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Problems with 1248 Gain Magnitude

Jeffry Merkley
Prodigy 70 points
Other Parts Discussed in Thread: ADS1248

Hello,

I have a question on the ADS1248.  We have the device working but the readings we are getting are much lower then expected. When I change the gain the readings change by the expected ratio, e.g. 128 is twice the hex code as 64.  I am running a 4 wire RTD that will be reading from -80 C to 300 C.  So I need approximately 128 mV at full scale. I am using the internal reference so 128 mV with a gain of 16 should be very close to full scale even with offset errors.  I currently have a precision POT in place of my RTD so I can adjust the input.  I have a DMM across my pot to read the voltage.  The code I am getting at 128 mV input is 0x0E7670 with a gian of 16.  So about 1/8 of full scale.  Changing my gain I get the readings below:

Gain                    HEX output

128                      3B9679

64                         1DCAC5

32                         0EE699

so these seem to be the correct ratio between gains but no where near the magnitude they should be.  I can turn the pot up to around 400 mV and the code will rise with it until I saturate and get 7FFFFF.  The other issue is that a gain of 32 and 16 give almost the same code. I have included a rough schematic of what I am working with.

0045.ADS1248 Schematic.pdf

Thanks for your help.

Jeff

  • 0
    TI__Guru* 98175 points

    Jeff,


    I would start by using a DMM and measuring back the input voltage and the reference voltage directly to see if there's something wrong in the setup. With the readings being lower than expected, the inputs are really lower, or the reference is higher in voltage. I think the readings you give are for a 128mV input, but I wanted to make sure that you checked the reference as well.

    In your description, you mention that you're using a Tek scope to read back the data. Can you post a scope shot of the first byte of that transaction (or at least to the first byte of the ADC data)? I want to make sure that you're correctly getting the right flavor of SPI (where the clock dwells low, and data is clocked in with the falling edge of SCLK). You probably have it correct, but I want to make sure. I would also read back all of the registers to make sure they are what you expect them to be.

    Do you run any calibrations when changing the gain?

    You might also want to measure between AIN2 and AIN3 in gain=1 to make sure that you get 0.1V as well (as long as you expect 300uA current through 350Ohm resistor).

    In the description, you say that you're using a 300uA source. Is this external? Can you use the internal IDAC instead and disconnect the 350Ohm resistor from the external drive? Do you have any other components hooked up to the schematic not drawn on the description?

    I know there are quite a few things to check, but if you report back on these, maybe we can help get the correct data out of your ADC.



    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 70 points

    Joseph,

    Thanks for the quick reply. We actually got the device working. We were not running within the VCM voltage range. Once we provided a +/- 2.5 supply it started giving expected code.

    Jeff

  • 0 in reply to Jeffry Merkley
    TI__Guru* 98175 points

    Jeff,

    Great, thanks for sharing that. If you have any other questions, let me know.


    Joseph Wu