thanks all for your help.. I see it is a problem with the internal resistance of CD4066B while it has quite big 90 Ohm (according to my calculation in the schematics design) internal resistance.
Hence, my objective now is to fix what I have now (the PCB is etched), hopefully with a limited time of my thesis work period I could quickly fix the PCB I made by just changing several components (resistors, etc) instead of adding another few! Only if the problem still there, then I would redesign all my layout. So, I need your advice again here...
Regarding @crutschow 's explanation, I would like to ask several questions:
-On the LM317 side:
Let's take an example of determining the R1 for 10mA current source (since it is the minimum output current LM317 can deliver >> Might wanna change to LM317L as Boncuk said). Then the calculation would be:
1.2V : 0.010A = 120 Ohm (Is it correct?)
BUT not to forget the internal resistance of CD4066, so when I assume it to be 90 Ohm then to deliver such 10mA of current, the external resistor in one of four switches output must be substracted by 90 Ohm.
This gives 120 Ohm - 90 Ohm = 30 Ohm >>> nominal resistance value for one of four switches to give 10mA output current source.
When the voltage drop across the RLoad is maximum 5V-2V-1.2V = 1.8V, the maximum resistance the current source can supply is equal to 1.8V : 0.010A = 180 Ohm
FYI: The purpose of having this variable current source is to sense the different voltages of each E12 Resistor in two decades ranging from 1 Ohm - 8.2 Ohm to 10 Ohm - 82 Ohm. This is put at the RLoad terminal of my current source before amplified (the voltage drop across RLoad) with Instrumentation Amplifier as an input to 10bit ADC in uC unit. Since the maximum measured resistor is equal to 82 Ohm, from what I did above, 180 Ohm range is far away from 82 Ohm and it is adequate to supply 5V power to LM317. Even if I supply with 20mA current, the maximum allowable resistor is 1.8V : 0.020A = 90 Ohm, it is still in range!
The problem:
-I am trying to deliver not 10mA ++ but also 1mA, 5mA, 10mA, and 15mA (max) this is used to vary the voltage output of my current source circuit to be gained by the inst.amp (the inst.amp is also variable gain with four switches as CD4066 one) so the output to the ADC is always above 1V (resolution is 5mV, I took 1V because it is 20% of the maximum range 5V, hence increase accuracy).
I believe from what I read above, substituting LM317 to L type might work!
And my thesis title is: An automatic resistor measuring and sorting device using uC by the way...
Guys, I am very thankful if you could read and understand, my english is not very good since I am living in such a non-english-speaking country. I hope I could get some ideas here, thanks!
Correct me if I'm wrong!
thanks all!
Best regards,
Kelvin
Indonesia