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Original question:

DRV425: DRV425

DRV425: Use ADC Vcm as REFIN and Inverter Signal to Increase Dynamic Range

Rene Suc
Prodigy 20 points
Part Number: DRV425

Dear TI Experts,

at the moment I am using a DRV425 in my circuit with VDD = 5 V and REFOUT set to (VDD/2) and connected to REFIN (with a 27 Ohm resistor in series as recommended in the datasheet).

I use ADC AD4134 from Analog (as I need a finely programmable output data rate) and have Vcm set to 2.048 V (output). So ideally, VOUT of the DRV should also swing around Vcm of the ADC. I did not completely understand the DRV425 datasheet in this regard, so here are my questions:

1. Can I use the ADC Vcm output as REFIN diretly, so that VOUT of the DRV swings around REFIN/Vcm? I would then leave REFOUT unconnected. (see figure below)

2. I want to increase the ADC sensitivity/dynamic range by inverting the DRV VOUT signal and feeding both, original and inverterted signal into the ADC (see figure below). Do you have any recommendations on this (in addition to the proposed figure)?

Thank you! 

  • +1
    TI__Mastermind 32120 points

    Hello Rene,

    1.You can use the ADC Vcm to drive the REFIN directly and leave REFOUT pin unconnected.

    2.  Converting from single ended to differential will double your range but make sure that the noise or external fields are not a bigger issue.  You will still be limited by the output of the DRV425 both by noise and any other fields it picks up that are not the target field to be measured.

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Dear ,

    thank you for your really valuable input. Concerning point 2: I thought about what you have said and came up with a better solution I think.

    I would not amplifiy noise and effectlivey increase the SNR, if I use two DRVs in an antiparallel configuration, like this:

      

    My external AC magnetic field (0 - 10 kHz, 1 uT up to 10 uT) that I want to measure is far away, so it will be almost identical at the two DRV positions. Thereby I will also remove the DC offset and improve my AC field sensitvity.

    What do you think about it?

    Have a great weekend,

    Rene

  • 0 in reply to Rene Suc
    TI__Mastermind 32120 points

    Hello Rene,

    That will work but there a concern.  

    The noise you will get peak to peak at 10kHz if you filter is about 1µT from each one of the DRV425.  If you use the full bandwidth you will see more like 1.4µT per DRV425.     See this post.  Also I have verified these numbers on some DRV425

    When you combine the you should assume RSS addition of the noise.

    On another note: If you ever need to combine the signals analog without a differential ADC see the Bus Bar Theory of Operation.  The way you are doing it doubles your bandwidth.  So your option would be better.

    Regards,

    Javier