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Noise on ADC, MSP430F47183 using CC1200 with LTC1992 op amp. RF Noise.

Intellectual 260 points

Replies: 5

Views: 1446

I have a custom board using a CC1200, a CC2500, and a LSResearch TiWi module. I am sampling data from the ADC and am picking up noise during the 900MHz CC1200 transmit. It completely shifts my steady state voltage. It also appears that this is entirely radiated. I can put a cable on and move the antenna 50 feet away and it goes away. I can also put the board in an environmental chamber and run the antenna outside the chamber and see that the problem significantly disappears but when I move the antenna back inside, the problem greatly appears. I have tried filtering everywhere possible. I have added shields over every radio. Adding high frequency caps at strategic points. I am lost and would appreciate any ideas. 

5 Replies

  • Guru 248415 points

    I agree that it sounds like the issue is related to radiated power.

    - Does the shift/ noise vary with output power?

    - You write "shift your steady state voltage": Could you clarify which voltage this is?

    - Does some of your supply voltages drop when you start sending?

     OOO until November 25th

  • In reply to TER:

    - Does the shift/ noise vary with output power?

    Yes

    - You write "shift your steady state voltage": Could you clarify which voltage this is?

    Sorry, I meant RMS voltage. It will bump the sine wave coming off my Rogowski coil either positive or negative during the transmission.

    - Does some of your supply voltages drop when you start sending?

    No. They are rock solid measured on a 7GHz probe. 

    So, an update: It appears that the LTC1992 op amp has some ESD protection diodes inside that appear to be rectifying the signal. I have been working with LT to confirm this. It isnt rectifying the signal through the base/emitter junction like what normally happens in op amp rectification, but through these diodes. Now, if I take the op amp off the board and short ALL channels of the MSP430 sigma delta ADC, I still get this pulse. The only way I can see this happening is if the radiated noise is coming in and the ESD diodes inside the ADC (if there are any) are rectifying the signal and causing it to appear like a shift. I guess at this point, I am trying to figure out if anyone has had ADC issues with a 1W or higher radio blasting the MSP430 and how they fixed it. 

    I am very confident that the rails are solid, the ADC ref is solid (ground), and the inputs are clean since I shorted them. I think this is still a radiated issue, but I am tracking it down to rectification at this point. 

    Any more help would be appreciated. 

    Thanks a bunch. 

    -Joe

  • Guru 248415 points

    In reply to Joseph Orender:

    From your description and from the results it looks like a radiated issue. The question is where/ how the radiated energy enters the circuit. If you use a shield the energy is more likely to enter the circuit outside the shield if not through the ground. Have you tried to probe to find out which nodes are more sensitive than others/ pick up more energy? Without having schematic and layout it is a bit difficult to comment.

     OOO until November 25th

  • In reply to TER:

    Yes, it is entirely a radiated issue. I guess what I am trying to find out is what the inside of the MSP430F4187 ADC looks like so that I can determine where the rectification is occurring. I can inject this noise through the top of the body of the chip even with all inputs shorted. I understand how to debug the power plane, ground, and signal rails and shield them, but I did not expect that the ADC would behave the way it is. 

  • Guru 248415 points

    In reply to Joseph Orender:

    Since you at the moment are interested in MSP430 details I would recommend posting a edited version of the question in a MSP430 forum.

     OOO until November 25th

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