Our group is desiging an ecg circuit. So my question is, to protect the patient by making an isolation circuit, what didodes should I use?
Also, we are thnking about using diodes to protect the ADC and microcontroller. The voltage range we want is between 0 and 3.3v. which diodes?

 

Diodes do not provide patient isolation! Is your ECG circuit battery powered?

Ken

 

no, but they would help prevent voltage going to the patient. for now, we are using a power supply.

 

Please post a schematic of your circuit. I would like to see where you intend to place these "isolation " diodes.

Ken (former Biomedical Engineering Tech)

 

not my exact circuit. but this is where I plan to place the diodes.

 

here is a design using protection diodes http://www.soa-world.de/echelon/wp-content/uploads/2008/12/elektor.jpg

 

If you are building your own ECG circuit it is imperative that you use batteries and not a power supply powered by AC mains.

 

we are going to switch to battery later anyway, beucase in the big picture, we are making a wireless stethoscope

 

here is a design using protection diodes http://www.soa-world.de/echelon/wp-content/uploads/2008/12/elektor.jpg

do u know why they used those values for the didoes, or does it not really matter?

 

to protect the microcontroller, adc, which diodes should I use?? the adc can handle up to 3.3 volts.

 

Those diodes provide protection to the opamps' inputs, not to the patient. The fact that output common is connected directly to external devices leaves the patient vulnerable to external voltages.

Ken

 

If I were building an ECG amplifier I would use two fresh 9V batteries and skip the voltage regulators.

 

You should not use a power supply unless you have total isolation between it and the patient. This includes, for example, using transformers to supply power to the ECG amplifiers and opto-couplers to pass on the signal for processing after amplification.

Use battery power it's safer!

 

You don't need diodes for protection. And it depends on what you are trying to protect.
You needn't protect your circuit but patient protection takes top priority.
It also depends on what you are trying to achieve. Are you planning on digitizing the PQRST complex or are you simply monitoring heart rate?

To monitor the pulse, simply use the ECG amplifier followed by a voltage comparator driving an opto-coupler and into the MCU input. No projection diode is necessary.

 

k. thanks! We are able to read the ecg signal for now. It's not necessary to protect our ADC, microcontroller but its better to have extra protection. At voltages less then zero, we'd want it to clamp at 0. and voltages greater then 3.3, to clamp at 3.3.

 

If you use an opto-coupler your signal will automatically be constrained from 0-3.3V. No additional protection required. Plus you get isolation.