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ADS1115-Q1: Stability

Jonathan Navor
Genius 9265 points
Part Number: ADS1115-Q1
Other Parts Discussed in Thread: ADS1115

Hi Experts,

Good day! Please help us to assist our customer. Here is the full details of their inquiry below.

Using the Gravity a2d board to control a 200AH LiFePo4 Battery by measuring 4 cells to decide what action to take.
Using a Rpi ZeroW controlling Opto relays for Charge, Load etc.
Measuring voltages from around 3.00 V to 3.60 V , but the wiring of this requires a resistor divider
network to keep within the voltage limits of the device.
So, at the lowest theoretical value:
Cell1 would be 3.0 V relative to Negative
Cell2 would be 6.0, Cell3 would be 9.0 and Cell4 12.0 v.
By using a resistor divider network of metal film resistors I bring cells 2 to 4 down to 3.0 volts.
Cell 1 is direct to AN1 with 1k ohm to Negative
Cell 2 is with 1k in series and 1k to negative
Cell 3 is with 2k in series and 1k to Negative
Cell 4 is with 3k in series with 1k to negative.

Not using Gain and reading 500 samples of which I remove the High / Low samples and then average the rest. (Typically 498 samples are averaged)
At the moment using continuous sampling, I read each channel and then wait 30 seconds and do it again.

From reading TI forums , resistors over 2k are not recommended as they alter the accuracy.
My software corrects that with a calibration value. Accurate mV Multimeter value / Average Count = ie: 0.1261725
Using a different divider on AN4 I get the actual Battery voltage as well.
When running it is stable within 2 counts (as long as there are no loads or charging currents)

HOWEVER, comparing the reported value in the morning versus later in the day I am getting Total Battery errors
amounting to 0.1 V.  Now the Temperature difference is only about 15-20 degree C.

Trying to ascertain what I have done wrong.
Will do more test like heatgun on resistors and looking at the stability of AN1 (No series resistor), but
other than that I am at a loss as to what is the cause of the drift.

I feel sure that the device (ADS1115) is Temp stable.

Looking for constructive comments as to possible causes.

Best regards,

Jonathan

  • 0
    TI__Guru** 113765 points

    Hi Jonathan,

    It would be very helpful to know the values being used in the resistor divider as well as the tolerance of the resistors used.  It would also be helpful to know the configuration register settings for the ADC.

    A 100mV shift would appear most likely to be in the resistor divider.  This would become more evident if the direct measurement value remains stable while the other shift.  It would also be helpful to know the actual conversion results as number of codes as opposed to calculated values.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Genius 9265 points

    Hi Bob,

    The customer found the resolution to this inquiry.

    The problem was a result of measuring the AIN pin whilst the device was powered down.

    I did not expect the ADS1115 to draw currents that would drop my divider voltage by that much.
    Measuring the voltages with the A2D powered-up solved the problem of the voltage divider doing what I expected.

    The second part of the problem was measuring each of the 4 Cells as a separate entity, which meant that if one cell went up,


    the second cell would give a false reading.


    This was solved in software by treating each cell with reference to the Negative.

    Thank you still for your usual support.

    Best regards,

    Jonathan