Opamp circuits for active sensors

 

While making all the projects, I've been using a lot of different opamp circuits and actually the basic concept of the circuits used, is often the "same".  In this article I will point out the most important opamp circuits used in combination with active sensors. These active sensors, like for example the acceleration sensor, produce a tiny DC-voltage change, which has to be amplified and tweaked.

Some reminders:
Opamp stands for 'Operational Amplifier' and is an ideal building block in electronics:
 1. It has a very high input impedance (resistance): so it doesn't take any current into it's input.
 2. It has a very low output impedance; so it can deliver a relative strong output current.
 3. The amplification (Au) is very, very high.

 4. In the circuits below, the powersupply of the opamps is not drawn. If you make use of an opamp, you will always need a powersupply to 'feed' the opamp. Most of the times, especially in audio world, the opamp needs a balanced power supply, meaning +voltage, ground and -voltage. When I work with sensors, I do use  opamps that also work with a single power supply (+voltage and ground). For example: LM358, TLC274, LM324.

For more information about the opamp itself, checkout the wikipedia or equivalent websites.


 Follower or Buffer circuit:

 

follower

This circuit is used to 'buffer' the output of a sensor. Often the sensor is placed far from the actual sensor interface, so long wires have to be connected to it. To avoid loss of the signal (the output of the sensor is not always capable of generating enough current), this circuit can be applied. The output of the sensor is connected to the positive input of the opamp. Because of the feedback from the output to the negative input, the Au (amplification) is 0dB, or 1x. Thus the output signal is not amplified, but just 'follows' the input. The opamp is capable of driving more current and a somehow longer cable will not be a problem. The sensor just has to deliver it's tiny current to the input of the opamp.


 Non-inverting amplifier

 

non inverting

The non inverting amplifier has an amplification which is always more than 1x (>0dB). The proportion between the both resistors Rf (R feedback) and R1 determines the amount of amplification. The output of the sensor is connected to the positive input of the opamp.

In formula: (Uout/Uin) =  Au = (1+ Rf/R1)


Inverting amplifier

 

inverting

 The ampification (Au) with the inverting amplifier only depends of the proportion between Rf (R feedack) and R1. The output is 180 degrees out of fase with the input; it's negative. This circuit is also known as the summation amplifier.

In formula: (Uout/Uin) = Au = - (Rf/R1)

 


Non inverting amplifier with offset adjustment

 

non inverting offset

 A lot of sensors do generate a nice little dc-voltage change, but do have some dc-offset on the output. With the ADXL202 chip for example, the filtered output is "dc-change"  (=AC) in reference to 2.5V.  When the sensor is tilted to one side, the output will be 2V and tilted to the other side 3V. This means a maximum change of 1V. It would be better to have the output swing a big as possible - a higher resolution. In the circuit above, the ampification will be around 2,5 -3 times (around 10dB). With the potentiometer of 10k connected between the plus and minus power, the output dc-offeset can be adjusted to zero.