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Debugging differential charge amplifier

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magic:

--- Quote from: SethGI on April 05, 2020, 10:21:39 pm ---Huh, I'll look into that. So if I were to substitute for an opamp with much lower bias current (i.e. TI TLV4120 w./ 0.3pA input bias) could I get away with minimal other changes?

--- End quote ---
That sounds like a CMOS device so have a look if noise specs are satisfactory, particularly at lower frequencies if you care about them. If you need less noise, JFET opamps could deliver that.

duak:
Here's a link to a design from Analog Devices: https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0350.pdf

I'm not privy to the transducer's specs or the overall performance targets so it's difficult to give detailed design advice.  I don't know if there are other devices that can utilize the same footprint.  It should be possible to kludge in an alternate device to test out changes so the PCB doesn't have to be respun right away.

Without knowing what the application is, I'll bet that any circuit that operates on only 3.3 V without some transient input overload protection could be overloaded.  I don't think a piezo can generate enough current to damage the input device, but a sufficiently large transient could cause the circuit to block up and take some time for recovery.

SethGI:
Thank you all so much for the help... this forum never ceases to amaze me.


--- Quote from: duak on April 06, 2020, 04:00:03 pm ---Here's a link to a design from Analog Devices: https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0350.pdf

I'm not privy to the transducer's specs or the overall performance targets so it's difficult to give detailed design advice.  I don't know if there are other devices that can utilize the same footprint.  It should be possible to kludge in an alternate device to test out changes so the PCB doesn't have to be respun right away.

Without knowing what the application is, I'll bet that any circuit that operates on only 3.3 V without some transient input overload protection could be overloaded.  I don't think a piezo can generate enough current to damage the input device, but a sufficiently large transient could cause the circuit to block up and take some time for recovery.


--- End quote ---

I also didn't think the piezo could reasonably damage it, and we're taking lots of steps to isolate this board (it's in it's own enclosure on it's own power supply). So, I'll look into transient protection on a future revision of the board, but for now I'll leave it as is.


--- Quote from: magic on April 06, 2020, 06:00:52 am ---
--- Quote from: SethGI on April 05, 2020, 10:21:39 pm ---Huh, I'll look into that. So if I were to substitute for an opamp with much lower bias current (i.e. TI TLV4120 w./ 0.3pA input bias) could I get away with minimal other changes?

--- End quote ---
That sounds like a CMOS device so have a look if noise specs are satisfactory, particularly at lower frequencies if you care about them. If you need less noise, JFET opamps could deliver that.

--- End quote ---

Does something like http://www.ti.com/lit/ds/symlink/ths4121.pdf seem like a good choice. It is a drop-in relacement (same footprint) so if I could get away with that, it'd be ideal. I have to respin the board at some point to fix a few digital hiccups, but this is the only 'breaking' issue - everything else can be easily fixed by hand.

Thanks again!


duak:
SethGI, The THS4121 has a much lower input bias current but it also has a minimum Common mode Input Voltage of 0.65 V (worst case - see p.4).  It could be made to work if the transducer was raised off DC GND by a volt or so - see the Analog Devices ap note.



Henrik_V:
My comments:
You want to use a charge amplifier (CA) , because your source is a charge source, and the charge amplifier is a virtual short (ideal 0\$\Omega\$) , so connecting cable capacity influence is minimized. You want the inner conductor to be at the same potential than the shield and any charge produced by the piezo should not build up a voltage, instead it is sucked by the CA.
If you use a coaxial connection (SE) and connect the shield to GND (makes sense ;) ) AND connect a charge amplifier , that amplifier absolutely needs a bipolar supply (or needs to be decoupled by an capacitor >>C_f ;) ). 

SE to Diff in the second stage...

The value of R2 (noise source) is questionable ....  again, the input  impedance of a CA is 0\$\Omega\$ (ideal)  , in reality I measured values between 10 - 600\$\Omega\$ for lab grade commercial CAs.     A protection current limiting resistor (if needed) should be directly at the OP input (in the loop) ...
And stop thinking with voltages ... think about some fractions of pC charge  ... (isolation , guarding, cleaning ... ) until you hit the inverting input of the OP.
And remember C2 (and C1) are prone to vibration pickup ... (and other caps on the board can act as piezo actors ....)   

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