Replicate Bode plot charge amp from article

crutschow

Joined Mar 14, 2008
34,045
The two look reasonably close, with the difference likely due to the model characteristics of the op amp you used.
The reference plot may have been done with an ideal op amp.

Do your sim with an ideal op amp in LTspice and see what you get.
 

Audioguru again

Joined Oct 21, 2019
6,608
You do not show white and black plots.

The orange plot shows a low frequency voltage gain of 120dB which is one million. Its gain is 1 at 1MHz.
The lowpass slope is 20db/octave.
Those plots are for an ordinary old opamp, not for the LTC6240 opamp.

The article is completely WRONG!
The Rf with Cf is not a highpass filter. It is a lowpass filter.
The Rin with Cin is not a lowpass filter. The signal source resistance with Cin is another lowpass filter and Rin with Rf sets the gain of the opamp.
 

Thread Starter

scottc19

Joined Nov 8, 2023
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ci139

Joined Jul 11, 2016
1,898
the CMOS inp.-stg. op.-amp.-s have usually quite large Vio - also see "Gain Bandwidth and Phase Margin vs Supply Voltage" diagram and the Fig.4 + consider appearing at the CMM input voltage range for your particular supply value

for LTspice you might try:
never tested https://www.analog.com/media/en/technical-documentation/data-sheets/14923f.pdf
maybe https://www.analog.com/media/en/technical-documentation/data-sheets/111214fb.pdf

PS! -- you might also want to complete your signal source by setting or specifying its parallel resistance
 
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Audioguru again

Joined Oct 21, 2019
6,608
The datasheet for that opamp does not show open-loop frequency response below 10kHz.
The compensation capacitor in opamps usually cuts frequencies above 10Hz.
 

ci139

Joined Jul 11, 2016
1,898
i tried it
aac-pz-inp -- tp.gif
note: below the actual formula for Av is
\[ A_V=-\frac{R_1}{R_2}\left({\frac{R_1}{R_0}+2}\right) \]
so
\[ R_0=\frac{R_1}{-\frac{R_2}{R_1}A_V-2}=\frac{R_1}{\frac{R_2}{R_1}\left|{A_V}\right|-2} \]
(the voltage points \(x_n,a_n\) are actually terminals of likely capacitive piezo elements)
aac-pz-inp -- bp.gif
 
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crutschow

Joined Mar 14, 2008
34,045
that brought it closer @crutschow, still missing the pole near 1.5hz
That1.5Hz rolloff is due to a combination of the GWB and max open-loop gain of the op amp.
If you use the LTspice opamp generic amp and change the gain to 1000k and the GWB to 1M (below), then your plot should be close to your black reference trace.

1699474622257.png
 
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