• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 182 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1248: incorrect output data

Patrick Geng
Prodigy 20 points
Part Number: ADS1248
Other Parts Discussed in Thread: ADS1298

hi @ all

I would like to use the ADS1248 for recording several RTDs and analogue
Voltages.

For getting familiar with handling the ADS1248 I built up a test setup.
unfortunately, the recorded data output code is not as I have expected.

The ADS1248 is put into operation under following condition:

AVDD = 3.3V

DVDD = 3.3V

I'm using directly connected AIN0 and AIN1 Inputs for Measuring the applied Voltage Level.

AIN0 = 500mV (stipulated via external circuit)

AIN1 = connected to Ground

As a reference the internal ref. 2,048V is being chosen.

Register settings:

address = Value

0x0 = 0x01 //set AIN0/AIN1

0x2 = 0x30 //onboard ref. selected

0x3 = 0x90 //PGA = 1, 2000 SPS

0x4-0x7 = 0x00  //no OFC applicable, transmitted byte-wise

0x7-0x9 = 0x400000 //FSC Gain = 1, transmitted byte-wise

0xB = 0xFF // no excitation current needed 

Expected Data output:

Output Code = 223 * VRTD/VREF

Output Code = 223 * 0.5V/2.048V

Output Code = 0x 1f 40 00  

 
Actual read back output Data =0x 00 00 00 f0

I know, by the use of offset and gain register the output data can be scaled, however the actual result is a significant deviation from expected value.

 

Could you please check my register settings, if chosen adjustments are sufficient for running a normal ADC conversation ??

 

Best

Patrick

 

  

 


 

 

 

 

 

 

 

  • 0
    TI__Mastermind 47625 points

    Hi Patrick,

    Thanks for your post and welcome to the forum! 

    The first issue I see is that the inputs cannot be tied to GND in a unipolar power supply configuration. In this case, you're using AIN0 as your variable voltage and AIN1 is the known voltage (GND) that you're measuring against. Due to the internal PGA, the inputs cannot be connected to GND as they require some headroom from the supply rails to operate. I would recommend tying your "known" voltage to mid supply (1.65V) instead of GND. Try fixing this and let me know what you see. 

    The second issue I see is the code conversion, This post covers code conversion for the ADS1298, however the same example should apply to the ADS1248. 

    Lastly, if you have not yet seen it, a Basic Guide to RTD Measurements: http://www.ti.com/lit/an/sbaa275/sbaa275.pdf may be helpful. 

  • 0 in reply to Alexander Smith
    Prodigy 20 points

    Hi Alexander,

    many thanks for your fast response.

    A good hint, and I guess as it is not possible bypassing the PGA, then shifting

    Voltage level of AIN1(current GND) is the only reasonable possibility. I give it a try and report than.

    best

    Patrick