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ADS1255 sampling cycle

Gergely Olah
Prodigy 170 points
Other Parts Discussed in Thread: ADS1255, OPA365, ADS7818

Hi!

I would like to measure a signal with an ADS1255. The signal contains peaks, and I have to measure the amplitude of the peaks. The peaks remain high some us-s, so I have to know when the ADS1255 starts sampling in the conversion cycle in order to position the peaks to this time interval (peaks fade away soon).

I have read the datasheet, but it says only the sampling time length, not the start of it (I would like to know when sampling starts in the function of DRDY* signal).

Thanks,

G.

  • 0
    TI__Guru**** 181401 points

    Hi Gergely,

    The ADS1255 is a delta sigma converter so essentially it's 'always' sampling.  You could use the /SYNC pin or a SYNC command to reset the internal filters and start the settling at a specific point in time - have a look at the Settling Time using Synchronization section on page 20 of the data sheet and take note of the settling times vs. data rate in Table 13. 

  • 0 in reply to Tom Hendrick
    Prodigy 170 points

    Thank you for your answer, I supposed it because I always got the sum voltage of the conversion cycle.

    Can you suggest an ADC which would be able to measure a signal with high resistance (MOhms) on the analog input and with a sampling time of about a few microseconds in 0..2V DC? (It would be the best to use an inside AD of an MSP430, but as I see, input resistance of MSP430 inside SAR is only about a few hundred Ohms).

  • 0 in reply to Gergely Olah
    TI__Guru**** 181401 points

    Hi Gergely,

    Tha's correct - you would essentially be integrating the pulse which is probably not what you want to do.  Most SAR converters require a low impedance input, so you might be looking at using an amplifier ahead of the ADC to buffer your input.  How many channels are you sampling and what sort of resolution are you looking for?

  • 0 in reply to Tom Hendrick
    Prodigy 170 points

    Hi Tom,

    Thank you for your reply. What I liked in ADS1255 is the integrated input buffer, what I enabled and my signal remained almost the same. I am trying MSP430 - SAR simultanously. It pulls the signal with an offset, but the absolute value of my signal does not matter, so it is working, but with too much noise. I need to measure 20-30 mV with a resolution of at least 9 bit (without noise), only one channel.

    Regards,

    Gergely

  • 0 in reply to Gergely Olah
    TI__Guru**** 181401 points

    Hi again Gergely,

    Take a look at the ADS7818 - this is a simple SAR that has a convert start input so it is in 'sample' mode until you pulse CONVST low.  If you know when your peaks are present, you can capture the instant in time - like a snapshot.  You'd still probably need a buffer though, perhaps the OPA365 would work for you there.

  • 0 in reply to Tom Hendrick
    Prodigy 170 points

    Thanks, I will check it.