• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 49 subscribers
  • Views 1396 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.

ADS1198: ADS1198

user4976161
Prodigy 40 points

Part Number: ADS1198

Hi - I am trying to understand the operation of the ADS1198 when driven by a single ended input voltage. I have the ADS1198 eval board and the EVM software. The channel is set to normal electrode and PGA gain = 1. If I connect INN input to 0V and drive INP to 1V should I read 1V? The ADS1198 EVM software is giving scaled results. For example if AVDD = 3V, Vref=2.4V and Vin=1V the read voltage is 1.7V, and if Vin=1.2V the read voltage is 2.05V. In another example if AVDD = 3V, Vref=4V and Vin=1V the read voltage is 2.3V, and if Vin=1.2V the read voltage is 2.76V.  In another example if AVDD = 3.3V, Vref=3.3V and Vin=1V the read voltage is 2.07V, and if Vin=1.2V the read voltage is 2.5V. What is causing the scaling? 

Thanks,

Diarmuid.

  • 0
    TI__Guru 69426 points

    Hello Diarmuid,

    Thank you for your post and welcome to our forum!

    What is AVSS connected to? I assume you are using a unipolar supply with AVSS = GND?

    The ADC will convert the differential input voltage between INP and INN. In principle, if INP = 1 V and INN = 0 V, the ADC output should read 1 V. However, there is another restriction on the input voltage due to the internal PGA: the input common-mode voltage. The PGA amplifiers are not able to swing to the rail, so the input common-mode must be kept at least 200 mV away from each supply rail.

    Try increasing INN to mid-supply (1.5 V) and connect INP to 2.5 V.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hi Ryan, 

    Thanks for the reply. Yes AVSS = 0V. I have run a number of tests to try to isolate the problem. When I set INN=1.5V and INP=2.5V I do indeed get Vout=1V. I ran two tests with INN=1.5V and INN=1.2V, and ran the tests at both VREFP=2.4V and 4V. Below are plots showing Vout against the differential input voltage, so for the case of INN=1.5V INP was swept from 1V to 2.5V in 0.1V steps. As can seen seen when VREFP=2.4V there is a good agreement between the differential voltage and Vout. However when VREFP=4V Vout is scaled by about 1.34. This is close to the ratio of VREFP/AVDD (4V/3V) could this have a bearing on the result? 

    I also done tests with INN=0V and swept INP from 0V to 1V in 0.1V steps. Again the results below show the same scaling when VREFP=4V. 

    I have run additional tests and made both AVDD and VREFP=3.3V on the eval board but I the scaling effect does not go away - in fact it gets worse.

    Any help appreciated.

    Diarmuid 

    ADCTestData.pdf

  • 0 in reply to user4976161
    TI__Guru 69426 points

    Hello Diarmuid,

    Apologies for not noticing this in your first post. When AVDD = 3 V or 3.3 V and AVSS = 0 V, you cannot use the VREF = 4 V setting. Essentially, the output of the reference buffer (Figure 26) is railing and unable to reach 4 V. If you probe the VREFP pin with respect to AVSS, you should see that the reference voltage is less than 4 V. The ratio of VREFP to "4 V" is likely the dominant source of the gain error in your plots for VREF = 4 V.

    Please review the External Reference section of the Electrical Characteristics table.


    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 40 points

    Hi Ryan,

    Many thanks again for your reply. Yep when I make AVDD=3V and set VREFP=4V the measured VREFP=2.96V. Also, if I make AVDD=3.3V and set VREFP=4V the measured VREFP=3.24V. However when I power down the internal reference and use an external reference results are a little confusing. If I make AVDD=3.3V VREFP=3V and set INN to some midscale voltage (e.g. 1V or 1.5V) again I get a scaling issue at the output.

    Also, at times the eval software can give erratic results - repeated voltage sweeps give differing output voltage readings. I am not sure if this is an issue with the software or the hardware. 

    Regards,

    Diarmuid.Figure 1 AVDD.pdf 

  • 0 in reply to user4976161
    Prodigy 40 points

    ADS1198FrontendTest.pdfHi Ryan,

    To get a better understanding on the ADC I have added a LDO regulator to provide a mid-scale reference voltage for INN and adjusted the PGA gain and re-run the INP sweep tests. Attached are the results - again I am seeing a 200mV offset in the readings. I want to use channel 1-8 as ADC inputs (without any ECG functionality) - I have turned off all WCT, RLD and pace functionality is there anything else in the setting that could cause the offset?

    Thanks,

    Diarmuid.  

  • 0 in reply to user4976161
    TI__Guru 61302 points
    Hi Diarmuid,

    What power and reference settings were you using for the last test?

    The output being limited to lower voltages as the gain increases is expected since the full-scale range of the ADC remains the same.