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TINA/Spice/ADS1258EVM-PDK: ADS1258EVM

amar tr
Prodigy 30 points
Part Number: ADS1258EVM-PDK
Other Parts Discussed in Thread: TINA-TI, ADCPRO, ADS1258

Tool/software: TINA-TI or Spice Models

Hi everyone

We are working with ADS1258EVM, this ADC is been evaluated for Accuracy and Resolution. During testing we are feeding 1mv, at the pin A1 and AICOM in the evaluation board.
The configuration followed is as mentioned below,

1) Mux
a)Autoscan option
A1

b) Switch delay time
0 microsec

c) Chopping
Enable external chopping.

2) Clock Source
internal.

3) Power and Ref

a) Power Supply
Unipolar

b) Idle mode
Sleep

c) VREF Source
DAC

d) SBCS
OFF

With this is Configuration we are getting 12-14 bit accuracy.
We need to know what is the maximum Accuracy which can be achieved with this ADC1258 ( specially in the Eval board) with ADC range set to 0-5volt.

We are feeding the input through the calibrator CA-71 and the source range is set to 100mv and feeding 1mv at input (A1 and AICOM).
Please find the below attached screenshot of Multiscope using ADCpro software.

the captured hex value for the fed input is:-

0x25EE6

0x25E75

0x25E56
this shows last 8bits are varing in the captured data.

1) please suggest how to identify the accuaracy of this ADC ADS1258.

2) let us know what is maximum accuracy achevied by this eval board at manufacturer test setup?

3) How to Convert the Obtained Hex value into voltage(Transfer function of ADS1258)?

Best Regards
Amar TR
Software Engineer

Multiscope_Screenshot.rar

  • 0
    TI__Mastermind 44550 points

    Hi Amar,

    Have you tried measuring other input signals?

    From the screenshot it would appear that the input signal is oscillating.  Generally, I recommend starting with a shorted input measurement (with both inputs connected to a mid-supply voltage  and checking that ADC output result is near 0V with good noise performance. With an input signal applied to the AADC it becomes harder to determine if the input signal or the ADC is at fault. 

    The best case noise performance of the ADS1258 is shown in Table 6 of the datasheet:

    Assuming you're using the DRATE[1:0] = 00h setting, you should be seeing about 2.8 uVrms. ...From the three samples you collected the standard deviation is about 75.8 codes or 24 uVrms (assuming a 2.5-V reference). This noise is higher than expected and would lead me to believe that most of this noise is coming from the input signal you're applying.

    To answer your questions:

    1. I believe your asking about the resolution, is that correct? Refer to table 6. This will show you the noise performance (i.e. Resolution) limits of the ADS1258, without the effects of input signal noise or reference voltage noise. Therefore, in practice you may observe addition noise due to external noise sources.

    2. Regarding noise performance, the ADS1258EVM should perform relatively close to the values shown in table 6. Regarding ADC accuracy... accuracy is harder to predict and will depend largely on your system implementation. To determine accuracy you would need to take into account the ADC offset, ADC gain error, ADC drift, ADC INL, ADC input bias current, external filtering, accuracy of your sensor, accuracy of any input circuitry, and the accuracy of your reference source - all of these factors can limit the system's accuracy. Additionally, accuracy can vary from system to system, and will also depend on how you decide to calibrate your system.
       
    3. Check out Table 10 in the datasheet... 1 LSB = Vref / 780000h. Once you've converted the ADC's output code to a signed integer, you simply need to multiply that integer by the LSB size to get the voltage measurement.

    Best regards,
    Chris