I made a quite faulty design on my project which ia a voltage measurement device in a Truck (24V) as I learned when tested. I used SMAJ littlefuse in the measurement channel- which lead through its leakage to diffrent voltage readings in between those two channels. Also the leakage is non linear- which provided me from fixing this with a scale value. So I digged in this great forum and found a very interesting thread:
In my project I would like to make use of the analog ins of the 328P to measure 0- 30V DC (calculated as 0-33VDC to have some margin). The 328P runs on 5V controlled by a Traco TSR 1 2450 (I guess I need to ask also about that one, later)
The above thread was super helpfull but I am still a bit stuck.
Here is what I wonder:
- I changed the design to two diodes per channel, I think I will need a diode with ultra low leakage to be sure not to alter my measurement signal, right? any suggestions?
- A cap was mentioned to get rid of AC ripple and to calm the input. Was it thought in the way I did it?
- Any suggestion on size for that cap? I still would like to use the data at approx 100Hz sampling, so I don´t want something that big that it averages out my measurement, right?
- Is the input impedance ok? Or should I chose other voltage divider rating? I saw something with 1Meg Ohm in series on the input - and thought that must also be a great source for noise, insn´t it?
- As I understood this diode approach needs a stable VCC rail, since I use a Traco TSR-1-2450 with a 0,01mF on direct out and a 10muF on the PCB to stabilize the VCC, is that rigid enough? Or shall I consider another approach?
With my first approach, with the littlkefuse:
I desoldered them and tried just the circuit. I found that the 0V and the 33V Point are quite accurate. But in between the thing looses this accuracy, and is of a few hundrets mV (like 800-1200mV). Of cause I only check with a scope, but I am afraid to maybe got something else wrong. At least both channels show the same behaviour.
Even if a 10bit adc has its limitations, I would at least expect it will work reliably, or am I mistaken and should look for some other ADC that will just work out of the box?