Electronics > Projects, Designs, and Technical Stuff
Debugging differential charge amplifier
SethGI:
Henrik,
Thanks for this, it's super helpful. I just want to clarify one thing on the schematic. What is that V12 doing? To me it looks like a voltage source (1.6V) shorted straight to ground, but I'm likely misreading it (on account of being a novice).
Also, what is Aol? Sorry, I'm a bit out of my depth here... thanks for all the help!
--- Quote from: Henrik_V on April 07, 2020, 09:04:09 am ---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 ....)
--- End quote ---
ehughes:
Why are you connecting one side of the hydrophone to circuit common? I think you might be affecting the DC condition on on side of the charge amplifier.
If you are doing fully differentlial, the GND connection is not needed and may be a counter productive.
This circuit (ignore values) has worked well for me and I have used it with a piezo sensor and a CS5366 ADC.
SethGI:
--- Quote from: ehughes on April 07, 2020, 03:00:39 pm ---Why are you connecting one side of the hydrophone to circuit common? I think you might be affecting the DC condition on on side of the charge amplifier.
If you are doing fully differentlial, the GND connection is not needed and may be a counter productive.
This circuit (ignore values) has worked well for me and I have used it with a piezo sensor and a CS5366 ADC.
--- End quote ---
Yeah, you're totally right about that. We're getting ready to do a new version without the GND connection (just need to prototype first). Was that the only stage you had, or was there more? Just for my reference. Also, how was noise performance?
Henrik_V:
--- Quote from: SethGI on April 07, 2020, 02:49:13 pm --- What is that V12 doing? To me it looks like a voltage source (1.6V) shorted straight to ground, but I'm likely misreading it (on account of being a novice).
Also, what is Aol? Sorry, I'm a bit out of my depth here... thanks for all the help!
--- End quote ---
V12 is your Vcc/2 .. , but if you have your powersupply onboard why not have a bipolar supply for the input amp?
Aol : (DC) open loop gain of the OP
Henrik_V:
--- Quote from: ehughes on April 07, 2020, 03:00:39 pm ---Why are you connecting one side of the hydrophone to circuit common? I think you might be affecting the DC condition on on side of the charge amplifier.
If you are doing fully differentlial, the GND connection is not needed and may be a counter productive.
This circuit (ignore values) has worked well for me and I have used it with a piezo sensor and a CS5366 ADC.
--- End quote ---
Should work, but your cable runs at VMID potential, prone to cable microphony or triboelectric effects. If your cable is moving/vibrating you quickly get unwanted charge signals ... can be annoying, depends on the application.
Navigation
[0] Message Index
[*] Previous page
Go to full version