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TSW54J60EVM: Problem with input offset on Channel 1

Part Number: TSW54J60EVM
Other Parts Discussed in Thread: LMH3401, ADS54J60,

SUMMARY: A problem has recently developed with our TSW54J60EVM.  On channel 1 (INA) there is an offset of ~ +1V on J1 INAM.  We did not previously have this behavior.  Based on our initial troubleshooting we replaced the LMH3401 but still see the same behavior.  The problem seems to be in the LMH3401 front-end section because the ADS54J60 shows a good signal on Channel 1 when isolated.  More details are given below.  Further guidance would be greatly appreciated.

We purchased the TSW54J60EVM and TSW14J56EVM about a year ago and initially tried to implement signal averaging (see prior posts).  After no success implementing the signal averaging approach we simply acquired continuous data and processed the long time series offline.  We also converted Channel 1 to DC coupling by removing C200, C201, and R12 as per the user manual, and C202 and C203 as per prior guidance from E2E (not in user manual).  The system was working successfully in DC coupling mode for many months.

The last few months the system was unused and when we used the system this past week there was a problem on Channel 1.  We could not get good signals/signal quality on Channel 1 (Function generator sine waves with 0.05V amplitude and 0.00V offset gave acquired signals with a negative offset of ~ -0.75V and extremely noisy signals ~ 0.14V p-p).  We felt that the only possible culprit for this problem was the LMH3401 and so we replaced that component, with no success.  The problem still remained.  After this we again made measurements along the input chain, took better notes, and these are summarized next.

We operate the system with the single-sided signals going into J1 INAM and J3 INBP (per user manual) and 50-ohm terminators on J2 INAP and J4 INBM.  With no signals connected and J1 and J3 open, we measured 0.972V across the J1 connector and 0.00V across J2-J4.  A corresponding capture using HSDC-Pro showed a Channel 1 signal ~ +0.97V with only four binary codes ranging from 64508 to 64511.  Probing the LMH3401 differential output by voltmeter from R29 (positive) to R33 (negative) we read -1.854V.  We then also added 50-ohm terminators to J1 and J3 after which J1 and J2 both measured 0.545V across their inputs with a corresponding 0.00V across R29-R33.

To isolate whether the ADS54J60 or the LHM3401 was at fault we opened the connections at C202 and C203 (which are shorted for DC coupling mode).  After this the HSDC-Pro software showed nearly identical signals on both Channels 1 and 2 with 0.00V average and ~0.4mV RMS noise.  This implies that the ADS54J60 is working fine and the LMH3401 is at fault.  Similar measurements (with and without terminators as before) gave the following voltages with J1 open: J1=1.012V, J2=0.806V, and R29-R33=-3.116V.

All this time, corresponding measurements on Channel 2 gave the expected 0.00V inputs, differentials, and captured signals.

If there is any relevant information missing please let me know.  Also, please let us know what our next steps should be.

Regards,

Greg

  • Greg,

    Are you still having issues with this EVM? In similar situations, we had problems with flux build-up over time and after cleaning the board in an ultrasonic cleaner, it fixed our problem.

    Regards,

    Jim

  • Jim,

    We purchased a second TWS54J60EVM board to continue our testing.

    The original board still has the same symptoms. We will try cleaning the original board as you suggested. Since we are currently using the new board in our testing it will take a few days before I could re-evaluate the old board. Forgive my ignorance, but if the board is not used and not modified (since ~ a year ago), how does flux build-up over time occur and affect the circuits?

    Greg
  • Greg,

    In your case it may be corrosion. Since you replaced the amplifier, if the board was not properly cleaned, it could start to corrode over time.

    Regards,

    Jim

  • Jim,

    OK, now I understand your logic; however, we only replaced the amplifier after we noticed the problem and only a day or two before the post. Replacing the amplifier did nothing to change the problem. The only modifications we made were to change to DC coupling, so corrosion might have occurred at the jumpered capacitors (C200, C201, C202, C203) and the removed R12. We will also look into this.

    Thanks.

    Greg