ADuC847: Noise perfomance

Document created by analog-archivist Employee on Feb 23, 2016
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We are using your ADuC847 microconverter to interface to a strain-gauge load
cell for a weighing application.
The load cell output is 0 - 10mV and is connected to Ain1+ and Ain-.
The ADC is configured for a unipolar, fully-differential, 0 - 20mV input with
the sinc filter set to maximum (SF=255).
The external reference voltage is half of the load cell excitation voltage,
generated using 2 x 10K resistors.
The analog input components are surface mounted on a double-sided PCB with an
analog ground plane underneath them.
The analog and digital 5-volt supplies are generated by 2 separate regulators,
and the analog and digital grounds are connected at one point only via a link.
We need to achieve a minimum of 16-bit peak-to-peak resolution for our
application, but the maximum we have been able to achieve, with the two input
pins shorted and using your WASP program, is about 14 bits.
The same test conditions on your evaluation board produced similar results.
Changing the input voltage configuration to 2.56 volts increased this value to
more than 17 bits on either PCB, but adding an external amplifier to produce
this voltage from the load cell reduced the resolution back down to about 14
bits.
We have tried various different values for the filtering components with no
improvement.

 

The noise performance of the ADuC847 (in chopped mode) is summarised in table
11 of the ADuC847 datasheet. As you can see, the typical resolution with an
input range of +/-20mV and SF=255 is 14.5 bits. On a unipolar scale of 0-20mV,
the resolution is halved giving 13.5 bits. If your full scale signal (from the
load cell) is only 10mV then the effective resolution is halved again to
12.5bits.

From the figures that you mention, the ADuC847 is performing to specification.

The only way to get better resolution from the ADC is to increase the input
signal level. This can be done by increasing the excitation voltage to the load
cell (consistent with the load cell's rating and the ADC input range). In that
way you can reduce the gain setting on the ADC to the point where you are
achieveing better noise performance.

Applying gain externally using an amplifier has a couple of major disadvantages
- the amplifier adds noise
- the amplifier has offset, which is not rejected by the ADCs chopping scheme
- the amplifier is likely to consume more power than the ADCs PGA.

From the filtering perspective, there are of course more sophisticated IIR or
FIR digital filters than simply an average of 5 results (you are actually
averaging and decimating).

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