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Original question:

AMC3302: Constant Noise in the output signal and suggestions to improve the PCB layouts

AMC3302: reduce noise in Current measurement

Ilamparithi Tamizhanangu
Prodigy 65 points
Part Number: AMC3302
Other Parts Discussed in Thread: THS4131

Tool/software:

Dear sir,

This is our initial version of the AMC3302 current measurement board. It produces constant 300 ~ 350mV noise signal in the BNC output of THS4131D amplifier.  Our expected maximum current measurement is 50A and the shunt resistor value is 1mOhm.

Schematic

PCB Layout

Initially I missed the filter in the above circuit. Based on the previous post suggestions, added the filter low pass filter in the final output (after THS4131D). (R = 1000 Ohm & C = 100nF, cut off frequency: 1.6 kHz).

Please find the below current waveforms for the AC and DC Current loads.

DC Load current: 1A

AC Load current: 200mA

Queries

1. The load current having constant noise of 200mA (in the voltage output measurement:40mV (after the 2nd Amplifier output))). THS4131 amp gain is 5. AMC3302 output 8mV noise output. Could you please suggest the solution to reduce the noise output in the current measurement. (In the revised version, I will move the low pass filter in the output of the AMC3302 (before THS4131 amp). 

2. We are getting the constant noise in the range of 2.8kHz to 3.5kHz range. This noise is always present the measurement output.

3. During the normal operation, the device AMC3302 required power input is 231mW (231mW / 5.5V = 42mA) (section 6.5). The maximum safety input current limit 309 mA (5.5V). I wish to add the fuse in the power supply lines to protect the isolation barrier.  During the normal operation, shall we add the fuse in power supply input current limit as 100mA? kindly suggest the suitable fuse rating for the power supply input lines.

4. Do we need to use the fuse after shunt resistor and before INP (pin 6) and INN (pin 7).? The amplifier output is connected to DAQ Module and PC. During the fault condition we need to prevent the damage to the isolation barrier.  Do we need to add the fuse in INP & INN pins? if yes kindly suggest the suitable rating for the same. 

5. During the fault condition, if the isolation barrier gets damaged, does it affect only the communication or it become open circuit or short circuit or possibilities for both? 

Thanks & Regards,

Ilamparithi Tamizhanangu.

  • +1
    TI__Expert 5915 points

    Hi Ilamparithi,

    Thank you for sharing your project. 

    One consideration of protentional error sources could be not defining the common-mode voltage. As noted in the datasheet, "Either INP or INN must have a DC current path to HGND to define the common-mode input voltage." Please connect INN to HGND first. What does the input look like?

    To clarify, where are you measuring this noise from? 8mV of noise after the AMC3302 and a total of 40mV of noise after the THS4131? Do you still see the 2.8kHz-3.5kHz noise since including the output filter with 1.6kHz cutoff frequency? 

    As far as fuse recommendations, we do not have specific guidance on what fuse ratings to use per application. Again, our recommendation for overcurrent protection would be incorporating an isolated comparator as described in this app note.

    If the isolation barrier is compromised, the signal could be corrupted and device performance will be unpredictable. This also poses a risk of electric shock or damage. Whether the device fails short or open depends on a variable of things and is difficult to ensure. This app note describes these failures.

    May I ask what shunt resistor you are using for this project?

    Best regards,

    Eva

  • 0 in reply to Eva Mason
    Prodigy 65 points

    Hi,

    Thanks for the answers. 

    After connecting the INN to HGND, the waveform improved a lot and now and waveforms measured properly. 

    we will follow the application note for over current detection and fault condition scenarios. 

    we are using the 1 mOhm shunt resistor (WSLP59311L000FEB) for this application.

    Thanks again for guiding us to solve the issue.

    Thanks & regards,

    Ilamparithi Tamizhanangu.