ADS1298/ADS1299 Difference of SNR

Hello Kato,

The SNR is different in those devices because the ADS1299 is a more precise device designed to measure EEG signals. Just like with anything in electronics, though, there is a tradeoff. The tradeoff here is with power. The ADS1299 is power hungry compared to the ADS1298, but it has to be in order to achieve performance.

It is possible to achieve better SNR with an instrumentation amplifier, but the devil is in the details. The amplifier will be able to gain the signal to have the signal be larger relative to the noise introduced by the data converter, but the amplifier itself introduces noise. To figure out the scheme that works best, you can calculate the sum of the input referred noise of the system.

In the case of the ADC on its own, the magnitude of the noise can be seen in those tables near the beginning of the datasheet. If you're looking at the situation where an INA is the front end, you can take the noise of the instrumentation amplifier and add it to the noise of the data converter divided by the gain of the INA (to make it input referred).

The topic of how to add noise sources is the topic of graduate level study on its own, but perhaps the datasheet of a given amplifier will give the magnitude of noise for a given bandwidth which is near to the bandwidth of the data converter.

Regards,
Brian Pisani

Hey Kato-san,

In general I tend to think the ADS1299 is a good part to serve as the front end for EEG measurement. The best way to determine if it will be good enough is to ask your customer how small a magnitude of signal they want to capture. From there take a look at the ADS1299 input referred noise numbers and see if they could still read the signal even in the worst case with that level of noise introduced by the ADC. Ultimately it will be up to them to decide if it's "good enough". If it is not i.e. if the signal magnitude is under the magnitude of input referred noise on the ADS1299 or only incrementally greater than it, then an amplifier must be used and the discussion can then shift to finding the right amp.

Regards,
Brian