I'm using AD8212 to measure current through led lamp.
The dropout voltage on R15 is 0V i don't know why.
See attached schematic and PCB layout.
there is no connection between GND_ISO and GND_48V.
Can you confirm that you are measuring directly across the resistor (between IOUT and GND_ISO)?
Couple of things to try and check out:
Is it possible for you to disconnect the AD8212 from the rest of the circuit? Please measure the voltage across the resistor when it is disconnected. While it is disconnected, you could also try having a single ground for the device and see whether that gives you the correct output.
Also please probe across the shunt resistor, what is the actual voltage at the input of the AD8212? Can you also check what voltage is across V+ and COM? It should be 5V.
Let me know what you find and we can go from there.
Yes, i measure directrly across the resistor (between IOUT and GND_ISO) = 0V.
The shunt voltage is 200mV.
it's not possible to disconnect AD8212 because as you know the footprint is quite problematic and i don't think i can manage to put it back on. this is a prototype board i made so only 1 is available.
First thing i'll do on sunday when i get back to work is to measure 5V across V+ and COM.
if it's OK than i will try to connect GND_ISO and GND_48V (i hope this will not affect the isolation to A\D).
buy the way i notice that in CN0218 there is a connection between GND_ISO and shunt GND (Figure 1)
but in the schematic i didn't find any connection only connector J5 to which i need to connect a shunt resistor , that is why i didn't connect it in my scheme.
what do you think ?
I have connected the GND_ISO and GND_48V and it's working now.
The voltage across V+ and COM is 5V.
Is connecting GND_ISO and GND_48V interferes with the isolation of the A/D ?
Sorry for the delayed reply.
I checked CN-0218 and the important GND to be separate is the grounds on the isolator.
On the figure above, it is Gnd-iso and Gnd1 that need to be separated. The grounding that you referred to (below) is the ground of the source and the ground of the analog circuitry.
It looks like by connecting your two grounds together, you are connecting the grounds on the isolator, which may cause problems in your system.
I've connected GND_ISO and GND_48V as you suggested and the output value from AD8212 seems OK.
as you can see from the attached files GND_48V is connected to DC-DC which it's output 5V & GND is for the MCU.
I don't think this is like connecting the grounds on the isolator because the dc-dc is a kind of an isolator himself, doesn't it?
BTW, i'm getting reading from the A/D with +3mA error, so it seems to work, but once in a while i gets errors from the AD7171, the status register is not equal to the desired pattern, maybe there is a problem with the software, i'm still working on it.
can you pls advice about the ground connection problem and how to fix it (if you see any)
In the above scheme the measure output from the shunt resistors goes into a LED lamp.
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