Design a system to measure the electroretinogram (ERG)

[benderlee]

Homework Statement

Design a system to measure the electroretinogram (ERG). Use a differential amplifier to maximize the CMRR, and condition the signal appropriately for a 16-bit A/D system with input range ±10 Volts. Give all component values.

Suggest an appropriate sampling rate, assuming that all noise has been sufficiently attenuated by the time the signal reaches the digitizer, and give the resolution of the signal in V/bit at the digitizer.


ERG bandwidth= 0-300 Hz

ERG dynamic range = ± 1 mV

Relevant Equations

(See attached image)

 

[berkeman]

Welcome to the PF.

Thanks for posting some of the relevant equations. Now please show us your work on the solution. Also, what do the sensors for an ERG look like, and where are they placed?

 

[benderlee]

I am truly stumped by this one, looking for more information online myself and was seeing if anyone in this great community knows anything about this, so far I found this:

https://webvision.med.utah.edu/book/electrophysiology/the-electroretinogram-erg/

 

[berkeman]

I am truly stumped by this one, looking for more information online myself and was seeing if anyone in this great community knows anything about this, so far I found this:

https://webvision.med.utah.edu/book/electrophysiology/the-electroretinogram-erg/

That's a helpful link. So it looks like one ERG electrode is placed on the cornea, and the other is attached to the patient's body for reference.

So start with the desired sampling rate (how is it based on the signal bandwidth?), and design the differential amplifier circuit and other gain elements and filter elements next. Are you familiar with designing digitizing systems, including anti-alias filters? That is all part of this problem you are working on.

You can also probably do some reading about ECG and EEG measurement systems, since they will share a lot of the same design components (they will have different bandwidth and pickup voltages, but the rest is pretty similar).

 

[benderlee]

So this is how the actual circuit would be set up

 

[berkeman]

So this is how the actual circuit would be set up

Did you draw that, or find it somewhere? A differential amp has two signal inputs, not three.

 

[benderlee]

This is a diagram from a textbook - attached is another diagram from another text. that is more or less the same with a ground, reference, and corneal electrode:

 

[benderlee]

Also FYI: in the ERG, the differential amp records the difference between the active electrode (attached to the cornea) and the reference electrode (attached to the outer canthus). The ground electrode is attached to the forehead.

 

[berkeman]

This is a diagram from a textbook

That's not a very accurate or informative diagram, unfortunately. Hopefully you are learning about differential amplifier design for biomedical applications from a better and more detailed textbook or application notes.

Also FYI: in the ERG, the differential amp records the difference between the active electrode (attached to the cornea) and the reference electrode (attached to the outer canthus). The ground electrode is attached to the forehead.

So the diff amp inputs should come from closely spaced electrodes at the eye, and the "ground"/reference connection comes from some other large part of the body nearby. The "ground"/reference connection goes into the overall amplifier circuit, not into the diff amp itself.

So show us what you have figured out so far. What resistor values have you chosen for your diffamp and why? What filter topology and component values have you chosen for your anti-alias filter and why?

What sample rate have you chosen for your ADC and why?