Sorry! I wrote wrongly abt the voltage at pin 6 of AD820. It should be V = 1.752V instead.
However, at pin 3 of AD820, V = 1.933V !
Added after 8 minutes:
IanP said:
Try different IL300F ..
If the problem persists increase the value of R1 from 49.9kΩ to ≈53kΩ ..
Regards,
IanP
I see no point in changing the value of R1 from 49.9kΩ to ≈53kΩ!
In fact, I think a 30KΩ should be used to replace both R1 and R3. This is because according to the page 5 of the datasheet of IL300F, for the K3(transfer gain) to be 1, it falls into group Bin F = 0.945 - 1.061. Based on page 10 of the datasheet, the resistance of the resistor should be 30KΩ.
Am I right?
I did try to replace the resistors from 49.9KΩ with 30KΩ, the voltage drop seems to be reduced, meaning that less voltage drop. However, the calculated output readings based on the input analog signal are still not correct. It is the same as if the resistors have not been changed.
Added after 16 minutes:
artem said:
Well this difference :
At pin 5 of IL300F, V = 1.818V
At pin 1 of AD8601, V = 1.826V
could be clarified by high offset votage up to 7 mV (worst case ) (data sheet AD8601) , influence of input bias should not be as it is up to 1 nA (worst case). In addition you can have little bit measurement error , that could somehow explain remnant 1 mV (ot maybe more ).
To explain this difference :
At pin 6 of AD820, V = 1.933V
At pin 5 of IL300F, V = 1.818V
please measure voltage at :
a. R3
b. R1
c. pin 3 opf AD820
What is precision of R1 and R3 ?
Yet, the difference pin 6 of AD820 pin 5 of IL300F
is not much important but pin 3 AD820 to pin 5 of ILF300F.
Although photodiode is raither current source , it could be better to normalize power sources to same voltage level - make them equal AVCC and +V_ISO.
Decreasing of R2 will make sence if AD820 experience saturation when output being set to rail voltage . Till that no decrease is usefull from static point of view, but could change transient behaviour (which is not important to us).
The precision of R1 and R3 being used is 1%.
It is unlikely to make AVCC and +V_ISO equal since the circuit is designed in such a way to isolate 2 parts of the circuit. I did notice that there is a voltage difference between the analog ground, AGND, and the ground of isolated circuit, IBP_GND. The voltage difference is about 0.01V.
I would like to know if I use a high resistance resistor to tie both AGND and IBP_GND to balance both grounds, would it void the isolation design?
Once again, thanks for everyone trying to help me with this!
I hope to learn as much as possible with you guys.
Best regards,
Eric Mar