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ADC0808/sensor reading error?!!

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Thirdium

New Member
Hello all,
I've designed a crkt, to read sensor values, long story short, i have a rabbit MC, a bunch of sensors and ADC0808.
Used also opamp to amplify some senor outputs, the problem is this:
when i connect GROUND(0) to any input of the ADC0808 i get an output reading of 0.67 Volts! (Vcc=5, GND=0, Ref+= 5V, Ref- = GND).
the other weird thing is that alll the outputs for the temp and other sensors are exactly the same as input(no 0.67v shift), yet only for the Pressure sensor i get a 0.67V shift...if i measure the input using a voltmeter i get 3.9v, but the output reading of the adc is 4.5V!!), note also that all components are connected to the same Ground rail, so no difference in ground).

The pressure sensor i am using is "MPX4250A".

i'd appreciate any help...thanks!
 
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Sceadwian

Banned
Not a clue sorry.
 

Thirdium

New Member
c'mon guys! somebody out there can help me! its my graduation project, this is really gettin on my nerves, i have to start designing the PCB phase :(
 

Thirdium

New Member
any suggestion is welcomed...... coz its confusing me.... is it possible that conenctin the Rabbit microcontroller directly to the ADC0808 is applyin some kind of load?


thanks
 
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Sceadwian

Banned
You're getting different readings on a meter and the ADC, maybe the ADC is bad? Is the output from the pressure transducer a steady voltage or pulses? Pulses will screw up an ADC's reading big time.
 

Thirdium

New Member
You're getting different readings on a meter and the ADC, maybe the ADC is bad? Is the output from the pressure transducer a steady voltage or pulses? Pulses will screw up an ADC's reading big time.
the output from the Pressure is steady, aslo if the ADC is bad why all of the other readings are correct?.
again the weird thing is that when i connect the ADC input to the ground i get 0.67V, but when connecting any other sensor with an output over 0.67v i get a correct reading (no dc shift), except for the pressure sensor i get a 0.67v shift:S! its crazy, i can solve it by hardware, but solvin the problem without knowin what exactly the problem is! is not quit engineerin, and it is an issue when i discuss the project :'(
 

Sceadwian

Banned
Can you post your schematic?
 

ccurtis

Active Member
I looked at this too, and I am at a loss for an explaination. The only things that come to mind are power supply bypassing, power supply noise, ground loops, the usual suspects. But, your results are good when most sensors are connected. Are you sure that nothing else is changing when you remove sensors to ground the inputs? What do you get for an output code with a floating input? The data sheet speaks of a potential DC offset if the input has filtering capacitors and a high source resistance. That might account for the pressure sensor, but not for the offsets when the inputs are grounded.
 

Thirdium

New Member
Thanks for the reply, well there are 2 notes to take in consideration:-
1. For testing issues (before desiging a pcb) the rabbit is on the development board, meaning the voltage supply is takin from that board, and the supply for all other components(adc,sensors,opamps.etc) on a bret board is takin from an external battery(+9v and +5v regulator), but there is no difference in grounds(voltmeter measuring).
2. Also there is a capacitor connected between the Pressure sensor output and ground ( 470 pF), i remember that i tried to remove it and nothing changed .

i'll post the schematic once im done, hope it'll help!
Thanks alot.
 
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Thirdium

New Member
here is the schematic...well most of it, didnt include the microcontroller, signals are connected directly to the ADC, (ADD A,ADD B, ADD C, ALE, D1-D8, Start, EOC). Output Enable (OE) is connected directly to +5 v.

 
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Sceadwian

Banned
You might want to put a decoupling cap on the VREF+ line to ground right next to the chip, a small electrolytic. I don't see anything blatantly wrong there (at least at the ADC) so I'm clueless =)
 

Thirdium

New Member
i told u it is drivin me crazy :p, anyways since my Vref+ is connected to the +5v rail, u can see that there is a ceramic capacitor 01uF connected between +5v and gnd.
thanks
 

Sceadwian

Banned
Doesn't matter Thir. The VREF should have it's own bypass cap right at the chip, it's not in the data sheet for your chip but it's common practice with ADC's.
 

ericgibbs

Well-Known Member
Most Helpful Member
i told u it is drivin me crazy :p, anyways since my Vref+ is connected to the +5v rail, u can see that there is a ceramic capacitor 01uF connected between +5v and gnd.
thanks
hi,
Have you tried a 47uF cap from the 7805 output to 0V.

Can you say what tempr range you have designed for with the 1K and 54R on the LM35,, just interested.:)
 
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