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Topic: Trying to adapt a schematics with single power opamps to dual power ones (Read 878 times) previous topic - next topic

amundsen

Hello,

I am trying to build this circuit, coming from an article about gesture controllers for music (http://cnmat.berkeley.edu/system/files/attachments/currentmodefinal3.pdf)

This circuit is supposed to allow one to get simultaneously x, y and z values  for a 4-wire resistive touch-screen (actually the author uses a trackpad but it functions with the same resistive principle).

Here's the description of the circuit: "The solution developed by the author is to provide a small constant current through the touch pad array inverting op-amplifier shown in Figure 8. This is similar to the approach Moog employed on his 926 duo keyboard. The inverting input is a virtual ground at the same potential as the non-inverting input. The current through the pad resistors is established by two constants in the circuit, the summing node input resistor R and the source voltage reference applied to one leg of this resistor. The three measurands (XYZ) are computed by subtraction of the three op-amp outputs. The op-amps should be selected to have low input offset voltage. External noise sources are controlled by limiting the op-amp bandwidth with a low value capacitor from its output to the summing node (C)."

There is a raw schematics in the article, duplicated here in the first picture.

I have good op-amps at hand (AD620) but they need dual power supply while those shown in the schematics use a single supply. I have adapted the schematics to those op-amps with the addition of a special IC providing dual supply from a single one I do also have at hand (Maxim 680), which should allow me to connect the circuit to an Arduino.

However I am not sure my adaptation is correct. Can someone give an opinion about it (please see second picture) ?

Thank you in advance.


MarkT

The problem with dual supply is that your opamps can then push -5V into the Arduino and fry it (I'm assuming -Vs = -5V?) - best not to do things this way if you can, you'll need to add protection resistors on the outputs (10k perhaps) to avoid burning out the Arduino inputs.

Its hard to compare the two circuits since one is a nice circuit diagram and the other a wiring diagram.

Using some low voltage rail-to-rail opamps would be better I reckon (certainly less complexity in power supply, but you might need to generate a 2.5V signal ground).
[ I DO NOT respond to personal messages, I WILL delete them unread, use the forum please ]

Docedison

The pencil drawing used bi-polar power supplies... The Grounded + input will give you some problems trying to realize that circuit... one other point... the symbols you use for the op-amps are grossly incorrect... use the right ones, they were created for a reason. Rather than being able to see at a glance what is intended your drawing has to be interpreted. and much was lost in your interpretation.
If your schematic tool won't reproduce the correct symbol, find one that will. IMO

Doc
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