ADS1299 Circuit problem

Shivaprasad Bhat

Other Parts Discussed in Thread: ADS1299

Hi all,

We are using the following circuit from the OpenBCI project to make our own EEG acquisition system using ADS1299. The problem is we are not able to get ceramic capacitors of higher values (such as 1uF, 10uF, 100uF etc.). We tried with electrolytic capacitors. But we got all 1s in the registers (sometimes random values too) when we read using the Arduino.. What could be the reason for this strange behavior ?

over 8 years ago

0 Brian Pisani over 8 years ago

TI__Mastermind 38745 points

Hey Shivaprasad,

The type of capacitor will not affect the ADS1299 functionally; only performance will be affected. Instead I would look to make sure everything is powered on correctly. Probe the power and VCAP pins and report the voltages to me so I can verify.

In addition, it looks like your START pin is not driven to a state. This will prevent you from reading conversion data reliably, though it is probably unrelated to your inability to read registers.

Regards,
Brian

0 Shivaprasad Bhat over 8 years ago in reply to Brian Pisani

Prodigy 75 points

Hi, We checked the values of VCAP terminals. It's as follows

VCAP 1 = 1.2V

VCAP 2 = 2.5V

VCAP 3 = 6.8 V

VCAP 4 = 285.3 mV

And powers :

DVDD - 3.34V

AVDD - 4.92V

0 Tran Ngoc Viet BME over 8 years ago in reply to Shivaprasad Bhat

Prodigy 95 points

May be you solder reverse capacitor. I suggest you should measure the current of your circuit

0 Shivaprasad Bhat over 8 years ago in reply to Tran Ngoc Viet BME

Prodigy 75 points

We have connected the negative terminals of all the electrolytic capacitor towards the ground. Isn't that correct?

0 Tran Ngoc Viet BME over 8 years ago in reply to Shivaprasad Bhat

Prodigy 95 points

ok, if you need to help from me, free contact.

0 Brian Pisani over 8 years ago in reply to Shivaprasad Bhat

TI__Mastermind 38745 points

Hello Shivaprasad,

Please capture your register read routine SPI signals using an oscilloscope or logic analyzer and post them here.

Brian

0 Shivaprasad Bhat over 7 years ago in reply to Brian Pisani

Prodigy 75 points

Din and Dout

Drdy and CS

0 Tran Ngoc Viet BME over 7 years ago in reply to Shivaprasad Bhat

Prodigy 95 points

I think you should use saleae logic for debug program of SPI. your spi signal look incorrect.

0 Shivaprasad Bhat over 7 years ago in reply to Tran Ngoc Viet BME

Prodigy 75 points

Yes. The logic waveform does not look proper at all to me as well. But unfortunately, we don't have access to a logic analyzer

0 Brian Pisani over 7 years ago in reply to Shivaprasad Bhat

TI__Mastermind 38745 points

Hello Shivaprasad,

Those definitely do not look like normal SPI communications. Is this what happens when you attempt to read the registers?

Regards,

Brian Pisani

0 Shivaprasad Bhat over 7 years ago in reply to Brian Pisani

Prodigy 75 points

Until you asked for the SPI signals , we didn't really check ( we blindly trusted OpenBCI code).. So don't know if these are the waveforms everytime. I guess I will have to try fixing spi first. I will see once again and let you know.

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ADS1299 Circuit problem