Calculate parameters of active high pass and low pass filter given for EOG circuit

The circuit is from here.

The opamps are : OPA2335 (dual channel because of horizontal and vertical electrodes.)

I don't exactly understand how to calculate R and C values based on the 0.5 high pass and 35Hz low pass in these configurations. I'd know how to do it for passive circuits but this just confuses me and the datasheets don't help either.

I don't exactly understand how to calculate R and C values based on the 0.5 high pass and 35Hz low pass based on these configurations. I'd know how to do it for passive circuits but this just confuses me and the datasheets don't help either.

The CR high-pass filter uses an op-amp buffer following it. That buffer does nothing but provide unity gain i.e. it doesn't affect the CR high-pass response and, on its output, it faithfully reproduces the signal from the CR network.

The next stage is a Sallen Key 2nd-order unity-gain low pass filter. There are literally hundreds of web resources on this type of circuit but, the only thing that is unclear from the diagram is the Q factor. You need this to estimate the resistor and capacitor values but, from experience, I would expect Q to be between 0.5 and 0.8.

Try this Sallen key calculator: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm

• How do I calculate the R & C values, does any pair work, or is the selection dependent on other factors? Jun 20, 2023 at 14:36
• Any idea what the purpose of the two potentiometers are for the second amplifier and the R & C values for the active feedback circuit? Jun 20, 2023 at 14:37
• @Roshan use the link in the answer and play around with values and, if you scroll down the link the site produces bode plots for each run you perform. Generally, try and keep resistor values between 1 kohm and 1 Mohm. Keep capacitor values above 10 pF and below 10 uF as a general rule. Jun 20, 2023 at 15:20
• I believe those potentiometers adjust signal gain and DC offset. You might want to raise a new question for the R and C you talk about. I have half an idea but not really enough and, other folk's answers might be revealing. Jun 20, 2023 at 15:23
• @Roshan if we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. Jun 20, 2023 at 15:24