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Original question:

DRV425-BUSBAR-EVM: Reproducing accuracy measurements

DRV425-BUSBAR-EVM: DRV425-Q1 Output Noise

Frank Meyer
Prodigy 50 points
Part Number: DRV425-BUSBAR-EVM
Other Parts Discussed in Thread: DRV425-Q1, DRV425

Hello Javier,

's data had 20 mVpp of noise with up to 40 mVpp.

The DRV425-Q1 data sheet says

-  Noise is typically 17 nT rms from 0.1 to 10 Hz

-  Noise density at 1 kHz is 1.5 nT/sqrt(Hz)

These values don't correlate to the data shown by the original poster.

The RC filter you're advising has a cutoff at ~ 16 kHz.

This raises two questions:

1.  Does the DRV425 generate substantial noise beyond 1 kHz?

2.  Or was the noise in the original data due to grounding/layout?

  • 0
    TI__Mastermind 28460 points

    Hello Frank,

    Just want to make sure we are discussing the same post.  https://e2e.ti.com/support/amplifiers/f/14/t/641235

    In this post the issue was not noise of the part but rather the input to the ADC and the sampling rate. 

    If you are curious about the noise see this post below as it has more detail about noise as the specific frequency you are interested in.

    https://e2e.ti.com/support/amplifiers/f/14/t/606174?DRV425-What-is-the-minimum-resolution-of-DRV425-is-it-possible-to-mesure-a-current-of-1-mA-magnetic-field-of-0-1-uT-

  • 0 in reply to Javier Contreras4
    Prodigy 50 points

    Hello Javier,

    Yes, that is the post I was asking questions about.   Thanks for the quick reply. I'm very interested in this part, and have the EVM on order.  We've used Hall Effect devices but they're very noisy so I am sensitive to this performance.  So I've some follow-up questions.

    The poster said he used a TI ADC to measure Vout and Vref, then subtracted Vref from Vout.  

    You said the problems were 1) input to the ADC and 2) sampling rate.

    1. I don't have a schematic but it looks like the output of the DRV425-Q1 is the input to the ADC.   So what did you mean by the 'input to the ADC'?

    2.  The data sheet for the TI ADC shows a 200 kSa/sec sampling rate.  If he's sampling "DC" that should be sufficient.   What did you mean by sampling rate?

    Thanks,

    Frank

  • 0 in reply to Frank Meyer
    TI__Mastermind 28460 points

    Frank,

    This was an issue with sampling of the ADC.  From my recollection and looking at my emails this had to do with the sampling rate of the ADC and not having a capacitor close to the input of the ADC.  Note that most output of amplifiers cannot drive large capacitance so I suggested a 1K Ω resistor in series.  The issue in this situation was not noise from the DRV425. 

    I have verified the device at about the full bandwidth and I did get results similar results to the plot in the post I mentioned.  A peak-to-peak of about 1.4µT.  The bandwidth of the DRV425 is about 47 KHz depending on the shunt resistor used.  For noise estimations use the plots shown in the post I mentioned.

    For the ADC sampling rate it has to do with charge distribution as it samples.  You typically want it about 20x the capacitor of your sampling cap of your ADC.  The documentation for the MSP430 was not clear on the sampling capacitor so I overestimated in there applicaiton. Now this is for a SAR type ADC.  There is a training series that goes into more detail.  The sampling rate determines how fast that capacitor has to recover and charge before it needs to get sampled again.  The training link below is better at explaining this in detail.

    https://training.ti.com/ti-precision-labs-adcs-introduction-sar-adc-front-end-component-selection?context=1139747-1128375-1139106-1128643