I am trying to build the circuit below taken from TI application document. The changes i want to make are , i want to use 3.3V supply and since i am interfacing it with cc2430 , i don't need the ADC. But i want your help in identifying the changes that i should make on the circuit based on the requirements . I also would like to know what kind of resisters i should use, type, power specification.
waiting for your kind response....
The main change you will have to make is scaling the gain of A3 such that with the largest ECG signal amplified by the INA326 it will not saturate into the output rail. In this design the 2.5V pedestal/reference is generated by the REF3125. Since your supply is now 3.3V your half scale changes to 1.65V; therefore, you will also have to divide down your +2.5V voltage reference (using a 1.5:1 divider such as a 1.5M and 1M ohm to ground) or create a buffered divide by 2 off your supply.
I hope this helps.
Thanks for quick reply,
I have some doubts:
Can i give supply of 3.3 v to the REF3125 and ADS8321 and still have Vref = 2.5v and the ADS8321's working ?
the other think i would like to know is?
Which components should i remove if i can use the ADC of cc2430? The ADC can accept voltage < 3.3v.
If you scale down your supply to 3.3V, you need to scale down your reference voltage to 1/2 scale as well for best performance. This means that your reference and your pedestal should change from +2.5V to 1.65V. You should remove the ADS8321 if you are using the ADC of the cc2430, but you will still need all the rest of the components to provide the 1/2 scale reference to your ADC.
I was working on something else for a while. I have implemented the Ecg amplifier with all the changes you told me. Now i want to test it but i don't have ECG simulator. How do i test it? Is it safe to test it on humanbeing?
waiting for your help
Please keep in mind that the recommendations that we give are only with regard to the analog and supporting circuitry that will make an application such as ECG work. I cannot advise you on anything with regard to patient safety. There is protocol and procedure outside the scope of this forum that must be followed in this regard.
If you would like to test your circuit, you can put a function generator between RA and LA i.e. 10mV-PP and see if you can get Vref + 10mV-PP*Gain at the output of the INA and the rest of the following gain stages.
I got ECG simulator and the result of the test are:
First time i test it i was getting a a waveform which looks like square wave and i thought
it maybe due to saturation and increased the supply from 3V to 5V. This time i got good result.
But the next day i was getting square wave even if i made the supply to 5V. After some time
the OPA2335(A3 and A4 in the figure) got burnt. I got it soldered again but still got burnt again.
in case it helps to change the gain (which you suggested) I used 7.5k resistor instead of the
5k connected to A3.
I have sorted out the problem which was burning the OPA2335. But i still have another problem.
The ECG amplifier is working good for supply voltage nearly equal to 5 volts but when i try to reduce the
supply voltage to 3.3 it doesn't work.
I nee your help
Could you post a link to that application note? I have been looking for it for 2 hours and cannot find it.
you can get the medical application guide in this site
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