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Conversion Formula

ismail sadiq
Prodigy 85 points
Other Parts Discussed in Thread: ADS1298

Hi 

I wanted to know how to convert the hex data acquired from ads1298 ECG FE to volts?

What is the exact formula of conversion for both leads and test data.

  • 0
    TI__Guru**** 180901 points

    Hi ismail,

    What is your reference voltage and PGA setting on the ADS1298?  This will influence the 'codes to volts' conversion.  Please take a look at the equations on page 26 of the data sheet and see if that helps you see what it is you need to do for your specific implementation.  If you would like to post the details on your PGA settings and reference voltage, we can try to help you with a specific equation.

  • 0 in reply to Tom Hendrick
    Prodigy 85 points

    My reference voltage is 2.4V.and PGA gain is 6.

  • 0 in reply to ismail sadiq
    TI__Guru**** 180901 points
    Hi Ismail, Sorry for the delay in getting back to you. The ADS1298 has a BTC output and that can complicate the SW a little in trying to convert hex to voltage. The easiest way to go about this (IMHO) is to take the raw code and convert the BTC data to straight binary. You can do this by 'XORing' the data with 0x800000. Full scale positive is 0x7FFFFF, XORing that with 0x800000 would give you 0xFFFFFF. Full scale negative is 0x800000, which yields 0x000000 when XORed. Your code to voltage input relationship is essentially [code-1 (in decimal)*LSB size]/gain. The LSB size is given in page 30 under the Format section as VREF/(2^23-1). If you need to compensate for polarity, simply take the result and subtract your reference voltage.
  • 0 in reply to Tom Hendrick
    Prodigy 70 points

    Hello Tom and Ismail,

    The code to voltage input relationship may be

    code * LSB size / gain ?

    Best regardes.

    YJ

  • 0 in reply to Tom Hendrick
    Intellectual 285 points

    Hi Tom.

    As you said above, if i got a "800000h" from ADS1298, i changed this to  voltage is -2.4V/6=-0.4V?

    Is that right? I used internal test signals  DC . And my gain was setting to 6.

    Another question is about the internal test signals. The internal test signals amplitude is 1V or 2V,and the frequency is 1HZ or 2HZ. But when i chose the DC test signal ,the amplitude also is decided by the TEST_AMP bit(1V or  2V)?

    Last question, if i set my ADS1298 in 1KSPS, and i just use one channel which set to the internal test signals ,just DC,  what code will i get from the ADS1298 output pin?

    Waiting  for your reply.

    Appreciate.