LTC6810 - 100~200mv difference

Hi Sir/Mdm,

I am trying to hock up a 4S battery pack monitoring system using LTC6810, below is my schematic, one issue is seems the accuracy is not there, when I connect 4S 3.337V cells, the LTC6810 output is around 3.187V, and all the four channels are the same situation. 

One more issue is the Vref2, as per datasheet, it should be 3V. but my measurement using multimeter is 2.84V, also got >100mv difference, is it normal?

Pls help to check whether my schematic design got some issue? appreciated.

Modified the statement.
[edited by: ELON at 12:57 PM (GMT -4) on 30 Sep 2021]
  • 0
    •  Analog Employees 
    on Sep 30, 2021 1:53 PM

    What is the voltage at V+ (after the 100ohm filter resistor)?  Vref1 and Vref2 can drop below spec if V+ is below 12v.

  • Thank you very much for your reply, I just measure using my multimeter, before 100Ohm resistor, it is 13.34V, after the resistor (V+), it is 13.33V. seems no problem, any other possiblilities I should take note?

  • 0
    •  Analog Employees 
    on Sep 30, 2021 6:36 PM in reply to ELON

    Did you have the REFON bit set during this measurement (i.e. this was done at the same time as Vref2 is 2.84v)?  The ADC and reference each consume some current to power and I want to be sure that current isn't dropping the V+ pin below 12v.

    A few more debugging questions:

    1. Do you have any loads on the VREF2 pin?
    2. Is this lower voltage also present on VREF1?
  • Hi Mvrablic,

    Thanks a lot for your prompt reply. I directly used the sample code (using the linduino board and the latest demo code) to do the test, not sure REFON bit set or not... I have tested many times, the V+ pin is very stable and above 13.2V, even during the sampling period.

    1. for VREF2, I linked to GPIO3 and GPIO4 through 2 10k resistors, intended to measure two temperatures using NTC externally. below is the schematic, any issue?

    2. The VREF1 is stable, 3.294V

    3. By the way, for isoSPI, I use the HM2100NLT as below, may I know Whether the HM2100NLT is fit for LTC6810, since it's not from the suggested list for 6810?

  • 0
    •  Analog Employees 
    on Oct 4, 2021 4:31 PM in reply to ELON

    Looking at the schematic again I notice the VREG transistor configuration is slightly different than the datasheet.  Can you add the 100ohm resistor and 100nF cap in datasheet figure 36?  This may improve VREF stability.

    Regarding your numbered items:

    1. This is the correct configuration for use with a 10k NTC on GPIO3 and GPIO4.
    2. This is higher than expected despite being within spec.  It may be fixed by the filter change suggested above.
    3. I think this will work based off of the datasheet, but we have not done tests with it ourselves.

    If the VREG transistor filter doesn't fix it, then I still think it may be a voltage sagging for a very short time during measurement.  Have you tried with a higher cell voltage (4V+) or shorting R1?  If you don't want to test with real cells you can use a resistor ladder attached to a power supply with 1k resistors or similar.  I still worry that V+ is dropping for a very short time when the ADC turns on which would impact accuracy.

    Are you planning on using the discharge functionality of the S pins?  If so make sure it follows datasheet figure 41 or figure 40a if so.  I'm not seeing where the S pins are connected to in the attached schematic.