Voltage to Current Converter using Op Amp

Hi friends, in this post we will see how to convert voltage into current using simple circuitry. In most of the cases we get the output of measuring devices in the form of voltage. It is not good to transmit this output voltage to the destination directly. Due to addition of noise and wire impedance the output voltage may get corrupted. So in such cases we have convert that voltage into current form. So let us see voltage to current converter.

Voltage to Current Converter using Op Amp

Following circuit shows the voltage to current converter using operational amplifier. It consist of simple resistance connected to the inverting and non inverting terminals of op amp.

Voltage to Current Converter
Voltage to Current Converter

In this circuit the load is grounded and the current through the load can be calculated as follows.

I_{1}=\left (\frac{V_{in}-V_{2}}{R} \right )

I_{2}=\left (\frac{V_{o}-V_{2}}{R} \right )

The current through the load is given by,

I_{L}=I_{1}+I_{2}=\left (\frac{V_{in}-V_{2}}{R} \right )+\left (\frac{V_{o}-V_{2}}{R} \right ) =\frac{V_{in}+V_{0}-2V_{2}}{R}

The gain of the amplifier is

\left(1+\frac{R}{R}\right)=2

So

V_{0}= 2V_{2}

Substituting this value in above equation we get,

I_{L}=\left (\frac{V_{in}}{R} \right )

Thus the current is directly proportional to the applied voltage and the resistance used in the circuit. it should be noted that all the resistances used in the circuit are equal to R.

See alos: Current to Voltage Converter

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Signal Conditioning Circuits for RTD

Hello friends, today we will see Signal Conditioning Circuits for RTD. We know that sensitivity of RTD depends on the temperature resistance coefficient of the metal used for RTD. The value of this coefficient is very small and thus RTD requires the amplifying circuit which is the first signal conditioning circuit for RTD which we are going to see in next post.

Linearization of RTD:

Another signal conditioning circuit required for RTD is linearization circuit. We know that temperature vs resistance curve of RTD is non linear and therefore for wide range of measurements we need to use Linearization circuit for RTD.

Signal Conditioning Circuits for RTD
Linearization of RTD

See also: How to Linearize RTD Output.

Lead resistance elimination:

We know that RTD is a low resistance device that means it has a very small range of resistance and therefore there should not be any lead resistance effect on the output of RTD i.e. Lead wire resistances should not be added with the RTD resistance. Therefore another signal condition circuit for RTD includes lead resistance elimination.

Sensor fault detection is also one of the important signal conditioning circuits for RTD. Sometime due to corrosion of connecting leads RTD may get opened. In such cases signal conditioners may indicate some finite voltages. So to avoid these wrong readings we have to design a signal conditioning circuit for RTD.

In short There are four main signal conditioning circuits for RTD as follows:

  • Bridge amplifier
  • Linearization circuit for RTD output
  • Lead resistance elimination
  • Sensor fault detection circuit

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Signal Conditioning Circuits

We know that signal conditioning is a process in which signals from different sensors are transferred into a form necessary to interface with other modules of system.

For example, we know that thermocouple produces very low output voltage and this voltage is not sufficient to operate the other controlling modules. Therefore there is need to amplify such signals. For this purpose we use different signal conditioning circuits. In case of thermocouple, we have to use amplifier, linearization circuits, etc. the purpose of using linearization circuits is that, thermocouple has non linear characteristics but in most of the cases we need linear controlling action.

Signal conditioning circuit
Measurement System Block Diagram

 

Signal Conditioning Circuits:

There are different types of signal conditioning operations such as amplification, filtering, isolation, linearization, excitation, etc. we will discuss all these operation one by one.

Amplification

We know that most of the sensors produce output in the form of change in resistance, voltage or current. All these parameters are having very low strength i.e. very small voltage in case of thermocouple, small change in resistance in case of RTD, etc. Therefore we have use current or voltage amplifiers in case of sensors which produces output in the form of current or voltage.

If the sensor produces output in the form of change in resistance (such as resistance thermometer) we have to use bridge amplifiers. We can make use of operational amplifiers to amplify the signal.

Filtering

Another important signal conditioning circuit is filter. As mentioned earlier most of the sensor produces very low output and therefore electromagnetic noise may get added in the original output. To remove the electromagnetic noise from sensor output we have to use different filter circuits. Filter circuits eliminates noise i.e. undesired frequency components from original signal without affecting it.

Active filters, passive filters, bypass filters are the common types of filter circuits.

Isolation

Isolation circuits are required to differentiate signals from unwanted common mode voltages. Another advantage of isolation circuit is that, it protects measuring devices (sensors) if high voltage is applied to other circuit. It also breaks ground loops.

Linearization

There are many sensors which produces non linear output such as thermocouple, thermistor, etc. linearization circuits are used to convert non linear signal into linear one. It can be achieved by varying the gain of an amplifier as a function of input signal.

Excitation

Another signal conditioning operation is current or voltage excitation. Signal conditioning circuits provide the required voltage or current excitation to some passive sensors such as strain gauge, RTD, etc.

 

In the upcoming posts we will see signal conditioning of RTD, thermistor and thermocouple.

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