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ADS1248

Candi-jojo
Prodigy 130 points
Other Parts Discussed in Thread: ADS1248

Hello There,

I am having a problem measuring all analog intput using the multiplexer. I am using a Ratiometric principle, but instead of a 3-wired method, i have all analog input connected. Talking about like 6-wire principle, permit me to say so ;-)

Each time i conduct sampling of all channels using the multiplexer  to interchange the channels, i get an error of about 7% in all sampled channels except  the first channel.

If i disconnect all channels(take out all PT-100 Resistors ), leaving only one at a time while sampling, then i get values without the 7% error.

Possible Problem:

There could be a leackage of current through the other analog input channels when connected, even while no current flows through. Is this current able to course the 7% error?

Somebody help me out. 

  • 0
    TI__Guru** 113765 points

    Candi-jojo,

    There are a couple of ways that I can see how this is connected, so can you send a schematic of how you have the RTDs actually connected to the ADS1248?  It may be an issue of settling after the MUX switch. 

     In your original configuration, do you see an issue if you measure each RTD seperately over an extended period of time with no MUX switching?

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 130 points

    I tried pasting a Picture of my connection, but this is not working.

    Thanks a lot for your concern.

     

    voltage-4-wire.doc
  • 0 in reply to Candi-jojo
    Prodigy 130 points

    Ok, now it worked with a word file.

    Like i said earlier, if i disconnect the other PT-100, then it samples ok regardless of which channel that is.

  • 0 in reply to Candi-jojo
    TI__Guru** 113765 points

    Candi-jojo,

    Ok, I think there was a mistake or two in the drawing.  What is the actual current that the IDACs are producing. It appears the voltage across RBIAS is about 2V, and with a 833 ohm resistor the total current would be about 2.4mA.  The IDAC current outs don't fall in line with those values. 

    I'm also not sure how to interpret the data.  Maybe you could simplify the data as to the exact readings, such as the voltage across AIN0 and AIN1, AIN2 and AIN3, RBIAS.  

    I have another question, the first RTD drawing has three resistors and the second RTD has four.  What is that fourth resistor and how is it connected?  For the second RTD does the IDACs feed the inputs to AIN2 and AIN3?

    Thanks,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 130 points

    Hello Bob,

    The diagram is just to show you what method i am using.  for sampling, i am using the external refference voltage( Vref0 ).  To every channels being sampled, the two currents sources with 1mA  each are  connected. this  current sources sum up  to 2mA, which flows through a refference resistor of 1KOhm, making a refference voltage of 2V.

    on the Channels, i have connected to AIN0 : 100Ohm

                                                                        AIN1:    21Ohm 

                                                                        AIN2 :    325Ohm 

                                                                        AIN3 :    450Ohm 

    The voltage across AIN0 and AIN1, AIN2 and AIN3 then look like this:  

                                                                       AIN0 :   100mV

                                                                       AIN1:     210mV

                                                                      AIN2:      325mV

                                                                      AIN3:      450mV

    When i sample through this channels, i get values like this:

                                                                         AIN0  : 412077

                                                                        AIN1:    823296

                                                                        AIN2 :    1290240

                                                                        AIN3 :    1761280

  • 0 in reply to Candi-jojo
    Prodigy 130 points

    Hello Bob,

    I found my mistake. It has nothing to do with the module(ADS1248), it was a definiton problem in my debugger.

    Thanks for your effort,  i appreciate.