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ADS1220: ADS1220 Voltage variation issue with filter circuit

Part Number: ADS1220

capicitor C422 report.xlsx

Hello 

using the ADS1220 to get the load cell data but facing the issue of voltage variation as voltage is mV (measurement range 0.800mV-10mV) 

  

Using the configuration in my design as well as in ads1220evm

when i remove the capacitor (CDIF) i get the better results my voltage is also constant

what issue it can cause if i remove this capacitor and what are other values you recommend i should use in my design 

also posting voltage results with and without capacitor in EVM board in excel file 

Regards

Anil Kumar 

  • Hi Anil,

    I'll do my best to help you with your issue while the primary support is enjoying the holiday. With this in mind, please know that responses may be delayed. 

    I see that with the differential capacitor installed, the max-min variation is 140uV and without, 5uV. 

    What are the inputs connected to for this test? Are they connected to the load cell or floating? 

  • Hi Alexander Thank you for response 

    Inputs are connected to the load cell these results are for the 1.5kg weight put on the load cell 

  • Anil,

    I will take over for Alex as I am the normal support person for this device.  I have a additional few questions and comments about your setup.

    1.  What PGA setting are you using for your circuit?  Are you using PGA bypass mode?  If you are using PGA bypass mode the input will have transient current that  may interact with the input filter.  If you have the PGA enabled the input current is small and the input is a relatively high impedance.

    2.  Normally, the reason for using the differential capacitor 10x larger than the common mode capacitors is to avoid the translation of common mode signals to differential.  The way this happens is that any tolerance in the common mode capacitors will attenuate one of the common mode signals more than the other and translate the common mode signal to differential.  So you would normally expect eliminating the differential capacitor to make things worse (especially for large common mode input signals).

    3.  You mention that the results get worse with the differential capacitor.  How much worse?   Can you post some data.  Do you think the bad behavior is from noise pickup or is the circuit just acting strange.  How long are the leads to the bridge?  Are they shielded twisted pair.

    4.  What is the bridge output signal range.

    5.  Can you review section 5.2.2 in the users guide on "Bridge Sensor Connectors, J1 and J6"?  I want to make sure that you are following the recommended connections in the guide.

    Best regards,

    Art