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ADC architecture choice

Question asked by Ahygoon on Aug 21, 2012
Latest reply on Aug 23, 2012 by MaryMc


I schould design a load cell signal chain. My configuration is:

- load cell: 0.55mV/V at 5000kg

- load cell supply: not define yet: 5V give a poor sensibility. I think 8V is good because my system schould work between 9V - 16V.

I can not go higher than 8V regulated. (or solution are welcome)

- I have several load cell connected: (3 to 8) and I want to read each load cell signal separatly.

- I need to have 1 kg resolution.

- In the best case I need to read 1 convertion / channel per 10ms. in bad case in 100 ms.


For low input signal (as in my application) I schould use a gain of 128 + reduce the ADC reference voltage to ~1.2V.

This way I got a ADC range of +- 10mV. But I schould divide the sensor supply by 8.

- 1 - How can I estimate the noise of the ADC for other reference? (like 1.2V ^^)


- 2 -  I need an advice concerning a quick noise estimation.

Using only the ADC part, I have a noise of 121nV. It means I schould add this voltage to the signal I give at the output.

=> 0.55mV/V under 8V for 5T => I got ~0.9  kg / free noise count. @ 960 Hz


Using an amplifier befor the ADC (AD8226 for example) It adds 22nV / sqrt(Hz) at it input.

=> I anderstand as: if I considere 1kHzbandwidth (with a passive anti aliasing RC filter befor the AD8226 inputs) It means I add 0.6µV noise before amplification. That give lower performance than without the amplifier.

Where is my error?


- 2.1 - How does the setling time works? Schould I wait this time before each convertion?


Regarding the previous points, I also consider an architecture using 2 AD7797 (single channel 24bt ADC with fied gain (128)) and multiplexer (ADG1409).

The output frequency is 123 Hz, but by using 2 parts, I can have the data from all my channel in a good time.

- 3 - Do you have advices for an architecture that match my objectives?

A lot of solution semms to be possible, but may be they are not all pertinent.