LT1461 output ringing

Hi, I'm using the LT1461 4.096V version as an ADC reference for the AD7989-5. I have the output decoupled with a 10uF X5R 50V ceramic capacitor. Input power is +5V.

For my noise test my DC input voltages are

   VIN- = 3.96V

   VIN+ = 0.13V

I perform a burst of 256 readings 70 times and 4ms apart. I then overlay the 70 burst readings and look at my SD of counts to determine noise levels. On the past 2 revisions of this design the noise test data was always flat with a Gaussian distribution of noise. For this rev I noticed ringing over the duration of 256 readings and it happens during every one of the 70 bursts. The graph below shows the 70 plots overlayed. Please note I am reading all 18 bits and throwing away the bottom 2 LSBs. 

Some probing around with a 1:1 probe revealed the ADC reference was ringing at the start of each burst. The oscilloscope screenshots appear below. The blue trace shows the 256 conversion pulses and the purple trace is the AC component of the reference output.

Adding another 12uF directly across the ADC ref and gnd pins dropped the ringing by about 50% but I was only able to completely eliminate it by adding a 4kOhm load on the reference output. I know other references such as the LT1460 require a DC load but I wasn't aware one was needed with the LT1461. Does this sound right or am I masking a problem?

Here's the relevant section of my layout. The next layer down is solid ground plane.

Thanks,

Paul

  • 0
    •  Analog Employees 
    on Aug 9, 2018 3:30 AM

    Hi Paul,

    Thank you for sharing the data plots and layout picture. As you stated on past 2 revisions of design the noise data was flat, can you please share what changes were made in this revision?

     

    Fast settling time and good transient response requires a reference that can both source and sink current. As per the LT1461 load regulation specification in the data sheet, the LT1461 can source up to 50mA but cannot sink current. One solution is to add resistive load to provide a path to sink current from the REFCAP when responding to a transient event such as the dynamic switched load presented by the ADC reference input.

     

    Does this sound right or am I masking a problem?

    Yes, adding a resistive load is a solution and the exact value will depend on the ADC used and the size of the reference cap. From your testing is appears that 4kΩ is the correct value in this case.

    Regards,

    Anshul

  • Hi Anshul,

    That certainly makes sense. Thanks very much for the explanation. I totally missed the detail that the LT1461 can't sink current. I just figured it was buffered so I'm all set.

    I believe the difference from the previous revision was IN+ was always greater than IN-. I changed the input driver so now IN- may be greater than IN+, as noted in my noise test configuration. Sounds like in that case the reference is sinking current which was causing the ringing. 

    Thanks!!

    Paul