Optical Isolation amplifier (2)

[Godknows]

I have a question and this time it is about the cascaded system (V-F converter + optocoupler + F-V converter + a noninverting amplifier - gain of 2, + a lowpass filter).

This is what I had when I applied dc offset to my system so as to check its linearity:

However, when I included my ac signal, I had the following response:

The above frequency response should resemble a low pass filter. I do not know why I am having such response. Thanks in advance for your feedback

Circuit diagram for the above frequency response:

 

[moffy]

Why don't you try a PWM circuit instead? It would be very linear when modulated.

 

[Godknows]

Thanks. But what is wrong with this configuration? They work respectively. The only challenge now is that I was unable to make them work together. That is where I need assistance with at the moment. Thanks in advance.

 

[Godknows]

What if you show me the sort of PWM circuit that may work

 

[moffy]

Have a look at: https://www.ti.com/lit/ds/symlink/ne555.pdf and the pulse width modulation circuit. I would use a current source instead 0f a resistor to drive the cap to give linear response. If you use a 556 then the other 555 can be used to generate the clock.

 

[Tony Stewart]

There is insufficient debug info with your results.

Connect both circuits and verify transfer function of each component.

Does V>F work?
Is pulse sufficient into F>V ?
Does F>V work?

 

[Godknows]

Thanks. But the main reason why I am using the V-F is for the linearity so that the optocoupler's nonlinearity doesn't affect the signal

 

[Godknows]

These were the individual testing:

optocoupler's response:

Thanks in advance

 

[Godknows]

Also, how do you calculate the threshold voltage of the stand-alone V-F converter? Is the cutoff frequency for it calculated theoretically this way: fc=1/2*pi*Rt*Ct ?

Is it true that the circuit could work for very low frequencies? It seems not by the response of this circuit. I did not just know why I got that kind of response.

The V-F circuit [of the LM331 - https://www.ti.com/lit/ds/symlink/lm331.pdf] on the datasheet suggests that its linearity ranges from 10Hz to 10KHz. How can I adjust it so that its linearity ranges from at the least dc to 250Hz?

Thanks

 

[Godknows]

Thanks in advance.

 

[moffy]

Have a look at: https://www.ti.com/lit/ds/symlink/ne555.pdf and the pulse width modulation circuit. I would use a current source instead 0f a resistor to drive the cap to give linear response. If you use a 556 then the other 555 can be used to generate the clock.

Still struggling with the V->F and F->V. The PWM solution is much simpler.

 

[ronsimpson]

I made a PCB for this isolation PCB but have not put the parts on the board yet. I think it does what you want in one IC.

 

[Godknows]

Thank you very much for your swift and helpful response. I have been able to cascade the VFC&FVC (LM331) together and got:

The above cutoff frequency (25Hz) is too small compare to what I need at the time. The cutoff frequency should be up to 100Hz minimum. What component can I alter in order for me to get this cutoff frequency?
Page 9 and 11 are the location of the circuit diagram.
Thanks in advance!