I am taking a battery voltage, dividing it down and sampling this with my microcontroller's (MSP430) analog to digital converter (ADC).
I want to determine the overall (total) error that will be in this part of the system and translate this to a voltage on the scale of my original signal. (i.e. Total error will be +/- 0.020 V of the original 24V.)
A.) Let's say I am measuring a voltage of 24V coming into my system.
B) I am dividing this down with a resistor divider using resistors with a tolerance of 0.01%. R1 = 59k and R2 = 5k (Ratio is 0.078125). A voltage of 24V on the battery will then get divided down to 1.875V.
C.) I am using the internal “2.0V” reference of the microcontroller as a reference for the ADC.
The voltage here looks to fluctuate +/- 1.5% from the nominal 1.98V with VCC powered with 3V.
Datasheet snippet here:
http://i1228.photobucket.com/albums/ee460/MrBlueCalifornia/ADC_Refernce_zpsdhh1bkhw.jpg
Will this tolerance change if Vcc = 3.3V instead of 3.0V?
D.) The microcontroller that I am using (MSP430) gives some specs of the 12-bit ADC.
Integral Linearity Error = +/- 1.7 LSB
Differential Error = +/- 2.0 LSB
Offset Error = +/- 2.0 LSB
Gain Error = +/- 2.0 LSB
Total unadjusted Error = +/- 3.5 LSB
Link to datasheet snippet:
http://i1228.photobucket.com/albums/ee460/MrBlueCalifornia/ADC%20_erros_zpsy3dfpq7w.jpg
I am not sure if I should add up all the errors here in the ADC section but I feel I should add up all the errors in this section:
Total ADC Error = 1.7 LSB + 2.0 LSB + 2.0 LSB + 2.0 LSB + 3.5 LSB = +/- 11.2 LSB
Since I am using a 12-bit ADC I belive the total error here would be 11.2 LSB’s / 4096 = 0.273 %
E.) My total error would then be
Resistor 1 (0.01%) + Resistor 2 (0.01%) + ADC Reference for Conversion (1.5%) + ADC Error (0.273%) = +/- 1.793%
F.) If I change my ADC reference voltage to a 0.5% part then this works out to:
Resistor 1 (0.01%) + Resistor 2 (0.01%) + ADC Reference for Conversion (0.5%) + ADC Error (0.273%) = +/- 0.783%
Voltage reference: ±0.5%
G.) Now if I have a +/- 0.783% error and I compare this to my 24V that I am measure I could be off as much as +/- 0.187 V.
Does this look correct? Thanks for any feedback.
Thank you.
-Mike
