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

Written by j.t. barett
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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

### Things you need

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

## 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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