Why load cell value through AD620 showing 650mv at zero?

[Sharagim]

Hi,
I have a load cell which is plugged to AD620(for gain I use 50ohm).
It seems it is working properly but when I connect Ref of AD620 to GND I see it is showing about 650mv and for a few first kg(0~3kg) it wont show any changes at output then I use lm324 for Ref voltage and when it is at higher than 750mv I see all is working ok.
How is it possible to make output of AD620 start from 0v?

Thanks

 

[FvM]

Looks like you are operating AD620 with single supply. As you can read from the data sheet, it's not intended for this usage,

 

[Sharagim]

Yes you are right and when I use a 7660 to make -5V this problem gone.
Now I am getting some unstable values from the AD620 (As I mention I use 50ohm as gain).
for example ADC value showing 172 after some seconds it will become 171 and again .....170.....169.....168..... , Some capacitive like changes !!!

 

[FvM]

Now I am getting some unstable values from the AD620 (As I mention I use 50ohm as gain).
for example ADC value showing 172 after some seconds it will become 171 and again .....170.....169.....168..... , Some capacitive like changes !!!

To understand the meaning of these numbers (how much input referred drift does it represent) one need to know the ADC resolution and full scale range,

Generally speaking, there may be many reasons, e.g. load cell drift, variation in excitation current. Shorting the AD620 input can show if the drift is external or internal to the amplifier and ADC circuit.

 

[Sharagim]

thx for reply.
My load cell is 50kg S-Type and the the micro is 12bit res. as you see my gain in about 1000.
I don't need full 50kg range , It is important for me to have about 5gr resolution.