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Integrator Help Please

A

Anonymous

Guest
Hi all,
I built the front end circuit that Eric posted with the output through 4066 sample gates into the differential integrator that Eric also posted. When I power up the circiut the output of the integrator is at about -.5V and sticks there. Then when I bring metal near the coil, as it gets closer the voltage still stays at -.5V until the metal gets close enough to the coil it reaches a point where the voltage then shoots up. Then it behaves properly in that it shows a positive voltage continuously and it increases as metal is brought near the coil and decreases as the metal is pulled back away from the coil. Its like its stuck at -.5V until its got enough signal and then shoots positive. Has anyone experienced this? Its annoying to have to bring it near metal to make it work properly and its going to be a real problem when I hook up a PIC chip to it. I am using a TL072 for the integrator op amp on +5V VCC and -5V VSS. Tonight I thought I'd play around with the component values in the circuit some more but I really don't know if thats the problem or if it lies somewhere else. Any ideas would be appreciated.
Thanks a lot,
Fred
 
Hi Fred,
I'll come back with more hints later, but try grounding the input to the gates. If the integrator is working OK its output should be approximately zero. If this is OK try a longer sample delay, say 100uS and see if it comes off the -5V. If so, you may be sampling a positive overshoot in the coil circuit at the shorter delay. This will drive the integrator negative.
Eric.
 
Thanks for the feedback Eric. I will try shorting the inputs like you suggest. Acutally its not going to -5V but -0.5V on startup and it doesn't budge until metal gets close to the coil. Then after that it floats around above zero and as you move the coil around it fluctuates with the changing noise level which I can also monitor on the output of the NE5534 front end. It won't do that though on power up until metal is brought near the coil. Its quite strange. I am using a PIC to generate the timing pulses and have provided no way yet to change the pulse delay. Its fixed in my code at about 20uS. I will change that to 100uS to see if it affects it. I'm intending the final use for this as a boat tow so its going to end up with a much longer pulse delay than I have now anyway. Also, Once the detector is working properly, I plan to have a pot on an A/D or 2 pushbuttons to allow the user to vary the pulse delay.
Thanks,
Fred
 
Hi Fred,
Sorry, I missed the decimal point. The output of the integrator can sit at any level due to the coil characteristics and whether there is any metal nearby. In a completely metal free area, the coil does not have a flat response, particularly just after the recovery period i.e. 15 - 25uS. There is generally an overshoot due to the coil/screen/cable capacitance which makes the integrator sit negative. With plus and minus 5V supplies, I try to get the integrator to sit within + and - 2V to give plenty of headroom for signal.
When you say the integrator is stuck on -0.5 but frees itself after a piece of metal is brought up, it looks like some sort of unstable situation. Do the supply rails change? Are the supplies to the 4066 + and - 5V and also is the gate drive pulse of this voltage range?
Eric.
 
Hi Eric,
Of course I'm sitting here at work right now where I can't check the things you are asking. I will verify tonight and get back to you the results. I really appreciate you taking an interest in my problem.
I think you are right that there is something unstable. Once metal is brought near the coil, its like the output is unstuck (for a lack of a better way to put it) and then it sits around 2 volts (no metal near the coil) slightly moving as I wave the coil around and then it increases in voltage until it peaks at about 4.3 volts or so as I bring metal near the coil. It comes back down to around 2 volts as I remove the metal from the proximity of the coil.
Again, thanks and I'll let you know what I find after this evening.
Fred
 
Thanks Eric! On your post regarding my 4066, it made me think about that part of the circuit. I had actually used a 74HC4066 for the gates and of course it is constrained to TTL logic level voltages. My thoughts were that I would replace it with a 4066 and then switch the ground pin to -5V. When I replaced the 4066 I tried the circuit again and for reasons unknown to me it worked. With the circuit working, it uncovered a bug in my PIC code but its actually detecting metal and displaying it on the LCD. Its these small little wins that are so gratifying.
Thanks so much,
Fred
 
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