• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 46 subscribers
  • Views 1142 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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 CMRR - test setup from SBAA188 - how to get 96dB ?

Marcin Boczar
Prodigy 20 points
Other Parts Discussed in Thread: ADS1298

Hi,

I'm trying to conduct test as desribed in SBAA188 ( www.ti.com/lit/an/sbaa188/sbaa188.pdf ).
In "Table 1. Summary of Measurement Results" there is 96dB measured CMR.
I'm not able to get this resut in any way. Max what I get is about 82 - 84dB. This result I can get only with very short cables (about 5 - 6 cm long) and no Rp/Cp.
I'm using ADS1298 EVM and software for it. Test configuration setup is as described in document. EVM is battery powered (4xAA) and connected by USB to battery powered laptop. Can you write something more about test configuration ? Is cable shield drive populated on PCB and used (not tied to AGND) ?. On the picture I see some plate under EVM - it is ground connected ? Test was conducted in some "clean" environment (in my case 50Hz AC Line noise is bigger than signal from generator)?

Best regards,
Marcin

 

  • 0
    TI__Mastermind 38745 points
    Hey Marcin,

    There is nothing special about the setup besides what Venkatesh describes in the document. The shield drive was not populated. The environment was probably ESD safe but I doubt that it was shielded from overhead lights. Is your setup exactly what is described in the document? Including the electrode modeling, capacitive coupling, cable impedance, etc? What value of Rp are you using to try and achieve 96 dB of CMR?

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Prodigy 20 points

    Hi Brian,

    My setup is almost the same as in datasheet - I'm using only CH2, other channels are powered down / shorted internally - not connected to Vcm as described. Everything else is the same. Only Rp I use is 10kOhm protection resitor in cable snap. On FFT there is huge spike on 50Hz (AC line frequency) even wihout signal from generator - on the graph it is almost 1mVp-p.This signal is induced in cables and RLD is not able to filter it (I think so). I was trying to use some shielding - grounded metal plate under cables / board but it is not working at all. I need to perform those tests withoud active shield drive in cables but for now it is impossible.

    I will appreciate any hints for that.

    Best regards,
    Marcin

  • 0 in reply to Marcin Boczar
    TI__Mastermind 38745 points
    Hey Marcin,

    It could be that the 50 Hz signal is coupling into the system differentially. If that is the case then common-mode rejection won't help. To explore this, you could look at the two inputs using an oscilloscope. If you can see the 50 Hz wave easily, then it leads me to believe the signal is coupling differentially. Unfortunately, I cannot contact the author of this document to see if he had a special way of setting it up that he did not say in the app note.

    Regards,
    Brian Pisani