Tool/software:
We have designed 4 analog inputs +/-10V and 2 port for four wire PT100. We have placed as per TI guidelines TVS protection diodes in each ports and also place MOVs to earth on each lines, still we are facing issue that IC gets damaged in conducted immunity test while testing 4 wires PT100 port. Is there any way to protect and passed CI test in class A?
Thank you.
Neel Shah
We have used TI guideline for PT100 = 4 wires low side reference and as mentioned above also used protection components.
We got below issue in CI testing.
Please suggest how could we do through by adding more protection components or through any software algorithm or giving programmable delay for removing aliasing effect?
Thanks in advance.
Hi Bob,
We have considered your application note and same things are being applied in our design. We have passed Surge and EFT but face challenge in Conducted immunity test.
Due to confidentiality, I could not share schematic and layout files.
Could you please share your thoughts that we may do through by adding more protection components or through any software algorithm or giving programmable delay for removing aliasing effect?
Please refer below snaps for anti-aliasing filter settings. Could you please guide us on this for 4 channel aliasing effect solution(which is shown in above my chat disturbance between 1.17MHz to 1.6MHz)?
One last question is that in ADS124S08 IC is having any Butterworth filter design inside the chip which can be coded through software? Is it true?
I am looking forward to hearing from you.
Thanks in advance.
Best regards,
Neel Shah
Hi Neel Shah,
Unfortunately you are not giving me much to work with. Think about what you are asking me. It is essentially 'I have a problem, tell me how to fix it'. I don't know precisely what you are seeing so it is difficult to determine what action is actually taking place. I at least need to know what configuration settings you are using, and the raw data you are seeing (which means just ADC output codes without any conversions to temperature, voltage, etc.). It would also be helpful to know if measurements return to normal after a period of time or if the device requires a reset or power-cycle to correct the issue.
The programmable conversion delay is primarily used when switching mux channels to prevent charge distribution from one set of channels to the next ones selected. The mention of the integrated anti-aliasing filter is referring to the filter that reduces the noise of the internal switching frequencies. It will do very little to any external noise. See the circled sections below which are the filter components:
The programmable conversion delay only takes place when switching mux channels when in continuous conversion mode, and when using single-shot mode and starting a conversion.
The filter sections shown above are the only analog filtering components. The ADS124S08 is an oversampling device where the analog input is converted to a modulator bit stream. The modulator data is passed to a digital filter which process the data via a sinc or FIR filter depending on the configuration selected.
What I think is happening is you are seeing an analog affect prior to the signal entering the ADC input pins. Any capacitance or inductance will act as a storage element. If that is the case, then you will need to limit the amount of stored energy. One method is to use ferrites in series with the input filter resistance to increase the overall filter resistance when high frequency energy is present. One common mistake is to use too low of resistance or the wrong filtering frequencies. In some cases customers will only use 100MHz filter peak filtering values. If you are doing this you may need to add 1000MHz ferrites in addition to your current solution.
Another consideration is any exposed, unshielded wiring can pick up RFI/EMI. One area often neglected is if wiring is shielded, it must also be properly terminated and care must be take when connecting the sensor to the PCB. Small loops of wire at the connection point can defeat the shielding.
Best regards,
Bob B
Hi Bob,
Thanks for the response.
In our application there is total 6 Ports ADC. 4 of them are used for measuring +/-10V and 2 ports for measuring temperature through 4 wires PT100 sensor which is shown in above block diagram. These data communicated through ADC's SPI to MCU
These data are reding through our software web tool. Master devise and Analog measurement device communicated through RS485 device.
As you said, it is possibly to solved through adding ferrite bead in series for the solution of +/-10V measurement. But in PT100 ports we are facing issue that noise are going into ADC and randomly values has been changed and then IC got damaged. It seems through current source channel or through reference REFP0/REFN0 noise is entering in ADC.
Current source 1mA, ODR = 4K, Fc = 1.046K set through sinc filter.
Our design is similar to below snaps for PT100. We have just added schottey diodes at current source path and MOVs and TVS placed at input paths to the earth, still noise is injected in ADC. While other +/-10V input ports are not injecting noise in ADC same protection has been at input side (MOVs and TVS). Could you please guide me on this?
Please note that we are not using EM clamp for testing, we are using CDN device because as per IEC 61000-4-6 suggests CDN for noise injection for the Analog input port testing. We have too little power consumption of device in few mA. Should we use EM clamp for certification?
if you need any more information then please let me know.
Previously I have asked one query that ADS124S08 IC is having any Butterworth filter design inside the chip which can be coded through software? Is it true?
I am looking forward to hearing from you.
Thank you in advance.
Best Regards,
Neel Shah
