An Easy Circuit For Use With LVDTs

Here is a little circuitry you can use to experiment with LVDTs. Don't be put off by the number of op-amps. They come four on a chip, and a chip costs less than a dollar.

The circuit functions by taking the absolute value of each LVDT secondary separately. This is done by a clever op-amp and diode combination (not my design, unfortunately) that uses the op-amp to eliminate the forward voltage drop of the diode. 

The two absolute value signals are combined. Notice that the diodes are reversed between the two circuits. One circuit takes the absolute value while the other takes the negative of the absolute value. When combined, the two signals are effectively subtracted. The top right op-amp performs this function plus providing a little gain.

The combined signal is passed through a passive low pass filter (the 3.3K resistors and .1 ufd capacitors) before being further amplified and offset. The SPAN pot provides a degree of gain adjustment while the OFFSET allows you to shift the zero point.

And How Well Does It Work?

For exact performance (whatever that means) the 10K resistors in the absolute value circuits should all be the same value. The 20K resistors should be precisely twice the value of the 10K resistors. It's the ratio that is important rather more so than the actual value.

 So, if you happened to have a supply of super-exact 12K resistors, you could use them in place of the 10Ks, and replace the 20Ks with two 12Ks in series.

I didn't do that.

I purposely used regular tolerance (5%) carbon film resistors to see what the effect would be. I know by using an oscilloscope that the absolute value circuits are not balanced quite right.

Nonetheless, I'm getting quite reasonable performance. Using the roughly made LVDT described on the LVDT page, I'm seeing a peak deviation from linearity of about 2.6% of full scale. No parts selection, no balance adjustment and a LVDT already known to be a little short of perfect.

So, if YOU build an LVDT and take care to position the windings evenly, and use an ohm meter to select the 10K and 20K resistors, you should expect to see linearity better (perhaps much better) than 1% of full scale.

Signal Source

The LVDT should be excited with 2 or 3 volts at about 1 kHz. If you don't have a signal source, here's an easy one to build. This is a buffered state variable oscillator. Once again, op-amps are cheap...

If you are experimenting with more than one LVDT at a time, simply duplicate the buffer and connect it to the same node as the first.

This circuit can run off the same voltage regulators as the first circuit.

  1 Oct 2003

It's possible that I'm not as smart as I think I am. (Occasionally, I have moments when I know this to be true. Fortunately the feeling passes quickly.) Although I have tried to make this information as accurate as I can, it is not only possible, but also quite likely, that errors lurk within. I cannot and do not warrant these pages to be error free and correct. Further I accept no liability for the use of this information (or misinformation). If, after reading this, you are still interested, please be aware that the contents of this site are protected by copyright (copyright © 2002, 2003, 2004, 2005, 2006, 2007, 2008 by John M. Powell). Nonetheless, you may copy this material subject to these three conditions: (1) the copyright notice is copied and presented along with the material, (2) the copy is used for non-commercial purposes, and (3) the source of the material is properly credited. And of course, you may link to this page.