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iNA326 OFFSET AND RRO BEHAVIOUR

Nicholas Schlensky
Prodigy 120 points
Other Parts Discussed in Thread: INA326, TINA-TI

Hi

I want to use a INA326 to generate a +/-2V signal on a CM of 2.5V. The inputs are connected to a) 0-2.5V DAC and a 1.25V ref (derived from two 10k resistor off 2.5V ref).  The 2.5V ref is connected to the ref pin. The ina326 is configured with 200k/200k//1nf for a gain of 2 as per datasheet

1) Despite the input signals being stable at 1.249.9V and 1.249.9mv the output drifted by about 20mv. The reversal of the input pins fixed this problem. Why?

2) I expect the output to be able to provide +/- 2V Vref=2.5V (0.5-4.5V) but i cannot get this to work to this voltage, pin 5 voltage when measured shows the correct value so this seems to be the output buffer. The attached sim indicates what i am doing . The buffer is not connected to any load. Why would the buffer have trouble reproducing pin 5 voltage from 0.5V to 4.5V ?

Any help or insight would be apreciated

regards

Nicholas SchlenskyINA326T-TESTVOLTAGE GENERATOR.TSC

  • 0
    TI__Mastermind 25865 points

    Hello Nicholas,

    Thank you for providing the simulations...they are very helpful.

    It seems as though both of these circuits work in theory. The circuit on the left in the simulation inverts the DAC voltage (e.g. when VDAC=0, Vout=5V) while the circuit on the right does not (e.g. VDAC=0V, Vout=0V).

    I do have a couple observations, however. First, please note that the output swing of the INA326 is limitied to 75mV from the rails as discussed in the data sheet. This brings me to my 2nd observation: the gain in your simulation is set to 2V/V. If you want a 4.5V output given VDAC=2.5V (like the circuit on the right), the gain should actually be 1.6V/V.

    Vout=((Vin+)-(Vin-))*G+Vref -> 4.5=((2.5V-1.25V))*G+2.5 -> G=1.6V/V.

    I have attached a TINA-TI simulation of my interpretation of the circuit you're looking for. I understand that there is ~14mV when you use R2=200k and R1=250k. This is likely because of the simulation model. When you increase these values to R2=400k and R1=500k the simulation gives excellent results.

    /cfs-file/__key/communityserver-discussions-components-files/14/nicholas.TSC

  • 0 in reply to Pete%20Semig
    Prodigy 120 points

    Thanks pete,

    I noticed yr reply whilst i was discussing this with a collegue.

    I will update my cct board imediately.

    Again thanks for yr reply

    Regards

    Nick