noise or feedback problem with my ckt i guess

[scud21]

Hi

Im using the adc of pic16f877a and my ckt was all doing fine until i noticed that the signal from the oscilloscope where some kind of distorted when a LED lights up (i used LED to see if my program works). to give you background of what i am doing, my group is making an Automated External Defibrillator. the signal is a heart rhythm coming from a heart simulator. well back to my problem, like what i said earlier the signal is become distorted. it's like becoming a wave with an area. I used an ECG ckt and clamping ckt and looking from the output, it was really desirable but when i interfaced it to the PIC and then a LED lights on any port, the signal becomes distorted. i thought there was a feedback current coming from the LED. i already used optocoupler/optoisolator and also try increasing the resistor in series with the LEDS but it didn't work as well. can anyone suggest on how to solve my problem

regards,
scud

 

[eblc1388]

Is the PIC power supplied by a stable voltage regulator, fitted with suitable bypass capacitors? Have you checked the impedance of your clamping circuit feeding the PIC?

It seems the signal impedance is very high in your case, otherwise it would not be affected by the small current of a LED. A buffer between the signal and the PIC pin is required if the impedance is higher than 2.5Kohm as recommended by the PIC datasheet.

You would need to post the schematic of the clamping circuit, A/D input to PIC and the PIC power supply arrangement for additional comments.

 

[penoy_balut]

Check also your circuit layout and grounding.

 

[tunedwolf]

I reckon it's a power supply oscillation due to lack of decoupling for the micro. Make sure the power supply for the Pic is well regulated and decoupled.
And as has been suggested already, make sure your high input impedance is properly terminated into the pic interface. It could also be lack of drive, or perhaps an ac component of the signal your feeding the Pic with causing the oscillation.

 

[scud21]

I reckon it's a power supply oscillation due to lack of decoupling for the micro. Make sure the power supply for the Pic is well regulated and decoupled.
And as has been suggested already, make sure your high input impedance is properly terminated into the pic interface. It could also be lack of drive, or perhaps an ac component of the signal your feeding the Pic with causing the oscillation.

thanks for replying,

I used LM7805 as my voltage regulator with a 1N4001 diode rectifier connected to the output. I'll check the high input impedance termination solution.

regards,
scud

 

[scud21]

Is the PIC power supplied by a stable voltage regulator, fitted with suitable bypass capacitors? Have you checked the impedance of your clamping circuit feeding the PIC?

It seems the signal impedance is very high in your case, otherwise it would not be affected by the small current of a LED. A buffer between the signal and the PIC pin is required if the impedance is higher than 2.5Kohm as recommended by the PIC datasheet.

You would need to post the schematic of the clamping circuit, A/D input to PIC and the PIC power supply arrangement for additional comments.

thanks for replying,

i'll check the bypass capacitor in my volatge regulator and also the input impedance. My input impedance i think is more than 10Kohm.

regards,
scud

 

[redserpentone]

scud21:
I missed something obviously.
Why would you rectify a 5v regulator with a diode (.7v) ??

RedSerpent1

 

[scud21]

scud21:
I missed something obviously.
Why would you rectify a 5v regulator with a diode (.7v) ??

RedSerpent1

the 1N4001 rectifier diode i used has a parameter of 1 ampere/50V maximum..the 0.7V diode you were saying is just a simple silicon diode..please look on the internet about different kind of diodes

 

[Nigel Goodwin]

the 1N4001 rectifier diode i used has a parameter of 1 ampere/50V maximum..the 0.7V diode you were saying is just a simple silicon diode..please look on the internet about different kind of diodes

The 1N4001 is a silicon diode just the same, and has a 0.7V forward voltage drop - which is presumably what 'redserpentone' was refering to?.

 

[redserpentone]

am familiar with the 1N4001 the workhorse of industry for many a year.

If you connect in parallel a !N4001 to any LM7xx you have a 0.7 volts power supply considering that the input impedance of your circuit is well bellow of the diode's impedance. Sorry little diode.
this circuit would make a nice coffee warmer though.
Check on Internal resistances and 'swamping' of power supplies if you are connecting the diode in parallel with the LM7xx, if not ...I missed something.

rs1

 

[eblc1388]

If you connect in parallel a !N4001 to any LM7xx you have a 0.7 volts power supply considering that the input impedance of your circuit is well bellow of the diode's impedance. Sorry little diode.
this circuit would make a nice coffee warmer though.
Check on Internal resistances and 'swamping' of power supplies if you are connecting the diode in parallel with the LM7xx, if not ...I missed something.

rs1

Why have you automatically assumed that the OP had connected the diode in parallel instead of in series with the output?

Perhaps he might just want to get 4.3V instead? Or the diode is indeed connected in parallel but to protect reversed polarity? In both cases it is fine.

 

[scud21]

Why have you automatically assumed that the OP had connected the diode in parallel instead of in series with the output?

Perhaps he might just want to get 4.3V instead? Or the diode is indeed connected in parallel but to protect reversed polarity? In both cases it is fine.

L. Chung was right guys

 

[scud21]

Thanks to all of you guys, specially to L. Chung and tunedwolf it works now..^_^