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ADS1278: Analog input decoupling capacitor value

Part Number: ADS1278

Hello TI Community,

I just remarked what seemed to be a discrepancy between the recommendation in the datasheet of the ADS1278 (SBAS367F) and the electrical drawing of the evaluation board of this component (SBAU197A).

In the datasheet, it is written the following statement (page 37, point 6):

A 1nF to 10nF capacitor should be used directly across the analog input pins, AINP and AINN [...] Capacitors from each analog input to ground can be used. They should be no larger than 1/10 the size of the difference capacitor (typically 100pF) to preserve the ac common-mode performance.

However, in the last page of the circuit diagram of the evaluation board (page 22, Channels 5-8 Analog Connections), although the value of the capacitors between AINxP/AINxN is compliant with the statement (2.2nF), the value of the capacitor between the analog input and the ground is not (1.5nF instead of less than 220pF).

Is there really an issue here or did I read the datasheet wrongly?

In any case, could you please explain why the capacitor values are not below 220pF?

Thanks a lot for your help Slight smile

  • Hello,

    The Delta-Sigma ADC drive theory involved here is to size the differential capacitor to achieve the desired internal charge-transfer and settling requirements which yield datasheet THD+N performance from the ADC.  Then the common-mode (to-GND) capacitors are sized such that the cutoff frequency is at least 10x higher than the differential cutoff frequency.  Setting the differential cutoff frequency lower than the common-mode filters helps reduce the translation of AC common-mode noise from converting to differential signals due to mismatches in component values of the on-board capacitors.

    f(-3dB_differential) = 1/(2*pi*(2*R)*Cdiff) 

    f(-3dB_common-mode) = 1/(2*pi*(R*Ccm) 

    You are correct about the values on AIN5-8 on the EVM and when driven differentially the values would be Cdiff = 2.2nF and Ccm <= 220pF.  The board was designed a few years back, but I think they were set this way possibly for intended use with these inputs as single-ended signals where the AINN signal is connected to a DC mid-supply signal.  Even then, if there was common-mode noise on the AINP/N signals there would be higher risk for Common-mode-to-differential conversion on these channels with the capacitors set the way they are and a Ccm <220pF would be preferred as you noted.

    Without the on-board drivers the AIN5-8 channels are unlikely to perform as well as the AIN1-4 channels which have the on-board THS drivers. 

  • Hello Collin,

    Thanks a lot for this really quick and complete answer Slight smile

    One last question though : what if an analog input is used as single ended with AINN directly connected to GND?  Differential and common-mode would be the same.

    Then what decoupling capacitor value should be the best to place? 2.2nF? 220pF? Both? None?

  • Hello,

    I wasn't as clear as I could have been in my last reply when I'd mentioned about needing the on-board drivers to get to full performance, but you'll need circuitry more similar to the inputs on AIN1-4 where the inputs are actively driven with an amplifier which, in combination with the input RC charge-bucket, charges the internal sampling capacitor properly in the acquisition timing. 

    This relates to your question, because even in a signle-ended configuration it would be recommended to drive the inputs as shown in the image below.  If common-mode filtering was added, it would still be balanced and should be sized at least 10x, preferably 20-40x smaller than the differential capacitors.

  • Hello again,

    Thanks for these recommendations Slight smile

    I'm going to update my design accordingly.

    Have a nice day!