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REF2025: 1/f Noise Reduction

Part Number: REF2025
Other Parts Discussed in Thread: ADS1271, OPA2140, REF5025, LM4140, REF2030

Dear All,

In one of my application has very low noise budget. I am using REF2025 as reference to INA and 24-Bit ADC.

The input to INA is in the range of 50uV to 1mV roughly.

I am getting ~ 30 uVp-p noise in my ADC counts. After oversampling and doing the average of 1000 samples of ADC counts in each cycle of 200 ms, I am still getting significant variations of ~ 5 uVp-p in ADC counts. I am able to reduce this variation further after averaging more number of samples but it slows the response.

Although, there are several other factors too for noise in the circuit as, INA output noise, thermal noise & current noise produced due to source resistance, ADC output noise etc.

I have connected REF2025's Vref to ADC ADS1271 and Vbias to AD8220 directly. I have also used 100nF and 2.2uF X7R bypass caps on Vref and Vbias.

I was wondering if someone can help me out in designing the REF2025 buffer filter to significantly reduce (<1uVp-p) 0.1Hz to 10Hz noise. I have OPA2140 in my mind, if it can be used in buffer for Vref and Vbias both of REF2025.

Thanks & Regards,

Harish KS

  • Hi Harish,

    It is not possible to reduce 1/f noise using filters with reasonable values. Is the 30uVpp a combination of the 1/f noise and wideband noise?

    The only way to affect 1/f noise is to choose a more accurate reference. Since you are open to using a OPA, I would recommend to use a REF5025 over the REF2025 for the ADC and then you can resistor divide it down to get the 1.25V for the AD8220.

    The REF5025 will offer an improvement in 1/f noise and you can use larger caps to reduce the wideband noise.

  • Hi Marcoo,

    I always appreciate the prompt response on this forum.

    Marcoo Z said:

    Is the 30uVpp a combination of the 1/f noise and wideband noise?

    Yes.

    Marcoo Z said:

    I would recommend to use a REF5025 over the REF2025 for the ADC and then you can resistor divide it down to get the 1.25V for the AD8220.

    After seeing the datasheet REF5025, I am sure it would certainly reduce the noise significantly.

    Although, I would like to request you to please check if it would be no problem if I use resistor voltage divider for Vbias of INA.

    Since, I can calibrate out the initial accuracy of the voltage reference, please suggest if LM4140A (with tantalum 10uF & 0.1uF caps at output to reduce wideband noise) would be better over REF5025?

     Thanks for the support.

    Regards,

    Harish KS

  • The reason I did not recommend the LM4140 is because it only works from 0C to 70C. The ADS1271 is usable in a longer temp range. If this ADC is only going to have a ambient temperature between 0C to 70C, you can use LM4140.

    I prefer the REF50 due to the TRIM pin where you can add a capacitor to reduce noise. Also the REF50 can handle large capacitors so you can have 20uF or more which will can help make a lower frequency pole. You can also use a RC filter into a op amp to further reduce the wideband noise.

    I would directly connect the REF50 to the ADC and then use a voltage divider into a op amp to make a stable 1.25V reference for the INA.

    The other alternative is to make a RC filter with a pole around 100Hz or lower to reduce the wideband noise of the REF2025 which can help reduce that 30uVpp noise.

    Also check your power tree. If you have a switcher before the VREF, use better bypass caps or other LC filters to make sure the input voltage to the VREF is also stable.

  • Hi Marcoo,

    Thanks for the advise. I would definitely check first REF2030 with RC LPF with pole below 45Hz to reduce AC line frequency coupled noise in the input signal. After that I will compare the results with your first suggestion of REF5025 + Resistor divider & low noise op amp buffer for biasing.

    After reading your recent reply, reminds me that due to the unwanted power dissipation of linear regulators, I have used MCP16301 DC-DC converter to drop down the 24V DC supply to 5V. However by using low ESR bunch of caps on output, noise is in the mV range (~10mVp-p checked on scope). 

    I understand that noise in power supply can be coupled to low voltage signals.

    Would you recommend to replace DC-DC converter with LDO? If yes, can you please recommend one.

    Vin(max)=30V, Iout(max)=250mA, Vout=5V

    Regards,

    Harish KS