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I need help on a circuit I've designed to detect hits on a piezo sensor on a drum. It uses an opamp with 2 channel, on goes to a schmitt trigger, the other to the ADC input on the XMEGA microcontroller. Once the interrupt for the schmitt trigger is raised, the reading is done on the ADC and then a reset pin activate a transitor that flushes the excess signal.

Schematic

All of it works super fine as expected but there one little catch: I'd like to increase the sensivity of the schmitt trigger by a fixed value. The opamp I'm using to feed the schmitt trigger gets maxed out when I apply the maximum gain on it. On the side of the ADC channel, all is good, the signal is within good range (about 0V-4V).

Now here is the question: is there a way I can use this exact(ish) circuit and maybe add a resistor or 2 somewhere or remove one to bias the input of the schmitt trigger of let's say +1-2V? Since the threshold of this particular schmitt trigger seems to be about 2.2v when Vcc is at 5v but I want to detect signal to about 1-1.5v as well.

My background is software/firmware development so I'm a bit lost on what to do to solve this problem without having to redo the whole things (which works fine but isn't sensitive quite enough to my taste)

Schmitt trigger used is: SN74LVC1G17 Opamp used is: ADA4075

Here is what the board looks like (it if may be of any help), so there is room to solder jumpers or resistors around without having to older a new set of board:

enter image description here

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  • \$\begingroup\$ Consider making R18 smaller. \$\endgroup\$ – analogsystemsrf Aug 8 '18 at 3:45
  • \$\begingroup\$ The SN74LVC1G17 is non-inverting buffer, while the schematics shows it as inverting. Is this a misprint, or else? \$\endgroup\$ – Ale..chenski Aug 8 '18 at 3:59
  • \$\begingroup\$ Yes, that's the closest symbol I could I on kiCad. The part is really what the datasheet for the SN74LVC1G17 states. \$\endgroup\$ – Kévin Isabelle Aug 8 '18 at 4:01
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    \$\begingroup\$ Please make sure the schematic match's the parts used, else it really throws us a curve ball and we waste our time going in the wrong direction. \$\endgroup\$ – Sparky256 Aug 8 '18 at 4:07
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    \$\begingroup\$ You should be using correct symbols when asking the question, it is misleading. If you want to detect the beginning of your signal as fast as possible, you probably should use some sort of rectifier, before hitting the Schmitt trigger. \$\endgroup\$ – Ale..chenski Aug 8 '18 at 4:08
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If you can make sure the output signal from the OP-Amp is never higher than e.g. 3V, you could lower the supply voltage of the schmitt trigger gate to 3V with a voltage divider of two resistors. This lowers the switching levels.

Caveat: the output swing is also 3V only then, but it should still work with a +5V µC.

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  • \$\begingroup\$ Yes, 3 volts should trigger my 3.3v XMEGA input pin pretty well. Follow up question: what happens if I supply the schmitt trigger with around 3V and the input goes to 5v? Is there any danger for the chip? \$\endgroup\$ – Kévin Isabelle Aug 8 '18 at 3:28
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    \$\begingroup\$ The protection diode on the input will become conductive. If your voltage divider on Vdd is reasonably high-ohmic, that's not going to kill the chip. \$\endgroup\$ – Janka Aug 8 '18 at 3:31
  • \$\begingroup\$ Ok, so I use a pot to try it out, I'd better use something around 100k right? \$\endgroup\$ – Kévin Isabelle Aug 8 '18 at 3:37
  • \$\begingroup\$ Anything above 10kΩ should be fine. It depends on the current you expect on the output. Driving a CMOS µC input should never be a problem. Put a 10kΩ resistor between the pot and +5V to be on the safe side. \$\endgroup\$ – Janka Aug 8 '18 at 9:35
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Your main restriction to getting better performance is the ADA4075 and its power rails. The data sheet suggests it can't be used on power rails below +/-4.5 volts and you have it on +5 volts and ground i.e. you are not powering the thing correctly. May I suggest that if all you have is a 5 volt rail, you use a 5 volt rail-to-rail device.

Even if the power requirements are met, you also have a further issue and that is the common-mode input voltage range. For the ADA4075 it is specified as +/-12.5 volts when powered at +/-15 volts and this means that you cannot rely on signals that are within 2.5 volts of the power rails you apply. Given that you only have a 5 volt rail this is a serious nail in the coffin for using this op-amp.

And a further nail is the output voltage range. This is specified as typically +13 volts and -14 volts (on a +/-15 volt rail) so there are potential problems here too.

May I also suggest that after you change the op-amp you get rid of the Schmitt trigger and use the "proper" op-amp as a comparator with variable threshold level and variable hysteresis.

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  • \$\begingroup\$ Thanks for the tips Andy! I'll check into other parts meetings thoses specs. \$\endgroup\$ – Kévin Isabelle Aug 8 '18 at 19:19

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