ADS1299: ads1299. Input shorted. To big value

Ildar Rakhmatulin

Part Number: ADS1299

Hello. Sorry, I didn't find full information about acceptable noise value when input shorted in ADS1299.

(in this link, a similar problem like and my situation, but Mr. Brian said - OK e2e.ti.com/.../547117

If I shorted the channel input (by register CH1SET - 0x01) I receive the next result for the channel -
867
866
865
862

if convert to volts = 461 mkV it is too much. (error = 862*2*4.5/16777216)
if I shorted the input channel external by cable I receive similar value, with PGA gain = 24 my result will be 20 mkV. But it is too big.
Which value I need to receive during input shorted for the correct measurement EEG signal?

over 5 years ago

0 Alexander Smith over 5 years ago

TI__Guru 55232 points

Hi Ildar,

Are the results for your channel in hex or decimal?

https://e2e.ti.com/support/data-converters/f/73/p/772488/2856893#2856893

0 Ildar Rakhmatulin over 5 years ago in reply to Alexander Smith

Prodigy 230 points

Hello Alex. It is decimal format

Ildar.

0 Alexander Smith over 5 years ago in reply to Ildar Rakhmatulin

TI__Guru 55232 points

Hi Ildar,

Thank you for confirming.

To confirm what you are seeing, with a gain of 1, you have 461uV of noise. However, with a gain of 24, you are only seeing 20uV of noise. Or is this from the same calculation, and you have rounded 19.2 (=461/24) up to 20?

In the code to volt equation you are using, I do not see where you are dividing by the gain. Are you dividing by the gain for the gain of 24 result? If not, 20uV/24 = 0.7uV, which is within datasheet specification.

LSB = (2 x VREF)/ Gain / (2^24 - 1)

0 Ildar Rakhmatulin over 5 years ago in reply to Alexander Smith

Prodigy 230 points

Alex. Sorry that I not clear wrote.

I have value - 461uV (+- 0.5 uV ) when the channel input shorted by register CH1SET (001 : Input shorted). This result without any gain. Can i use this value for offset? (error = 862*2*4.5/16777216).

When I shorted the input channel external by cable I receive similar value (900 (+-30) decimal ), with PGA gain = 24 my result will be 20 mkV (error = 900*2*4.5/24/16777216).

0 Alexander Smith over 5 years ago in reply to Ildar Rakhmatulin

TI__Guru 55232 points

Hi Ildar,

No worries, I just wanted to confirm a few points before offering recommendations.

Yes, this value can be used for offset. For a more precise offset measurement, you could average your several readings together to reduce the affect of noise. 865 is the average for the numbers below.

867
866
865
862

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ADS1299: ads1299. Input shorted. To big value