# Piezo into 0-5v ADC, with offset center voltage?

I want my piezos to have a center voltage at 2.5v for a 0-5v ADC input on Atmega328, so it can sense force in both ways - one way would be >2.5v and the other will be <2.5v.

Will this circuit work? Or any improvments?

Thank you.

• It probably won't work as the voltage buffer will just eat all the charge of the piezos, or are those resistors meant to be 1M$\Omega$? (as in megaohm, in LTSpice M/m is used as milli and meg is for mega and the circuit looks very LTSpiceish) – Arsenal Sep 2 '15 at 15:10

Slight improvement (less power consumption and fewer parts):

simulate this circuit – Schematic created using CircuitLab

U1 has to be very low input bias current, of course. You might want to add some series resistors to the non-inverting inputs in case the piezo voltage exceeds 2.5V.

• Thank you for the ciruit. This way I can use 4 opamps for 4 piezos. I have TL084, and it says low imput bias current in datasheet. Btw, if I only have +5v powering the opamp, the outputs would never exceed 5v? – Zilaihong Sep 2 '15 at 15:48
• TL084 is a poor choice for this- something with rail-to-rail input and output would be better. Also that is recommended to run from a single 5V supply. For example, the inexpensive MCP6L04. I recommend the series resistor to keep the op-amp protection networks from conducting too much current, but the output should not exceed the supply voltages even under some abuse at the inputs. – Spehro Pefhany Sep 2 '15 at 16:07
• Sorry for the noob question, but how (and how much R) do I add the series resistors? And is there any suitable opamp in DIP packaging? Thank you. – Zilaihong Sep 2 '15 at 16:20
• Enough but not too much. ;-) Maybe 10K. Check out MCP6004. – Spehro Pefhany Sep 2 '15 at 16:24

If the resistors R3 and R4 are 1MΩ( in LTspice; 1m = 1 milli Ω , 1M = 1 mega Ω ) your circuit will work. For some additional improvements, you can take a look at the frequency response of your circuit. To do that you will need to change your model a little bit and consider some additional capacitances. Take a lot here for a very intuitive guide. Hope it helps!

• In LTspice: 1m = 1M = 1 milli, the latest version will automatically convert 1M to 1m to be less confusing. 1meg = 1Meg = 1 Mega. (I just tried because you made me distrust myself) – Arsenal Sep 2 '15 at 15:39
• oh yes I mean 1 mega ohm. But as I type in 1M, it automaticly change to 1m after I hit the ok button, how exactly I enter 1M? And I will be using this sensor for manual squeezes so it would be mostly VERY slow AC signals. Thank you for the link, I will have a look at it! – Zilaihong Sep 2 '15 at 15:43
• haha it seems you are right, I just tried. @Arsenal – Zilaihong Sep 2 '15 at 15:50