I have this filter circuit, and I would like to calculate the cutoff frequency as I would like to change a few components. I have tried to calculate the transfer function, but I started to get confused about all the stages; therefore, I am now trying to simulate it using LTspice, although I am not sure why the magnitude starts at -46dB. Is that correct, or am I missing something? Is it correct where I have placed the probe? I am a bit lost as I do not see any difference in the plot if I place the probe at the input or the output.

A little bit of information in regards to the circuit: I have set the source voltage to be 10mV equivalent to the max output voltage at 5V excitation of a 2mV/V sensitivity loadcell and a phase of 90, but I am not even sure that is the right way to do it. As per source resistance, I have used the bridge resistance of 350ohm.

If it can help the ADC input is connected to a CS5530.


The Screenshot does not include the mouse so I have placed a label where the probe is

 

hi flox,
Please post your LTS asc file.

To get the actual dB value the AC should be 1V not 10mV
E

Added: you have the ADC Vouts shorted to 0V.

 

hi flox,
Please post your LTS asc file.

To get the actual dB value the AC should be 1V not 10mV
E

Added: you have the ADC Vouts shorted to 0V.

Thanks Eric,

Could you kindly explain why 1V?
I have attached the .asc for the filter; also, in regards to the ADC Vouts, that is how I have them on my PCB.

Here is my circuit:

 

hi flox,
This is what I see using a 1V AC.
Looks a typical response

E

 

Hi flox,
Which type of ADC are you using?
E

 

Hi flox,
Which type of ADC are you using?
E

Thanks for your help; I have tried, and it looks more like a sensible plot now.

I am using a CS5530 Delta Sigma. The circuit I have at the moment works quite well, and I can easily get 10000 divisions on a 3000 intervals rated cell, although when selecting the components, I have not accounted for the temperature performance of capacitors and resistors and the circuit is a little sensitive to temperature changes; therefore I would like to replace the capacitors that now are X7R with C0G although the largest capacitance in 0805 which I have mounted is 47nF therefore I have to redesign the filter to suit this capacitance.

Any recommendations or ideas are more than welcome!

 

Hi,
A basic temperature run in LTSpice shows very little change in the filter response

What is the L/C measuring?
E

 

Thanks Eric,

Could you kindly explain why 1V?
I have attached the .asc for the filter; also, in regards to the ADC Vouts, that is how I have them on my PCB.

Here is my circuit:

View attachment 309580

The reason the AC source is set to 1 Volt is so the output will be in dB with respect to 1 volt. When you use ANY other value, the output will be in dB with respect to the value of the source.

 

Could you kindly explain why 1V?

The Spice dB measurement is referenced to 1Vac, independent of the source voltage, so 1V output for a 1V input will give 0dB gain, which is what you normally want.

 

The reason the AC source is set to 1 Volt is so the output will be in dB with respect to 1 volt. When you use ANY other value, the output will be in dB with respect to the value of the source.

The Spice dB measurement is referenced to 1Vac, independent of the source voltage, so 1V output for a 1V input will give 0dB gain, which is what you normally want.

Thanks for clarifying this, make more sense now!

Hi,
A basic temperature run in LTSpice shows very little change in the filter response

What is the L/C measuring?
E

As per now is on a test rig with 1kg weight on, I am currently testing drifting cause by temperature, noise and also I want to see if my code is stable and there are no issues with timers rolling over etc.