How to Design a Current to Voltage Converter Using Op-Amp

Written by j.t. barett
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How to Design a Current to Voltage Converter Using Op-Amp
A simple op-amp circuit will convert current to voltage. (electronics 4 image by chrisharvey from Fotolia.com)

While a humble resistor can act as a current-to-voltage converter, it will give you a weak signal. By Ohm's law, a 1-amp current flowing through a 1K resistor gives you a mere millivolt of voltage. A simple op-amp circuit, on the other hand, turns the situation around by providing amplification. With the op-amp, you can turn milliamps of current into several volts. To design an op-amp-based voltage converter, you'll need to know the current being input to the op-amp and the voltage expected from it.

Skill level:
Moderate

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Things you need

  • Current signal specification
  • Voltage requirement specification
  • Calculator
  • Pencil
  • Paper

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Instructions

  1. 1

    Calculate the op-amp's feedback resistor value by dividing the maximum voltage you want from the converter by the maximum current going into the converter.

    Use this formula: Rf = Vo/Ii

    Where "Rf" is the feedback resistor value in ohms, "Vo" is output voltage in volts and "Ii" is input current in amps. For example, if you want a 1-volt output from a 1-milliamp input: 1/.001 = 1,000 ohms.

  2. 2

    Sketch an op-amp symbol on the paper. This is a triangle pointed to the right. The left side has two inputs with minus on the top and plus on the bottom.

  3. 3

    Draw the symbol for your current source to the left of the op-amp. You can use the standard current source symbol: an open circle with an arrow pointing up. Draw a horizontal line from the top of the current source to the op-amp's minus, or inverting input. Draw a short vertical line from the current source to a ground symbol.

  4. 4

    Draw a short vertical line from the op-amp's non-inverting, or plus, input to a ground symbol. Draw a resistor in a feedback path between the op-amp's output and its inverting, or negative, input. Label the resistor with the value you calculated in Step 1.

Tips and warnings

  • The voltage appears between the op-amp's output and ground.

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