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AD7190 with instrumentation amplifier

Question asked by dclm on Jun 9, 2011
Latest reply on Jun 16, 2011 by MaryMc

Dear Sir,


I am trying to design a circuit for a long time and could not accomplished yet. I need to measure a loadcell. With 2mV/V 10kg. In my application the problem is that there is a lot of dead weight on my loadcell.


I am using 10 kg loadcell, my product weight is maximum 1kg, and I need a 0,05gr resolution. that means 20000 counts. I can not use lower range loadcell because there is 2-3kg weight of cup that I am measuring product with.


Previously I have design a circuit with AD7190, I have plug loadcell directly, without instrumentation amplifier. And results were not good. I was no where near my target.


Lately I am thinking of a different alternative. I am planning to use +/- 5 V supplies. That will sum up 10V on loadcell, which makes maximum 20mV for 10kg. and 100nV for 0,05gr. With gain around 800, because my dead weight is 3kg. with 3kg 6mV on ad620, and output of ad620 4,8V change. I will use a DAC on REF pin of ad620 so that when there is dead weight on my loadcell output will be around 1 V. And when there is 1 kg product my output will be around 3V. Since AD620 is not a rail to rail in-amp My output can swing between +/- 4V.


My questions;


1-) Is this approach make sense? Since complete front end designs I need to have 1/200000 count in order to archive my target, which is hard for an engineer. Keeping in mind that I need to make measurements in 0,4 seconds in where there is vibration in environment.


2-) AD7190 is differential input and ad620 is single and output. Will it be problem just simply connect negative input of AD7190 to AGND.


3-) What about Reference pin on AD7190, where to connect positive and negative reference pins.


4-) Is it OK to use regular 7805 and 7905 voltage regulators or should I use lower noise regulators?


5-) Until know I was using opto-couplers when making connection with microcontroller. Since they consume a lot of power in order to drive them is it a extra source of noise for AD7190. Just using 100 ohm series resistors can be better.  Even though generally switchmode regulators with a lot of noise used for powering microcontroller.