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MSP430FR60471: Presence of DC in the amplified output of ADC

Kamlesh Bharodiya
Prodigy 180 points
Part Number: MSP430FR60471
Other Parts Discussed in Thread: EVM430-FR6047, MSP430FR6047

Hi Team,

We are working with Ultrasonic Transducers along with TI's USS. We have out own custom PCB based on EVM430-FR6047 schematics. We are facing issue in the signal output when no transducer is connected (CH0 and CH1 are kept open, except the RC circuitry as given below)

The signal with gain of 22.8db (No transducer is connected, is given below). As can be seen the signal strength goes above 400. This messes up with our algorithm to process the data. 

When the MSP430FR6047 is replaced with a newer one. We see that USS shows reduced signal strength (at same gain 22.8db and no transducer connected).

This behavior is seen in multiple PCBs. Currently, we are of the view that MSP is introducing some noise just prior to PGA. To verify this, we also checked that a Good PCB and Bad PCB (which gives high signals without any transducer) signals at the CH0IN and CH1IN pins. We are getting similar voltage waveforms as shown below (Blue curve is the CH0IN and similar is obtained on CH1IN for both PCBs). In out opinion, this means that the DC is introduced and amplified inside the MSP, somewhere. Can this be a case? If yes, How to resolve this as we are seeing this issue in multiple PCBs? 

 

We need urgent help to resolve this. 

Regards,

Kamlesh

  • 0
    TI__Mastermind 27505 points

    Hi,

    I do not think this will be an issue on your application. Have you seen a DC drift on the ADC capture when the transducers are connected with a proper gain setting? 

    The noise can be picked up and amplified in the MCU. However, we would focus on how to minimized the influence of the noise from hardware and software together. In yout case, the noise is not going to affect the results. 

    Best regards,

    Cash Hao

  • 0 in reply to Cash Hao11
    Prodigy 180 points

    Hi Cash Hao,

    When Transducers are connected (and Kept in Air), same/similar signal is getting captured. The envelope threshold for the algorithm is 250 and hence the algorithm detects flow in Air. So, there is DC drift when connected with Transducers in Air, But in Water - Not much Drift is present (may be in few 10s of mV). Please suggest any solution to resolve this.


    Regards,

    Kamlesh

  • 0 in reply to Kamlesh Bharodiya
    TI__Mastermind 27505 points

    Hi,

    Your signal is too high when you keep the transducer in air. You can lower the gain settings and use a higher signal checking threshold by changing this parameter USS_ALG_SIGN_VAL_THRSH_CHK to filter out the signal in air.

    Best regards,

    Cash Hao

  • 0 in reply to Cash Hao11
    Prodigy 180 points

    Normally, the signal is okay. We are keeping the envelope threshold above the signal obtained with highest gain (in Air). Anyways, we are getting this signal even when transducers are not connected and only in some cases. Mostly the signal range remains below 150 when no transducer is connected. In Some PCBs this value is very high (from our understanding and tests - it seems that this signal is introduced and amplified by MSP430 itself). How can we reduce that in the software or hardware?

  • 0 in reply to Kamlesh Bharodiya
    TI__Mastermind 27505 points

    I think it is more related to the hardware based on your description. Keep the ultrasonic signal trace as short as possible.  And place ground around those signal trace. Use a shielded wire to connect with the transducer might also help to reduce the noise. 

    Best regards,

    Cash Hao

  • 0 in reply to Cash Hao11
    Prodigy 180 points

    Hi Cash Hao,

    We are using shielded wires. The trace length is of the same length as given in the EVMMSP430 design. We are exploring any hardware OR software filters. Any more suggestions on this?

    Regards,

    Kamlesh

  • +1 in reply to Kamlesh Bharodiya
    TI__Mastermind 27505 points

    Hi,

    You can use the software filter in the USS library. You can find about how to use it in this document page 9. https://www.ti.com/lit/an/slaa837a/slaa837a.pdf

    Best regards,

    Cash Hao

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