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AnalogIn accuracy

Topic last updated 08 Nov 2010, by Michael Gulsch. 9 replies
mario raats
01 Nov 2010

Hi All,

After receiving my Mbed I started experimenting right away. I developed some PIC based measurement solutions and wanted to convert them to the Mbed.

One of the things I want to build is a pressure measurement device to measure altitude, that gave good results with the PIC.

After connecting the analog part to the AnalogIn pin of the Mbed, the readings fluctuated heavily. I found more topics about this, it is described there as static.

So I connected an AA battery, theorectically that should have a very low input resistance. The results are the same, fluctuations of >5%.

Is there something wrong with the implementation of the ADC??

Does somebody have the same experience??

Igor Skochinsky
01 Nov 2010

Search forums for "noise".

mario raats
02 Nov 2010

That is just the point. With an AA battery connected as short as possible to the input pin, there should not be noise. Internal resistance/impedance of a power source is close to zero Ohms.

If it is a noise problem. it should be in the design of the Mbed board, which makes it unuseable for Analog measuring.

Martin Smith
02 Nov 2010

Bingo!

If you want accurate analogue measurements use an external IC.

Ian Molesworth
02 Nov 2010 . Edited: 02 Nov 2010

Save yourself a lot of heartache. Use an SCP1000 barometric sensor. Fully compensated, digital output, down to 15cm altimetry resolution.

Take it from someone who has a tube full of sensor technics analog barometric sensors at around £60 each going to waste!

Niclas Oberg
02 Nov 2010 . Edited: 02 Nov 2010

Ian - is your SPC1000 running nice on NXP ?

I have a SPC1000 from diydrones and want to get it running here

//Niclas

Ian Molesworth
04 Nov 2010

Yup Mine is up and running.

Niclas Oberg
04 Nov 2010

Nice - but I have the i2C version and the code has to be modified - my readings were stable at 3276 degrees :)

Ian Molesworth
04 Nov 2010 . Edited: 04 Nov 2010

 

user Niclas Oberg wrote:

Nice - but I have the i2C version and the code has to be modified - my readings were stable at 3276 degrees :)

That's a little warm ( divide by 100? ) I'm using the SPI version from sparkfun. I'm not reading temperature since I get that and humidity from an SHT75

Michael Gulsch
08 Nov 2010 . Edited: 08 Nov 2010

Hello,

 

I am using the SHT75, but I am reading both, temperature and humidity. Is there a reason only to read humidity?

Have a look to my library: Sensirion

Greetings

Michael