ADS1230: ADS1230 stability issue

Hi Pranav,

Noise is often a problem with load cell measurements.  There are a couple of things that would be helpful for me to know and that is the Gain setting used and the Analog supply voltage being used.

You referenced Figure 33 in the datasheet and above the figure is a calculation for noise-free counts.  If you are using the same configuration as is used in the example, then you should see stable reading over about 95,000 counts.  If the full-scale output occurs at 100kg, then you should be able to achieve 100/95 gram resolution.  What you are telling me is noise is so high that you are only achieving a fraction of the potential resolution.  The largest noise source most likely is coming from the cabling from the load cell to the PCB.  This will act as an antenna for EMI/RFI and power line-cycle noise.  This cable should be properly shielded and terminated.

You will also benefit by having some form of analog input filtering.  What filtering components used will greatly depend on the source of the noise.  Sometimes a simple  RC filter may be the most beneficial.  This will become a low-pass filter which will help to limit high frequency content as well as act as an antialiasing filter.  I would try placing a 10nF to 100nF capacitor across the ADS1230 analog input pins as close as possible to the device.  I would also add series resistance in the range of 100 to 1k ohms to each analog input between the load cell cable input and the capacitor.

To keep the measurement ratiometric, you should also add similar filtering to the reference input.  PCB layout is also important.  There should be an effective analog ground plane, and there should be no digital signals crossing over or near analog signals.

Best regards,

Bob B

Hi Bob

Thanks for your valuable input to my issue. First of all I would like to explain various points that you mentioned in your reply.

DATA RATE---10SPS
GAIN----128
Power Supply---- I am using a common 12v 2A SMPS to power my project form which I drive 5V from a low noise LDO(LM1085) this 5V is used to power my MCU as well as the IC and load cell as well.

The MCU board and the ADS1230 have different PCB boards

There are few doubts.

1) How to separate analog and digital supply I have a common source?

2) What kind of filter is required for the reference Input of the ADS1230?

3) There is always going to be wire noise related to my projects and can only control the shielding part, I need to solve it in the designing part(PCB) or by software filtering. Can I use an LCR filter in analog inputs? is there a requirement to decouple AINP and AINN pin with a capacitor.

4) what type of capacitors will work good for noisy environments?

5) Is there a requirement of ground plane as I have a common source so digital and analog ground will be same and induce noise.

Regards
Pranav Dosajh

Hi Pranav,

I apologize for the delayed response.  Please see my responses below to your questions.

There are few doubts.

1) How to separate analog and digital supply I have a common source?  BB> There is no need to separate the supplies.  Good results can be achieved using a common supply.

2) What kind of filter is required for the reference Input of the ADS1230?  BB> Reference filtering should be similar to analog input filtering.  The key is to maintain a good ratiometric measurement.

3) There is always going to be wire noise related to my projects and can only control the shielding part, I need to solve it in the designing part(PCB) or by software filtering. Can I use an LCR filter in analog inputs? is there a requirement to decouple AINP and AINN pin with a capacitor. BB> I would not use pure inductors to prevent unwanted oscillations.  Instead use ferrites with the desired filtering properties for the noise you are trying to filter.  Other capacitive type EMI filtering is also available.  You should have a differential cap connected between the AINP and AINN outputs.  Any RC low-pass filter should take into account anti-aliasing as well as noise filtering.  For the cabling you need to make sure all shield terminations are handled appropriately and shield ground is not connected directly to analog/digital grounds. 

4) what type of capacitors will work good for noisy environments? BB> The best capacitors to use for any analog filters is either a film cap or ceramic C0G/NPO.

5) Is there a requirement of ground plane as I have a common source so digital and analog ground will be same and induce noise.  BB> To achieve the best return current path it is best to have a solid ground plane.  The analog and digital grounds must have a low impedance connection at the ADS1230.  I would suggest a solid common ground plane.  The key is to keep analog signals away from digital signals.  You never want the digital signals to cross into the analog domain.  You may want to review the FAQ section:

https://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/grounding-techniques

Best regards,

Bob B