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THS4531A Popcorn Noise ?

Richard Newman
Prodigy 150 points
Other Parts Discussed in Thread: ADS1251, THS4531A

I am using the THS4531AIRUNR to drive the differential inputs of an ADS1251 analogue to digital converter in a multi channel EEG amplifier as shown in the Partial circuit below. R24 is driven by the output of an instrumentation amplifier and the THS4531A converts the this single ended signal to a differential signal suitable for driving the ADS1251.

I have noticed that a couple of the channels displayed what looks like burst noise, more commonly called popcorn noise when the input to the instrumentation amplifier was shorted. This can be seen in channel 35 on the screen shot below - the sudden switching between two levels is a clear indication of popcorn noise. Channel 36 shows a trace without popcorn noise. Replacing the instrumentation amplifier did not remove the popcorn noise but replacing the THS4531A amplifier did.

The data was captured at 20000 samples per second then filtered using a second order IIR low pass filter with a -3dB frequency of 70Hz - a typical diagnostic EEG bandwidth. 

My question is whether the THS4531AIRUNR is known to have popcorn / burst noise or is there some other explanation? 

  • 0
    TI__Genius 13935 points

    Hi Richard,

    You are correct that does look like popcorn noise.  I will need to research this and get back to you.

    Regards,

    Loren

  • 0
    TI__Prodigy 150 points
    Hey Richard, good data below - what is the vertical scale on the one that is popping - what are those step sizes? Most likely current noise popping and need to work backwards from your 4.7kohm values to what the input magnitude is.
  • 0
    TI__Genius 13935 points
    Hello Richard,

    If possible could you send some failing devices to the following address?

    Texas Instruments, Inc.
    5411 E. Williams Circle
    Tucson, AZ 85711
    ATTN: Wayne Comeaux, N2-314

    We can test them to verify performance.


    Thanks
    Loren
  • 0 in reply to Michael Steffes1
    Prodigy 150 points

    Hi Michael,

    Attached is the CH35 trace scaled with respect to the input of the THS4531A. Vertical scale is microvolts, horizontal in seconds. Data acquired at 20000 samples per second then low pass filtered with 2nd order Butterworth IIR with a -3dB point of 70Hz.

    I expected the noise to be about 20uVpk-pk (if no popcorn) - with the THS4531A contributing 4uVpk-pk, the ADS1251 14uVpk-pk and the preceding instrumentation amplifier gain (G=11.4)  stage around 12uVpk-pk. For comparison, CH36, which does not exhibit popcorn noise, is shown below.

    Although the datasheet suggests 2kilohm feedback resistors for minimum distortion I chose 4.7kilohm to reduce power consumption as ultra low distortion is not required for diagnostic EEG. The ADC reference is 4.096V so it is possible for the THS4531A outputs to be each swinging greater than 4Vpk-pk. Thus consuming more power in the feedback and gain resistors than in the THS4531A. 

    During the noise test the preceding instrumentation amplifier inputs are switched to ground so the signal driving R24 (see previous post) should be close to ground.

  • 0 in reply to Loren Siebert 1
    Prodigy 150 points
    Hi Loren,

    Thank you for offering to look at the devices.

    I have arranged for the THS4531A chip used on CH35 of my initial post to be removed from the circuit board and be shipped to TI. This may take a day or two since it is beyond my skill set to get the chip off without damaging the chip or circuit board (I could manage it myself if was a DIP or SOIC but not a 2mm x 2mm QFN !)

    I will put another THS4531A in it place and verify that the popcorn noise has gone before I send the device. It is possible (but unlikely) that the instrumentation amplifier (not from TI) driving the THS4531A is at fault even though it has been screened for popcorn noise.

    regards,

    Richard
  • 0
    TI__Prodigy 150 points
    Hello Richard,

    Wayne was asking if this part showing apparent popcorn was heading his way? We are setting up to evaluate for different modes of noise to try and isolate on the issue - it would be very helpful to have this device. Thanks.
  • 0 in reply to Michael Steffes1
    Prodigy 150 points
    Hi Michael,

    The suspect part has been shipped today via Fedex.

    regards,
    Richard
  • 0 in reply to Richard Newman
    TI__Prodigy 150 points

    Richard,

    We did receive that unit back and ran it through ATE and our noise test flow. So far, can't see any issues, can you contact me directly at m-steffes.ti.com

    Can you contact me directly at m-steffes