step down transformer and pt100 amplifier.....

[jit_singh_tara]

Dear friends ,

I am using step down transformer of 350mA .with 2 separate windings :
a.) 9-0-9V
b.) 0 -12V

Now i am generating :
+5v , +8V , -8V from the 9-0-9 winding .
And i am generating some 14V from 0 - 12 winding .

Now i am powering up my mcu , 8 seven segment displays , switches from the +5V regulator(lm7805).
And i am powering up my pt100 amplifier which includes 3 op07s and one lm324(constant current generator 1mA for the sensor . with +8Vregulator(lm7808) and -8V regulator(lm7908).

Now when i run the entire ckt there is fluctuation of 4 adc counts of 16f886 i.e around 20 mV . Please suggest me how to get a stable reading.

When i checked the regulators's output i found that +8V value was fluctuating like 7.3 to 7.8 volts ... and since this was powering the op07s amplifiers ...so there is subsequent shift in the final reading . please suggest a good method to have a stable reading on these voltages ....


Going to get a 12-0-12 winding with 0-12 winding transformer tomorrow .in order to get full +-8 volts from the regulators......

PLease suggest anything.....

 

[crutschow]

Do you have the regulators properly decoupled with the recommended capacitors near the input and output pins of the devices? Sounds like they may be oscillating.

 

[jit_singh_tara]

well i tried 470uf at input , and 100uf at the output.....

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any recommended values ?......