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INA826EVM: INA826

Vladimir Gorelik87
Prodigy 80 points
Part Number: INA826EVM
Other Parts Discussed in Thread: INA826,

Dear All,

I have an output drift problem while evaluating INA826 with TAL220B 5kG load cell.  After power is applied, the output reading is correct.  However, shortly after it starts drifting to 0 rail and within minutes becomes 0.

Here is my setup:

TAL220B is a Wheatstone bridge with 1k resistors.

A 49.9 Ohm resistor sets Gain to 1000.  High gain is needed because load cell produces output in single digit mV range.

I am using a 10V DC rail to power both INA826 and the bridge.

Vref to INA826 is set to 5.6V

Interestingly enough – even though the supply is a single rail, when load is removed, the output signal changes to negative value. When this negative value is subtracted from positive drifting down output, the result is equal to initial output reading at power up.  

I am at loss and would very much appreciate help.

Thanks in advance!

  • 0
    Guru 140200 points

    Hi Vladimir,

    what is the output signal of INA826 directly after power-on?

    You wrote:

    "Interestingly enough – even though the supply is a single rail, when load is removed, the output signal changes to negative value. When this negative value is subtracted from positive drifting down output, the result is equal to initial output reading at power up."

    Sorry, but this totally confuses me... :-) Can you please explain this in detail again? Have you numbers?

    Can you take a digital volt meter and measure the voltages directly at the load cell?

    10V bridge voltage is a bit high. This could heat up the bridge unwantedly and cause an offset voltage temperature drift.

    Kai

  • 0
    TI__Guru 60610 points

    Vladimir,

    Under the conditions you describe the maximum differential input voltage is 4.25mV or-5.5mV - see below.

    Have you checked all the connections?  If you continue having problem, please provide circuit schematic of your application.

    INA826 has a non-standard pinout - please make sure you have taken this into consideration (see below).

  • 0 in reply to kai klaas69
    Prodigy 80 points
    Hello Kai,
    Directly after power-on with no load applied:
    Vin = 615uV and steady
    Vout = -90mV and drifting up to 0.

    I don’t understand where this negative voltage is coming from… It almost feels like a capacitor is being charged

    Thanks
  • 0 in reply to Marek Lis
    Prodigy 80 points

    2705.INA826.TSCHello Marek,

    Thanks for a fast response.

    Input levels stay below +4mV.

    I have attached a TiNa scematic. 

    I am using an eval board powered by single 10V rail with 5.6V reference and with 49.9Ohm resistor added to set G=1000. This setup draws 10mA from 10V - most goes into Wheatstone with INA826 supply current is very low.

  • 0 in reply to kai klaas69
    Prodigy 80 points
    Hi Kai,
    Yes, it is confusing to me as well...
    Without any load applied the load cell shows 615 uV (microvolts) using Rigol DM3058E

    With 5Lb load applied, load cell outputs 4.136 mV
    Regarding load cell voltage - 10V is the max recommended level. Here is the data sheet - cdn.sparkfun.com/.../TAL220B.pdf
  • 0 in reply to Vladimir Gorelik87
    Guru 140200 points

    Hi Vladimir,

    there's something going terribly wrong... That's what you should see when not applying a load:

    vladimir.TSC

    Have you swapped the -IN, +IN, -Vs and +Vs pins of INA826?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai,

    I have accidentally swapped -Vs and +Vs (+Vs appears at the bottom of the symbol I was reckless to overlook) in Tina and that is why simulation did not run. Thank you for catching this.

    However everything is correct on actual setup and I still have the same problem...

  • 0 in reply to Vladimir Gorelik87
    Guru 140200 points
    Hi Vladimir,

    connect the load cell to the supply rails and to the INA826 as shown in my simulation. Then short circuit the both inputs of INA826. Do you see now about 5.6V at the output of INA826?

    How do you generate the 5.6V reference voltage?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai,

    I found what was wrong. While poking around, I found that +10V was drifting down +Vs and up on -Vs in reference to the board's ground plane. It was quite silly - negative terminal of the power supply was not connected to ground on INA826EVM board. Everything is stable after connecting and I see no drift overnight.

    Thanks,

    You may check is as closed 

  • 0 in reply to Vladimir Gorelik87
    Guru 140200 points
    Nice to hear, that it works now!

    Kai