commone ground problem

[mshh]

common ground problem

I have a problem with my circuit , i control buck converter using atmega8 and data acquisition card . my circuit has also LM35 temperature sensor .in the beginning i used one power supply to give +12 v and +5 v using lm7805 and that makes the regulator got hot , the problem is coming up when i connected all the ground to just one point . the control and power circuit have the same ground and that increased the output of lm35 however it should be lower . when i disconnect the power circuit the output of lm35 is gonna to be alright . then i tried to use power supply for the lm35 and another power supply for the power circuit but in a vain . lm35 is just connected to capacitor and resistor that is existed in its datasheet for filtration then a voltage follower op amp. i display PWM using oscilloscope (terminal and its ground)i attached the power and control circuit .so please help.

 

[mshh]

I think that the problem is with my power circuit , because i removed it and i measured temperature it was ok

 

[D.A.(Tony)Stewart]

Your problem is not clearly stated. I understand you get excess power lost when grounds are connected and also false temperature readings. THe RC circuit in the LH35 does nothing as shown. Beware of spurious resonance, long inductive wires and poor grounds.

 

[mshh]

my problem is false sensors readings however sensors is followed by op amp and it is working well separately when i connect the power circuit. the RC circuit removed noise

 

[D.A.(Tony)Stewart]

False sensor readings could be from voltage drop on ground wire or inductive noise on wires.

Twisted Pairs helps to reduce radiating common mode noise from PWM's with power and Return wires.

I suspect then your schematic should be corrected to show Op Amp input connect to Cap instead.

 

[mshh]

I suspect then your schematic should be corrected to show Op Amp input connect to Cap instead.

what do you mean by Cap instead , there is 100nf on the input of the op amp to filter noise