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Problem with ADC (MCP3201) conversion of a simple resistor ladder network

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codewrecker2

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Hi, I am in a spot of bother with the ADC conversion of a simple resistor ladder network.What i have is a series of 150(1%) ohms resistors with every other terminal of the resistor connected to o/p of a hall sensor.Yes,the hall effect sensors and the resistors are equal in no. Sort of like this

--constant current source->-150-o-----150------150-------- . . . . . . .. o termination
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|xxxxxxx|xxxxxxxxx|xxxxxxxxxxxx|resistor
xxxxxxxxxxxxxxxxxxxxxxxxxHall1o/pxxxxxhall2o/pxxhall3o/pxxxxxx|
xxxxxxxxxxxxxxxxxxxxxxxxxx|xx|xxxxxxxxx|xx|xxxxxxx|xxx|xxxxxxxxx150
------gnd--------------------------o--|------------o--|----------o----|--------oxxx|
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx|xxxxxxxxxxx|xxxxxxxxxxx|xxxxxxxxxx|
-------9V-------------------------------o-------------o---------------o--------------o--


I have a equivalent R of 15K( 150x 100).This is what the circuit does.as i pass a magnet across each hall effect i get a voltage drop across these resisors which is linear throughout the length.It seems linear enough on all multimeters i used.But in the application i am using MCP3201 as the 12 bit A/D doing spi with the pic 16f689.

This gives me a linear(consistent with DMM) reading upto the resistance of 12 K.Once i switch the hall ON beyond the 12 K(equ. R) , i get a very low converted voltage (~0.02)whereas the multimeter still remains linear(~1.2V). Also when none of the Hall effect sensors are ON, i get very low voltage through the ADC which is nowhere near the DMM reading. Then if i switch a hall ON within the first 12K(equ.R), i get ADC reading which matches the DMM reading spot on.

Is there something i am doing wrong by having resistors in series?

OK one more thing i gotta disclose.When i turned to the internal A/D of my PIC (10Bit) ,i got accurate results all along even where the MCP3201 was failing.WHAT COULD IT BE??

Thanx, if you reached here reading this.Ciao
 

Zman

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ADC MCP3201 headache

please post a better schematics, it is totally unreadable...
 

codewrecker2

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Re: ADC MCP3201 headache

OK,here it is now.sorry for the delay.
 

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