This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1211/INA2126 Questions

Other Parts Discussed in Thread: INA2126, REF5025

Hi All,

I'm doing project on weigh scale application.

I'm using ina2126 and ads1211 in the analog part and im using one smps.

I'm getting following problems.

1. I'm not geting the stable counts from ads1211.

2. If i move any mobile(with ringing) to neer the pcb, I'm getting more counts from adc.

How to I remove the above problems?

I'm done the following mistakes,

1. I didn't use any filtering circuits at the output of ina2126. Reason is, I'm confusing with cut off frequency. How and which freaquncy I use to design the filter? I have just used RC filtr with cut off frequency 100Hz. Is it ok?

2. I didn't used seperate ground planes for analog and digital circuits. How can divide analog and digital grounds from single ground line?

Thanks,

  • Srinivasan,

    There are certainly many issue that contribute to the problems you are seeing, but I'll do my best to answer the questions.

    1. When it comes to stable counts, it comes down to noise and electomagnetic interference (EMI) filtering, and both can be helped by input filtering. However lets look at the other parts of the system. First, at 0.7uVp-p the INA2126 looks like it will be sufficently low in noise for this operation. Second, the reference is important as well. The reference is an input to the modulator just like the primary inputs. You want a low noise reference such as the REF5025 for this application.

    The input filtering will certainly help in the application. The cutoff freqency can be determined by how fast you want the system to run. In your case 100Hz sounds reasonable. Note that the RC filter goes from the signal inputs and filter a differential signal with a capacitor across the ADC inputs. There are other capacitors going from ADC inputs to ground to filter the common-mode signal. This pair of capacitors should be smaller by a decade to eliminate the effect of mismatch. The difference between their cutoff frequencies can make a common-mode signal look like a differential signal. By making the  smaller, you’ve moved the cutoff frequency a decade out, but any mismatch will be masked by the filtering coming from the capactior filtering the differential signal. This is illustrated by the input filter in the ADS1230REF schematic (which can be found in the ADS1230REF user guide). While the resistors are set at 0 so the user can choose the filter frequency, the differential filter cap (C7) is .1uF, and the common-mode filter caps (C29, C32) are smaller at 1000pF

    There may be other things to try to limit the effects of EMI. In a system, there may be EMI signals referenced to earth or chassis ground. Ideally, we want the EMI signal to find the earth ground quickly without disruping the signals on the board. Adding coupling capacitors between analog ground to the earth ground (and digital ground to earth ground).will help limit the effects of EMI (.01uF rated to 1kV should do it).

    A few other pointers for reducing EMI are:

    - Strive for a near perfect ground
    - Design for a differential signal environment
    - Eliminate common mode currents
    - Minimize the loops that act as EMI antennas
    - Minimize path inductance - especially ground
    - Use a continuous ground plane to minimize disturbances
    - Fill open areas on signal plane with ground

    I believe we have a presentation on limiting EMI on PCB layout. If you're interested, you can post the request and I can try to get it to you.


    2. The grounds can be separated fairly simply, Imagine two rectangles in a single plane that connect at one single point in the middle. Although these will share the same potential because they are connected, one side will be an analog ground and the other is a digital ground. The power ground will connect to the ground near the common connection (which we call the star ground). Generally, the analog ground will go under the analog signals including the analog pins of the ADC, while the digital ground will go under the digital signals including the digital pins.

    In the ADS1211, this can be imagined with the common ground connection near the center of the device, with the analog ground covering the top half of the device and the digital ground connecting the bottom half of the device (from pin 8 /CS on down and pin 18 MODE on down on the other side).

    Again, this concept may be included in the presentation.

    I hope this helps.

    Joseph Wu

  • Thanks Joseph,

    How can I have the presentation?

    I found Some readily available EMI/RFI filters in SMD package. If I going to use these filters Where I have to interface in my schematic.

    Can you verify my schematic? If yes,

    How can I upload my schematic? or Can you gine your mail id?

    Thanks,