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Electronics => Electronics => Topic started by: g44 on March 08, 2010, 02:12:16 AM

Title: A/D
Post by: g44 on March 08, 2010, 02:12:16 AM

I work with PIC microcontroller
I used some function in this microcontroller with A/D converter
My main use was measure changes in the potentiometer
My problem now is that I want to connect to the microcontroller an accelerometer
And I see that the accelerometer have output resistance of 32K ohms
In the datasheet of the microcontroller I see that I can only connect to him an approximate resistance of 10K ohms
What I can do that this will work ?
Connect a parallel resistor to the ground ?
Is it meter ?
The devices that I work with them are :
PIC 16F887
http://ww1.microchip.com/downloads/en/DeviceDoc/41291F.pdf (http://ww1.microchip.com/downloads/en/DeviceDoc/41291F.pdf)
The data appear in Zain
PIC 18F97J60
http://ww1.microchip.com/downloads/en/DeviceDoc/39762e.pdf (http://ww1.microchip.com/downloads/en/DeviceDoc/39762e.pdf)
The data appear in Zain
Accelerometer  MMA7361LT
http://www.freescale.com/files/sensors/doc/data_sheet/MMA7361L.pdf (http://www.freescale.com/files/sensors/doc/data_sheet/MMA7361L.pdf)
The data appear in output impedance

Title: Re: A/D
Post by: waltr on March 08, 2010, 08:07:11 AM

Good catch on the accelerometer output impedance and the PIC's ADC input impedance. If you connects the accelerometer directly to the PIC's ADC you will get an incorrect value.
What you need is an Op-amp voltage follower to provide a low impedance source to the ADC.

Title: Re: A/D
Post by: g44 on March 09, 2010, 01:38:00 PM

thanks waltr
can you give a site that explain how to build an Op-amp voltage follower

Title: Re: A/D
Post by: waltr on March 09, 2010, 02:34:15 PM

Just google "Op-amp voltage follower". lots of info.