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DRV8353: Gain setting amplifier

Paul Klarhoefer1
Intellectual 365 points
Part Number: DRV8353

Hi,

I have a question regarding gain setting, regarding the chapter 9.2.1.2.4.1:

Questions

1) Why does Av must be larger then 14? What happens if it is set to 10 in this example? What voltage would be the analog output of the amplifier at 0A and at 40A?

Now to my schematic:

I have a 3W, 7mOhm shunt resistor. My VREF is 3.3V. So my Vo is the same 1.4V like in your example. Let's say I would like to have 20A peak --> 14.1A RMS. So this would be less then 3W.

According to equation above would be my Av calculation: Av > 1,4/0.007/20 > 10. Can I go with 10? What happens if I choose 20 or 5 ? My ADC of the uC is limited to the 3.3V (VREF). If I would like to increase the amps in future: What would be your suggestion im amplifier gain? Or should I go with a lower shunt resistor value ?

Thanks in advance.

  • 0
    TI__Guru 61380 points

    Paul,

    We only have defined gain settings of 5/10/20/40, 10 is too small, so we must pick 20 or 40. The voltage can be calculated as Sense_current x Sense_resistance x CSA_gain.

    Per those calculations you can use 10 gain but I'm not sure if you have enough headroom. To fix this you could use a 5mΩ shunt or go to 20x gain.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Intellectual 365 points

    Hi Adam,

    thanks for replying.

    Sorry, if its a dumb question, but that's the thing that is unclear for me:

    Sense_current x Sense_resistance x CSA_gain: so, in the example provided:

    40A x 0.0025Ohms x 20 = 2V

    -40A x 0.0025Ohms x 20 = -2V

    So I would need 4 volt of range to measure on my ADC input on the uC, and when I have maximum 3.3V range on the ADC input on my uC, then this would not work, or do I calculate something wrong ? I assume that 1.6V of the ADC input are 0A and then in theory 3.6V would be 40A and -0.4V would be -40A. In reality it would be zero and VREF if it hits the limits, right ?

    From my point of view, this would only work if I lower the gain, not increasing...

    Again, sorry, if I miss something completely obvious here...

  • 0 in reply to Paul Klarhoefer1
    Intellectual 365 points

    Just one update push, because it is marked as "Ti think it is resolved". But it is not resolved. I still don't understand. See above post from me.

  • 0 in reply to Paul Klarhoefer1
    TI__Guru 61380 points

    Paul,

    Half your VREF would be the 0A point. VREF can accept up to 5.5V and down to -0.3V. For example you could set your VREF to 5V, 2.5V would be your 0A midpoint, then 2V above or below this would be your -40A to 40A end points.

    Regards,

    -Adam

  • 0 in reply to Adam Sidelsky
    Intellectual 365 points

    Thanks Adam.

    Ok, then I understood it correctly. However then the example provided in  chapter 9.2.1.2.4.1 is not correct, from my point of view. Because it says, that VREF is 3.3V. But with 3.3V you can not measure +40A or -40A. Because the zero point is 1.6V.

    The only way this would work with 3.3V is to reduce the Gain to 10 not to 20. Am I right ? Or can something go wrong with 10 V/V instead of 20 V/V ?

  • 0 in reply to Paul Klarhoefer1
    TI__Guru 61380 points

    Paul,

    You are exactly right, sorry for the confusion! A gain of 14 or less should be recommended, not more.

    Let me reach out to the engineer who wrote that in the datasheet and get clarification.

    Regards,

    -Adam