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Original question:

REF5025: Do input and output need to add π - type filter network？

REF5025: Effective noise bandwidth of REF5025

Wendy Wang1
Genius 11330 points
Part Number: REF5025

Hi ,

I use the REF5025 for the ADC reference input. In order to reduce the effective noise bandwidth of the reference noise, I added a R which is 6.9kohm in series with the Cout.

Without the 6.9k resistance, the total noise is 46.13uV. With the resistance, diagram shown as below, the total noise got to higher than 300uV.

If I change R1 to be 1.1ohm, the total noise is 67uV. I think based on f_-3dB=1/2*pi*R*C, the higher R goes, the lower the noise bandwidth and the total noise should gets lower. The result is out of my expectation, could you please help with the question?

Besides,  if a parallel capacitor 10uF is added, the total noise is reduced to 46.13uV again.

Could you please kindly help with the question?

Thank you so much!

Best regards,

Wendy

  • 0
    TI__Mastermind 19370 points

    Hi Wendy,

    To start out, the REF50 model does not have the output capacitor stability conditions that the real device has. I would recommend to follow section 9.4.1. regarding the output capacitor and ESR. I would recommend to use a ceramic capacitor and a total ESR less around 1Ohm.

    Figure 1 is invalid in real world application due to high ESR.

    Figure 2 is recommended and 10uF will lower the noise.

    Figure 3 I am unsure if we can trust this value due to the 6.9k ESR.

    Your f_-3dB=1/2*pi*R*C equation if you are using it as a RC filter. In this case it is not used as an RC filter. To use it as a RC filter you need to connect the output  between the R and C. I would only recommend a RC filter if you have a high impedance load input.

    The image below helps show how a higher ideal capacitance will lower noise spectrum.

  • 0 in reply to Marcoo Z
    TI__Genius 11330 points

    Hi Marcoo,

    Thank you so much for your help!

    Does the 'output capacitor stability' mean the output capacitor stability of the amplifier in REF5025? Could you please provide me with some documents about the output capacitor stability?

    Thanks & best regards,

    Wendy

  • 0 in reply to Wendy Wang1
    TI__Mastermind 19370 points

    Hi Wendy,

    Yes, section 9.4.1 discusses capacitor selection. If you want the lowest noise possible you need to use the largest capacitor or use an RC filter + op amp combination. The article below discusses this in more detail.

  • 0 in reply to Marcoo Z
    TI__Genius 11330 points

    Hi Marcoo,

    Thank you so much! And I have read the document you recommend. While I still have two questions. Hope it doesn't bother you.

    1.  In Part 2 of the document, the table 1 shows that higher capacitor leads to lower noise. Then why in part 3, the capacitor is set to be 10uF?

    2. Could you please kindly explain why 'To minimize noise, the recommended ESR of CL is from 1ohm to 1.5ohm'? Since I haven't find the answer in the document series. 

    Thank you so much for your help, Marcoo.

    Best regards,

    Wendy

  • 0 in reply to Wendy Wang1
    TI__Mastermind 19370 points

    Hi Wendy,

    It is not always practical to design with the largest capacitor due to size/cost/leakage or other factors. 10uF is a good balance for most applications. I have seen applications use the REF50xx with 1uF to 47uF so it is up to the customer.

    I cannot elaborate more on the 1ohm -1.5ohm ESR since it is just the way it was designed. The compensation loop of the device was created to be optimal with these level of ESR on the load. I would not increase the ESR higher than that value or else there can be oscillation.