| Electronics > Projects, Designs, and Technical Stuff |
| Charge Amplifier - Spice vs. App Note |
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| geo_leeman:
Hey folks - I'm messing with a charge amplifier design and found a great app note by TI: https://www.ti.com/lit/an/sloa033a/sloa033a.pdf Within it has some simple formula for the lower/upper rolloff points for the gain. (See attached). I made up a quick spreadsheet and modeled the circuit in LTSpice (granted, with a different opamp, but with high GBP and FET input) and my numbers don't quite agree. The lower point is close, but the higher side is way way off. I've tried with several different setups and always get this result. Am I missing something fundamental, am I mental, or a combination? |
| moffy:
Your input high pass filter (C2 & R2) has a 3db pt of about 36Hz. Your low pass filter/integrator(C1 & R1) has a 3db pt of about 14mHz. So what you are seeing is correct. Also the ratio of C2/C1 sets the gain in the flat region for these values. |
| sdouble:
remove your serie resistance and add a cap to ground to mimic your sensor cap. your feedback cap is huge meaning that the sensitivity of your apparatus will be rather low. |
| geo_leeman:
I've added a cap to ground and used a current source instead so now it should be *exactly* like the schematic in the app note, but I'm not getting anything that makes much more sense. The series resistor is in the schematic as Ri. See Attached. |
| SiliconWizard:
I think a correct way to model the charge source would be to use an arbitrary behavioral current source instead, and set the function to the derivative of a voltage source modeling the charge Q(t). The current is obviously the derivative of the charge. The derivative of a function such as: A*sin(2*pi*F*t+phi) is: 2*pi*A*F*cos(2*pi*F*t+phi). You instantly see that it's not going to be just a scaled version of the original, but its amplitude grows as the frequency grows. An AC analysis with a basic current source will thus not give you the same result! Attached is an LTSpice schematic of what I mean. It looks like it gives a frequency response pretty close to what's in the app note. (With your values: Flow ~ 0.145 Hz, Fhigh ~ 361 Hz.) Edit: added a picture for quick preview. |
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