Pic Voltmeterwith a small problem

[Keaster12]

Im trying to build a voltmeter using a PIC1F887 and MikroC to measure the voltage drop across a resistor in a tube amp project I'm working on. I want to be able to display this on a lcd screen to monitor the bias of the amp. The voltage applied to the grid of the tube is actually negative but I don't think that matters to my problem. The voltage drop range on the resistor is about .5-1.2v on my store bought meter it reads not problem but on the PIC voltmeter it reads it but seems to fluctuate a bit about .2 volt up and down continually and will never settle at one reading like my other meter.
The pic is set to reference Vdd

Anyone have any thoughts or ideas

 

[paulfjujo]

hello,


could you show a drawing , where do you get the measurment ?
you can not measure on the grid of the tube, to high impedance is needed to do that.
Is there only DC value on this resistor ( no AC or modulation)
Do you wait enough time for ADC capture ?
How many measures per second ?
post you code ...

 

[Keaster12]

hello,


could you show a drawing , where do you get the measurment ?
you can not measure on the grid of the tube, to high impedance is needed to do that.
Is there only DC value on this resistor ( no AC or modulation)
Do you wait enough time for ADC capture ?
How many measures per second ?
post you code ...

The measurement is taken across the 15.6R resistor that's between the cathode and ground. I'm not near the computer with the code at this time but I'm waiting about 1/2 a second between measurements and there's no delay between capture and display, didn't think I would need any. I did think high impedance could be a problem if it is, is there a lower it?

 

[paulfjujo]

hello,

Try to use a low pass filter , between R 16,6 and PIC ADC input , to keep only the DC average value.
example : 2,2Kohm and 1µF ..