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ADS1220: How to get stable 20 bits in ADs1220?

Part Number: ADS1220

hello Sir,

Actually i m new to ADS1220,

I am not getting stable 20 bits in ADS1220.Here is my circuit actually using developmental board:

So tell me what changes are required in circuit,(I haven't worked any of ADC ,it is my first ADC )

what are filter required to provide stable 20 bits and also what i have to required in coding ,some software algorithm

If this is not provide 20 bits stable then suggest me another ADC which provide 20 bit stable

https://e2e.ti.com/support/data-converters/f/73/p/725530/2677390?tisearch=e2e-sitesearch&keymatch=ADS1220%20stable%2020%20bits#2677390

and also i read this link : it told it is that it is providing 19  to 20 bit stable.

if there is any link which help to make 20 bit stable please let me.

Please help out this problem as soon as possible.

Pranav K Moudgil

  • Hi Pranav,

    The number of noise-free bits may vary depending on the value of 1 bit or code (LSB).  The value of 1 bit or code, is determined by the reference voltage for the ADC.  The ability to resolve to a specific value is based on the noise of the system and the ADC.  In your diagram you are making single-ended measurements relative to ground so you will be limited to gains of 1 to a maximum of 4 with the PGA disabled and bypassed.

    When using the ADS1220 with PGA disabled, you can get a basic understanding of the noise of the device and the ability to resolve to a particular level by looking at Table 3 and 4 on page 17 of the ADS1220 datasheet.  The best resolution occurs at the lowest data rates, so if we key in on the tables using 20sps we see there are 2 numbers. The number outside of the parenthesis is an RMS value, while the number inside the parenthesis is the peak-to-peak value.  Stable means noise-free or the values relative to peak-to-peak.

    Using the internal reference, we can see that at 20sps there is 18.22 bits noise-free using a 2.048V internal reference.  This is because there is 13.43uV peak-to-peak noise resulting in the value of one code of 244nV (see section 8.5.2 in the ADS1220 datasheet for the calculation). 

    If we were to use a 5V reference (as in your schematic), the noise of the converter would still be about 13.43uV, but the value of one code would change to 596nV.  As the size of the LSB has changed, the noise-free number of bits will change as well.  This becomes 22.5 codes of noise, which is 4.49 bits resulting in 19.5 bits of noise-free resolution.  So the conversion noise remains the same, but the number of bits increases based on the reference.

    The better question to ask is what value of noise-free resolution in the measurement is required for a particular measurement range?  For example, if gain can be applied the number of bits will decrease but the value of a single bit will also get smaller improving the resolution.  If you look again at Table 3 and a gain of 4, notice that the peak-to-peak noise reduces to 3.91uV which means that the noise is reduced allowing for an input to be resolved to this smaller voltage instead of 13.43uV seen at a gain of 1.  Table 4 shows that the number of bits lowers from 18.22 to 18, but the value of 1 bit also changes from 244nV to 6.1nV.  So the number of bits reduces, but the resolution of the measurement increases substantially.

    It is also possible to increase resolution from averaging, but what must be a major consideration is external noise as compared to the ADC noise.  The noise tables are showing the best case resolution using shorted input (0V input) where the noise of the reference and power supplies are not seen in the measurement.  Also, any external noise relative to the sensor and input cabling are not seen in the measurement.  Often RFI/EMI are large contributors to the noise and must be handled appropriately.  Any noise pickup on wires between your pots and the ADC inputs will become part of the measurement.  The wires will act as an antenna and will require additional handling to reduce the noise.  One method is to create a low-pass filter.  You are already showing a series resistance.  To create a low-pass filter you would need to add a cap across the ADC inputs.  I would suggest trying a value of 0.1uF as a starting point.

    In the end, you will probably find it difficult to build a low-noise prototype using a method with a bunch of wires.  A low-noise precision system requires clean power supplies with a well designed PCB that has a good ground plane.

    Best regards,

    Bob B