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Noise Problem with 6148EVM

Marcel Behlau
Prodigy 20 points
Other Parts Discussed in Thread: ADS6148, ADS1248

Hi,

I want to evaluate an analog part with an ADS6148EVM. (The Analog device is part of measurement system, with uses the ADS6148, too!). I connect the ADS6148EVM with a TSW1200 for the communication. The measurement clock (50MHz) is generated by an AD5920-3. (I’m using the AD5920 in the system, too!). For the best EMC, all parts are shielded!

I want to measure and evaluate the noise generation of the analog part, but I measure with the ADS1248 already a bit noise of 3 Bit, if the SMA-Socket is open and 6 Bit noise, if the SMA-Socket is shorted to Ground. A 50Ohm termination causes a 6 Bit Noise, too!

 I can't find any information about the bit noise performance of the ADS6148EVM. My 6Bit seems a lot. Is this normal? Are they some information, about the handling and measurement with the ADS6148EVM?

Noise Signal with open SMA-Socket

Noise Signal, SMA Socket shorted to Ground

  • 0
    TI__Mastermind 29220 points

    Hi,

    it is normal to see some idle-channel noise when there is no input to the ADC.  Your first set of plots look to be about normal to me for idle-channel noise.   The second set of plots seem to have excessive idle-channel noise, and shorting the SMA input of the EVM should not have any effect that i can think of.  The schematic for this EVM can be found in the User Guide for the ADS6148 EVM (You might have a newer revision board than shown in the User Guide, but the input circuit hasn't changed.) and shorting the input SMA to ground just means that the first transformer in the input circuit has both legs of the primary coil grounded.  Figure 31 of the datsaheet shows a histogram of output codes with idle-channel input, I beleive.  (the device you have in hand may have a little different offset and so the exact codes may vary, but the spread of codes seems to match your first set of screenshots.)  i don't know why your second set of plots has such a large amount of noise.

    Regards,

    Richard P.