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#PMOD ISNS20 Question about formula


Bl3nder

Question

Hello I Hope someone can help me,

 

I bought a pmod isns20 ... and have a question about the formula in this guide:https://digilent.com/reference/pmod/pmodisns20/reference-manual

I dont understand from  where does the formula come from -> Ima = 1000/89,95 .....   i dont understand the 89,95 i have read multiple times the Datasheets ( ACC772,adc7476) .... my calculation would be as follows:

 

lsb = 3,3V ( operating voltage) / 4096 ( bit range frome the 12 bit adc)

x = read out bit word

mv = (x-2048) * lsb *1000

Ima = mv/66 ( sens from the sensor)

 

what is my mistake ? Can someone please help with the formula in the guide ?

 

 

 

Edited by Bl3nder
mistake
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Hi @Bl3nder,

I feel like I have corrected the Pmod ISNS20 material multiple times over the years since this division by 819 originally introduced is complete bogus; I don't know why it keeps getting reintroduced into the material.

The proper conversion for the Pmod ISNS20 (which is a retired product and correspondingly will have limited support) should be:

//result1 = first byte from the embedded ADC
//result2 = second byte from the embedded ADC
//sensitivity_correction is a global variable set by the users whim
//offset is the measured value by the module when there is nothing connected to the module
//Sensitivity correction percent * (bit result / 12 bits * 3.0V reference - calculated offset voltage)/ (0.066V per Amp ratio) 
amps = (SENSITIVITY_CORRECTION / 100.0) * (float((result1<<8) | result2)/4096.0*3.0-offset)/0.066;

Thanks,
JColvin

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Thanks for your answer but I still don't understand it:

offset is a a measured value when nothing is connected  example  2048  -> 2042 but what your mean exact with the SENSITIVITY_CORRECTION  can you give me an example ? 

 

Thanks :) 

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Hi @Bl3nder,

The sensitivity correction is a user defined correction value that you can arbitrarily choose; it is not required to use by embedded chip, if you end up deciding that there is a need to make this sort of correction to the output value. Otherwise you can just keep this value at 100 in order to not introduce any changes.

Thanks,
JColvin

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