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Doubt regarding usage of filter at output of ADS1294 to avoid oscillations

Other Parts Discussed in Thread: ADS1294, AFE4300

Dear All,

Greetings. One of our customer is using ADS1294 for baby weighing scale and they are getting more oscillations in the output. schematics attached for your reference. kindly advise how to avoid those osciallations at o/p, do they need to add filter circuit at the o/p. pls help to advise the solution for this.

Rgds,

Aravind.ADS1294 schematic.pdf

  • Hey Aravind,

    I am not a weigh scale expert, but I will loop in one of my colleagues who is more familiar with such applications. In the mean time, I have a few questions regarding what may be happening with their circuit. First, what is the purpose of the resistor in the input path? Being on the device-side of the shunt capacitors, it would have little effect on the band of the input since the input impedance of the device is so high. If your customer's intention was to create an RC low-pass filter, the resistor should go on the opposite side of the shunt capacitors. In addition, LC circuits are known to possess resonance. What frequency is the oscillation occurring at?

    As a side note, we recommend pulling all unused analog inputs on this device to AVDD.

    It is interesting that your customer has chosen the ADS1294 for a weigh scale application. That device is typically used for ECG measurement. From the schematic, it seems as though they are not taking advantage of any of the ECG related peripherals. What was your customer's thought process behind using this device in particular? If you'd feel more comfortable, we can move the discussion about your customer off the forum as well.

    Regards,
    Brian Pisani
  • Hi Aravind,

    I am assuming that the scale is based on load cells at 4 corners, and the ADS1294 was selected as the device has 4 simultaneous converters for processing the data.  Is this a correct assumption?  As Brian has already mentioned the input filters are incorrect.  In fact the customer has created an oscillator circuit.  I would suggest to first remove the inductance and replace with 0 ohm resistors.  Second the input resistors should be moved to the other side of the capacitors.  Also, the differential capacitor value should be 10 times larger in value than the common-mode caps to reduce drift and noise.  For example, C25 and C26 should be no larger than 0.01uF if C23 is 0.1uF.

    The ADS1294 may not be the best device for this type of application as you cannot use a ratiometric measurement to reduce noise, nor can you create enough gain to overcome any external noise relative to the low level input signal.

    It would be best to know the desired data rates and desired resolution the customer is trying to achieve.  We may be able to come up with a better solution for the customer.

    Best regards,

    Bob B

  • Dear Bob,

    yes i also hope for the same reason only they selected this device ADS1294 for weighing scale, initially we suggested them to look into AFE4300 only.

    I will give yours and Brian's suggestions to customer and come back with their feedback shortly.

    Rgds,

    Aravind.