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:
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Things you need
- Current signal specification
- Voltage requirement specification
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.
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.
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.
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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