topic ADC output in LPC Microcontrollers

ADC output

lpcware — Wed, 15 Jun 2016 17:53:08 GMT

Content originally posted in LPCWare by Shoaib on Sun Sep 14 15:10:46 MST 2014
Hi everyone,

I am using LPC1769 to measure current in a DC motor circuit and had some queries regarding the ADC of the microcontroller. I am using LTSR 15-np as the current sensor in +/- 5A range.
The sensor gives an output of 2.5V for 0 A current and increases upto 4.5V for 5A current.
When the motor circuit is disconnected (ie no current in the circuit)the adc is a giving a value of 3298/3299. I was expecting a value close to 2048.
I did not change the reference voltage given to the VREFP pin. Do I need to give 5V reference for this range?

Re: ADC output

lpcware — Wed, 15 Jun 2016 17:53:09 GMT

Content originally posted in LPCWare by LabRat on Sun Sep 14 15:34:04 MST 2014

Quote: Shoaib
I was expecting a value close to 2048.

Why?

Quote: Shoaib
Do I need to give 5V reference for this range?

Vmax for VREFP is VDDA (see datasheet). That's probably 3.3V in your case.

Just use a simple voltage divider to reduce your sensor output and calibrate your zero current point (which is 2.5V [color=#f00]+- 0.625V[/color])...

Re: ADC output

lpcware — Wed, 15 Jun 2016 17:53:09 GMT

Content originally posted in LPCWare by Shoaib on Wed Sep 17 03:16:47 MST 2014
Thanks for the reply!

I somehow missed the point that I should limit the voltage to the adc upto 3.3V that explains the reading of 3298 as (3298/4095) is nearly equal to (2.5/3.3). Thanks for pointing that out!