Sample time delay insufficient for normal tolerance

[stur0067]

The time delay between i2c write and read operations is insufficient to cover the natural part to part variation of ADS1x15 devices. The consequence of this is that some parts return the previous reading whenever queried. This issue only becomes apparent if the gain value changes between two conversions as the value returned will be correct for the previous gain setting.

Currently a delay of (1.0/data_rate + 0.0001) is used. The datasheet for the ADC states that the sample rate varies up to +/-10%. So, to avoid this potential issue the delay should be changed to (1.0/(0.9*data_rate))

[ladyada]

hiya - we'll be deprecating this library soon and replacing it fully with the circuitpython version which will be supported (we're waiting on one more breaking interface change before archiving this library for good)
can you check out https://github.com/adafruit/Adafruit_CircuitPython_ADS1x15 and see if that does the right thing?

[stur0067]

This new library's implementation does properly handle the part to part conversion time variation as it actually queries the device to see when the conversion is complete. It does have a little bit of room for improvement though as the _read function waits in increments of 10ms if the _conversion_complete query immediately after the _write_register call returns false. I haven't tested it but I would guess that for all but the very fastest data rates on the ADS1015 this first check will always come back indicating incomplete, forcing a wait time before that next check that is excessive for all but the lowest data rate setting of the slower ADS1115. This penalty will severely limit the user's ability to make multiple rapid single conversion, likely 50-100 samples/second, even when they believe they are using a much faster data rate. I would recommend some modifications that changed depending on how closely you'd like to complete relative to the ADC's conversion completion. 1A. Add a sleep of 1.0/data_rate, the nominal conversion time, before the while loop checks for conversion completion 2A. Add a sleep of 1.0/(data_rate*1.1)), the minimum conversion time, before the while loop Followed by a change to the sleep time within the loop: 1B. A sleep of 1.0/(data_rate*0.10) should loop at most once for case 1A or twice for 2A above and adds minimal i2c traffic but fully covers the -10% sampling rate condition 2B. A sleep of 1.0/(data_rate*0.01) should loop at most 10 times for A1 and 20 times for 2A. This will let you more closely catch the ADCs completion For 2B you could even monitor the average number of additional iterations the specific device requires and add that much extra time to the sleep before the while loop. This will reduce the i2c traffic.

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On Sun, Nov 18, 2018 at 11:22 AM ladyada ***@***.***> wrote: hiya - we'll be deprecating this library soon and replacing it fully with the circuitpython version which will be supported (we're waiting on one more breaking interface change before archiving this library for good) can you check out https://github.com/adafruit/Adafruit_CircuitPython_ADS1x15 and see if that does the right thing? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#12 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ArCxbBEbWSU_BvHvdsPGcARiZdC-0zBFks5uwaVegaJpZM4Ynvtg> .

[ladyada]

ooh do you mind either submitting a PR or adding an issue to that repo? that way we will see it next time we do a sweep thru the librayr! it would be appreciated

[stur0067]

Sure :)

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On Wed, Nov 21, 2018, 9:50 AM ladyada ***@***.***> wrote: ooh do you mind either submitting a PR or adding an issue to that repo? that way we will see it next time we do a sweep thru the librayr! it would be appreciated — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#12 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ArCxbNmEh2SAXwvm9HoO883b5aDNxSBZks5uxYRYgaJpZM4Ynvtg> .

[caternuson]

Hi! We are deprecating this library. This issue is either fixed or may need updating for the new library:
https://github.com/adafruit/Adafruit_CircuitPython_ADS1x15

Please see issue summary here:
#13