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ADS1255 VREF and Measured Value

Daniel Vamos
Genius 3950 points
Other Parts Discussed in Thread: ADS1258, ADCPRO, OPA301

Dear TI Experts,

I need some help to understand some parameters / values of the ADS1258

Using the ADCpro sw, here is the result:

Q1.: As you can see the VCC and Vref values are about 2.5V. Could you tell me why? I suppose this value should be around 5V? I am using Unipolar power source (included in the package 6V power supply) 

S2 switches are in Center position for DAC ref. voltage. But When I set the switches to the left the result is the same.

The ADS1258EVM manual Page 8 - 5 Voltage Reference section says: The output of the dac is multiplied by to giving a range approx. 0-5V.

Q2.: Does it mean, the maximum value of DAC on the front panel is 2.5?

My second problem is related to VREF and the calculation of the measured value results from the ADC.

VREF: According to the ADS1258 datasheet, Vref = Vrefp - Vrefn. On the page 25. equation says:  1LSB = Vref / 780000h. - This is the way how I can calculate the Voltage from the ADC

Q3.: Is Vref in this equation equals to the REF value that I can get back from  the Channel Data format when CHID[4:0] = 1Dh ?  So If i want to get an accurate Voltage result, first I am going to divide the REF value (External Reference) by 7800000h and then multiply this value by the Channel Data value ( after the conversion from Binary2's).

Looking forward your kind reply.

  • 0
    TI__Mastermind 44550 points

    Hi Daniel,

    Welcome to the TI E2E Forums!

    Daniel Vamos said:
    Q1.: As you can see the VCC and Vref values are about 2.5V. Could you tell me why? I suppose this value should be around 5V? I am using Unipolar power source (included in the package 6V power supply) 

    The internal readings are scaled differently than a standard ADC conversion. Unfortunately, ADCPro is not applying the correct scaling to these internal channels. Therefore, you will have to manually do these calculations.

    Page 26 in the ADS1258 datasheet shows how to convert from ADC codes to external reference voltage. I have also done this calculation for you in this Excel file:

    0804.ADS1258 Calculator.xlsx

    Notice that a voltage reading would show about 2.42V, while the external reference voltage is close to 5V. Also note that the reference is less than 5V. The OPA301 reference buffer output is only supplied by 5V and is clipping near the positive rail.

    Daniel Vamos said:
    Q2.: Does it mean, the maximum value of DAC on the front panel is 2.5?

    The DAC output is multiplied by two at the hardware level, but the ADCPro software is accounting for this. When you set the output to 2.5V, the DAC will output 1.25V and the OPA301 will amplify the voltage to 2.5V.

    Because the OPA301 is only powered by a 5V single-supply, practically you can only use reference voltages of 0.2V to 4.8V.

    Daniel Vamos said:
    Q3.: Is Vref in this equation equals to the REF value that I can get back from  the Channel Data format when CHID[4:0] = 1Dh ?  So If i want to get an accurate Voltage result, first I am going to divide the REF value (External Reference) by 7800000h and then multiply this value by the Channel Data value ( after the conversion from Binary2's).

    Your procedure to calculate the LSB size is correct. However, the value you input into "VREF" will be your reference voltage (as long as it is not clipping)...

    The channel data plot is not calculating the voltage reference correctly, so don't use that value.

    It would be best to manually calculate the correct reference voltage, then use that value in your LSB calculation.

    I hope that helps!

    Regards,
    Chris

  • 0 in reply to Christopher Hall
    Genius 3950 points

    Dear Chris,

    Thank you very much for your detailed answers and sorry for my late reply.

    For ADCPro I will use the manually calculated VREF but in my application first I will query the internal VREF (I do the scaling myself)  and that would be used in the equation for the accurate result. 

    Thank you for your time, all the best!

  • 0 in reply to Daniel Vamos
    TI__Mastermind 44550 points

    Glad I could help! Let me know if you have any additional questions.

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Genius 3950 points

    Thank you! 

    Here is my new thread :)

    All the best!