Author Topic: Charge Amplifier - Spice vs. App Note  (Read 462 times)

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Offline geo_leeman

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Charge Amplifier - Spice vs. App Note
« on: April 22, 2019, 01:10:27 pm »
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?
« Last Edit: April 22, 2019, 01:12:05 pm by geo_leeman »
 

Offline moffy

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Re: Charge Amplifier - Spice vs. App Note
« Reply #1 on: April 22, 2019, 01:44:23 pm »
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.
« Last Edit: April 22, 2019, 02:21:20 pm by moffy »
 

Offline sdouble

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Re: Charge Amplifier - Spice vs. App Note
« Reply #2 on: April 22, 2019, 02:48:21 pm »
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.
 

Offline geo_leeman

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Re: Charge Amplifier - Spice vs. App Note
« Reply #3 on: April 23, 2019, 01:46:53 pm »
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.
 

Offline SiliconWizard

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Re: Charge Amplifier - Spice vs. App Note
« Reply #4 on: April 24, 2019, 01:30:55 am »
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.
« Last Edit: April 24, 2019, 01:40:19 am by SiliconWizard »
 

Offline geo_leeman

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Re: Charge Amplifier - Spice vs. App Note
« Reply #5 on: April 24, 2019, 01:28:20 pm »
That makes sense and I can get the proper output then - the only thing not in alignment now is that the gain should be 40dB and we're seeing around 155dB. Looking at the input to the op-amp we see it's at about 40dB in the 1-100Hz region. So I'm guessing we're still not quite capturing the charge source right?
 

Offline SiliconWizard

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Re: Charge Amplifier - Spice vs. App Note
« Reply #6 on: April 25, 2019, 12:21:00 am »
The gain given in the app note is 1/Cf, which in this case equates to: 20*log(1/22e-9) ~ 153 dB, which is what we get here in simulation.

Just consider it's not a voltage gain per se, it's the gain between the input charge and the output voltage.
 

Offline geo_leeman

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Re: Charge Amplifier - Spice vs. App Note
« Reply #7 on: April 25, 2019, 05:05:35 am »
The gain given in the app note is 1/Cf, which in this case equates to: 20*log(1/22e-9) ~ 153 dB, which is what we get here in simulation.

Just consider it's not a voltage gain per se, it's the gain between the input charge and the output voltage.

Ah, I see. Helpful PDF breadcrumb for myself and others as well: https://www.hamamatsu.com/resources/pdf/ssd/charge_amp_kacc9001e.pdf
 

Offline sdouble

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Re: Charge Amplifier - Spice vs. App Note
« Reply #8 on: April 25, 2019, 06:38:28 am »
your Cc is far too high.
i should be in the (tens to hundreds of) pF range
 

Offline geo_leeman

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Re: Charge Amplifier - Spice vs. App Note
« Reply #9 on: April 25, 2019, 09:10:54 am »
your Cc is far too high.
i should be in the (tens to hundreds of) pF range
Actually assuming Cc to be zero, the other cap is the sensor/transducer capacitance.
 

Offline sdouble

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Re: Charge Amplifier - Spice vs. App Note
« Reply #10 on: May 06, 2019, 07:21:41 am »
your Cc is far too high.
i should be in the (tens to hundreds of) pF range
Actually assuming Cc to be zero, the other cap is the sensor/transducer capacitance.
yep, my bad.
Are you sure that it is of 20 uF ? That seems rather high.
 


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