• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 46 subscribers
  • Views 270 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1298: Turing OFF the RLD when measuring the input impedance of the channels using ac sources

Milad Faizollah
Prodigy 30 points
Part Number: ADS1298

Hi,

We are using ADS1298 for EEG monitoring. We are biasing the body using RLD electrode during EEG monitoring.

To measure the impedance of each channel we use the ac current source and sink of the channels. For example, if we want to measure channel 1's impedance, we turn on the current source on its INP as well as the current sink on its INN and measure the voltage across the inputs. Dividing the voltage over the amplitude of the injected current gives us the impedance, which is fairly accurate. 

- My question is can we turn off the RLD while measuring the impedance of the input channels?

- Turning off the RLD during impedance measurement, the body should still be biased to mid-supply through the current source and sink. As they have high input impedance (500M-ohm), the input common mode voltage should still be close to mid-supply even when connecting a big load to channel's input, right? 

- What is your suggestion on this? I'm asking because in my test, the measured impedances seems to be more accurate when RLD is OFF. Thanks!

Regards,

Milad

  • 0
    TI__Genius 11550 points

    Hi Milad,

    Thank you for the post. I'll get back to you this week for your queries. 

    Thanks for your patience and understanding.

    -TC

  • 0 in reply to TCT
    Prodigy 30 points

    TCT said:

    Hi Milad,

    Thank you for the post. I'll get back to you this week for your queries. 

    Thanks for your patience and understanding.

    -TC

    Thanks!

  • 0 in reply to Milad Faizollah
    TI__Genius 11550 points

    Hi Milad,

    Thanks. Sorry for the delay. Please see my comment for your inquiries below.

    To measure the impedance of each channel we use the ac current source and sink of the channels. For example, if we want to measure channel 1's impedance, we turn on the current source on its INP as well as the current sink on its INN and measure the voltage across the inputs. Dividing the voltage over the amplitude of the injected current gives us the impedance, which is fairly accurate. 

    - My question is can we turn off the RLD while measuring the impedance of the input channels?

    >> You should still be able to measure the impedance of the input channels when the RLD amplifier is turn off. You will need to turn on both the INP and INN leadoff current source and sink. 

    - Turning off the RLD during impedance measurement, the body should still be biased to mid-supply through the current source and sink. As they have high input impedance (500M-ohm), the input common mode voltage should still be close to mid-supply even when connecting a big load to channel's input, right? 

    >> Without the RLD bias circuitry, the only path for the leadoff current is through the current source and sink on the INP and INN inputs. However, this configuration will work to a certain extend. It depends on the accuracy or current matching between the INP and the INN leadoff current sources. The PGA input (INP-INN) can tolerate the current matching to some extent before the PGA input saturates. The DC leadoff will be the worst case and the AC leadoff will be the best case. The switching between the current sink and source on the INP and INN inputs will help to keep the common-mode voltage close to mid-supply even though the leadoff current imbalance will want to move the common-mode voltage to the voltage rails.       

    - What is your suggestion on this? I'm asking because in my test, the measured impedances seems to be more accurate when RLD is OFF. Thanks!

    For a more reliable impedance measurement, it is recommended to have the RLD turn on so the leadoff current has a proper return path. 

    Please see the Planet Analog blog link written by my colleagues on impedance measurement for the ADS129x family.

    https://www.planetanalog.com/signal-chain-basics-149-how-to-accurately-measure-electrode-impedance-for-lead-off-detection-in-ecg-systems/

    Thanks.

    -TC