AD5248 2.5 kOhms characteristics

I need an accurate resistor  with more than 256 Steps. Thats no problem by using the AD5248. I can connect the two potentiometer/ or rheostaten with some other resisors. I dont want describe the circuit i'm using more precisenlty.

I'm interested in the Stepsize, the wiperresistance und the max. resistant of the AD5248 2.5 kOhm version. In the Datasheet, there is just described the 10 kOhm variant. I would be proud of some Information of the 2.5 kOhm variant.

As i sayed further, i need an accurate systhem. The tollerance of 20 percent aren't what i'm searching for, but there is no better component, so i must handle that. Does the tollerance describe the behavior of the AD5248 or an static devitacion of each single component ?

thank you for each help

  • 0
    •  Analog Employees 
    on May 2, 2017 12:31 PM over 3 years ago

    Hi Felix,

    The stepsize can be approximated by the following equation:

    stepsize= Rab/256

       where Rab is the total resistance between the A & B pins of the potentiometer

    Meanwhile the actual resistance, you can compute it by using equations 1 & 2(page 13) from the data sheet. this equation considers the effect of the wiper resistance to the resistance between W & B or W & A pins of the potentiometer.

    The wiper resistance has a typical value of 160 ohms and a maximum value of 200 ohms.

    Please refer to Table 1 on the data sheet for specifications on the 2.5kohm variant of the AD5248.

    The tolerance specified on Table 1 is the nominal resistor tolerance from A to B at 25degC. The -20% to +55% tolerance are the limit but not for all AD5248 devices. For a 2.5kohm variant you may get a 2kohm resistor up to 3.875kohm resistor.

    May we ask why do you need a tolerance better than 20%? how do you plan to use the potentiometers?

    Thanks and best regards,

    Rainier 

  • Thanks Rainier for the Answer.

    Im just wondering about the information of the Data Sheet, thats why I have written the text. I've got two AD5248 here, for testing reasons. The experimental wiper resistance is about 59 Ohms at the first and 60 Ohms at the second IC (I've set the step to zero for that). Another Discussion about the AD5248 supports that (look at:AD5248 lowest resistance , "datasheet for the AD5248 explains that there is a contact resistance on the wiper of around 50 ohms.")
    I can only see the wiper resistance of 160 to 200 Ohms in my data sheet, like you. Maybe there are some more devitaions to the 2.5 kOhm version.

    The experimental stepsize is about 9 Ohms at the lower speps (thats fine with that: Stepsize= Rab/256), but the sizes are getting smaller within bigger stepsizes (the last steps have only an area of 3 Ohms)
    I think that is described with the R-DNL, am I right ?  Unfortunatily, I doesn't understand that. How can I understand the unit "LSB" ?

    Im want to use the AD5248 for simulating PT1000 temperatur sensors with an accurancy better than 1°C an a value from -30°C to 110 °C. So I need values of 840 to 1430 Ohms with a step size lower than 1,5 Ohms. I have connected the Rheosthaten with two resistors in a voltage divider circuit for that. The Problem is, that i must know the resistance of the systhem in all relevant cases with an hight accurancy, so that the simulated Temperature fitts to the resistor (maybe 2 ore max. 3 Ohms deviation of each Rheosthat to the calculated valius is ok).

    feel free to tell me that it isn't possible, if you think that.
    But I would be more proud to hear Informations about the calculation of the devitations, if there are some.

    best regards
    Felix

  • 0
    •  Analog Employees 
    on May 4, 2017 7:27 AM over 3 years ago

    Hi Felix,

    The wiper resistance that you are measuring is still within specifications as it is smaller compared to the maximum of 200 ohms.

    LSB is the ideal step resistance which is ~9.76ohms for the 2.5k variant. The R-DNL specification is the error on these LSB. Ideally the step size is equivalent to 1 LSB but due to DNL in the resistance, the error on the step can be as much as 2 LSBs or ~19.53ohms. So the 3ohm step size that you are encountering in your setup is still within specifications.

    We currently don't have a potentiometer/rheostat solution that have a 3ohm or lower step size. Can you provide a simplified schematic of your application so that we could have a better understanding of what you are trying to do and provide a better solution?

    Thanks and best regards,

    Rainier