OT: looking for help out of the forum - SOLVED thanks

[Gabriel]

hello, i have a non programming related issue with to my latest project. if any one is interested in helping please contact me via private message as to not bother anyone else with this.

its a problem with a instrumentation amp built out of one LM324 (3amps + 1 buffer)... its powered by single 10V supply, it takes a differential signal from a GE DRUCK pressure sensor, and amplifies the result, and then on to the PICs ADC.

R = 1Mohm, Rgain= 50kohm trimpot set for ~70 gain (Rgain=28.5kohm)

sensor output 0 - 100mv .... I've set the gain so that when the sensor is at 70mv output my ADC gets ~5V. (as to get 1m depth = 1volt)

the problem is that when the sensor gets past a certain point, its output keeps increasing ( getting closer to 100mv) but the amplifier is stuck at 2.14 volts output.

so in other words the amp works flawlessly form 0 - ~30mv at which point the op amp stops amplifying and the output regardless of the input is stuck at 2.14 volts.

i get 20mv from the sensor i get 1.40 from my amp - ok
i get 30mv from the sensor i get 2.14 from my amp - ok
i get 40mv from the sensor i get 2.14 from my amp - WTF
i get 70mv from the sensor i get 2.14 from my amp - WTF

it cant be saturation since I'm no where near the maximum output which worse case would be ~: Vcc-2v = 8V...

gain is 70, which i don't think its excesive... and worst case scenario se sensor outputs 100mv my amp should put out 7V....still not in saturation

sensor signals are both 7.53V, the amp take care of removing this just fine.... or seems to do so.

thank you...

Gabriel
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CCS PCM 5.078 & CCS PCH 5.093

[vinniewryan]

What's your compiler version? Just kidding.

Are you getting anything between 1.4V and 2.14V from the OP amp? Such as 1.5, 1.6, 1.9, 2.1..?

I don't have time to review your math right now, but I would suggest you double-triple check the data sheets to make sure your calculations are correct to the OP AMP specs. Also you might try testing the output of the amp by simply connecting resistors from Vs/d to the input, and testing the output to see if you can get any decent increase in output. If your OP amp always peaks at 2.14 volts, it's certainly a problem with the circuit. Check your connections, pins, and do a simple test with the chip to make sure it's not bad.

Do you have a schematic of your circuit? It would be easier for me to see potential problems with a diagram of some sort.
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Vinnie Ryan

[SherpaDoug]

Make sure none of the 4 op amps is saturating. In that configuration it is common to have the middle nodes saturate before the output.

At least we haven't told you to check your fuses ;-)
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The search for better is endless. Instead simply find very good and get the job done.

[Gabriel]

hi, thanks for your help... ill check for that...

private messages please..... dont want to cause any trouble..

gabriel.
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CCS PCM 5.078 & CCS PCH 5.093

[Humberto]

Hola Gabriel,