:

$$R_1 = R_3, R_2 = R_4, R_S = R_{S'}$$

Given the resistor equations above are satisfied, the load current is given by the following equation:

$$I_{LOAD} = \frac{V_{IN}}{R_S}(\frac{R_2 + R_S}{R_1})$$

The output impedance is given by the equation (note that this does not depend on the above resistor equality equations to be satisfied):

$$Z_{OUT} = \frac{R_3 R_{S'}(R_1 + R_2)}{R_1 (R_4 + R_S) - R_3 (R_2 + R_{S'})}$$

The Howland current source is commonly used to build a 4-20mA current-loop transmitter.

The XTR111 is a current-loop transmitter by Texas Instruments. It can drive grounded loads. It requires both an external BJT and MOSFET. As of May 2016, this IC was US$1.28 (100). ## XTR117 The XTR117 is a current-loop transmitter by Texas Instruments. It cannot drive grounded loads. It only requires an external MOSFET. As of May 2016, this IC was US$1.77 (100).

DigiKey stocks a number of pre-built voltage-to-current converter ICs. First, navigate to the following component sub-section on www.digikey.com:

Product Index -> Integrated Circuits (ICs) -> Interface - Sensor and Detector Interfaces.

Then select the filtering options:

• Type: Current Transmitter
• Input Type: Voltage