topic Re: Vrefn in ADC equation in LPC Microcontrollers

Vrefn in ADC equation

vaque — Fri, 24 Mar 2017 11:12:04 GMT

Hi;

I am working about a LPC824.

In UM10800.pdf at page 322 "Perform a single ADC conversion using a software trigger" it says that The result of the conversion is (4095 x VIN)/(VREFP - VREFN). But I need to know why is that equation and not this other:

The last one indicates a FS between Vrefp and Vrefn.

In this terms, the minimum LSB is:

Can someone help me?

Regards. Daniel.

Re: Vrefn in ADC equation

ckphua — Thu, 30 Mar 2017 00:15:10 GMT

For single end ADC input, the equation (4095 x VIN)/(VREFP - VREFN) holds since this is a simple ratio metric ADC.

Not sure how the equation with additional -VREFN comes into play.

Re: Vrefn in ADC equation

vaque — Thu, 30 Mar 2017 10:24:41 GMT

@Chee Keong Phua, basically I need to know if result is always 0 for any value of Vin=Vrefn.

* The first previous equation is missing a parenthesis between (Vinn-Vrefn)

Thanks and regards. Daniel.

Re: Vrefn in ADC equation

ckphua — Thu, 30 Mar 2017 15:40:11 GMT

When VIN less than or equal to VREFN, the ADC Count is zero.

Re: Vrefn in ADC equation

vaque — Fri, 31 Mar 2017 10:29:18 GMT

@Chee Keong Phua ok, thanks so much. Thus, the equation in UM10800.pdf at page 322 "Perform a single ADC conversion using a software trigger" is wrong. The good equation is this:

result = (4095 x (VIN-VREFN))/(VREFP - VREFN)

Thanks and regards. Daniel.