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TLC4541: Multi-Channel Read out overlap intermittently

Chu Lee
Prodigy 80 points
Part Number: TLC4541
Other Parts Discussed in Thread: ADS8320

HI,

we have mux in front of TLC4541 for 2-channel, some time the TLC4541 readout OK, sometimes the readout overlapped . any help

Thanks

  • 0
    TI__Guru**** 178501 points

    Hi Chuanxiao,

    What is the blue trace in the graph you posted?  A first guess on what might be wrong here is that there is something wrong with your MUX control.  Can you just apply DC (maybe 1V and 3V) to the MUX inputs and see what that does?  You are probably going to need to insert a drive amplifier as well between the MUX output and ADC input at some point, but lets figure out the channel switching first.

  • 0 in reply to Tom Hendrick
    Prodigy 80 points

    Hi Tom

    Thanks for the message:

    the blue trace is FS of TLC4541, the SCLK & FS are from ADSP-2185

    we have the old version with ADS8320EB that use same MUX, but the DCLCOK and CS/SHDN are from FPGA, and Dout to DSP, there were no issue with this MUX

    we have 3-set of TLC4541, one-set for 2-channel, others are for 3-channel (X/Y/Z) which are X & Y are overlapped, and Z is Y (looks like TLC4541 cannot data out in 10KHz intermittently)

  • 0 in reply to Chu Lee
    TI__Guru**** 178501 points

    Hi Chaunxiao,

    I meant the blue trace in the first graph, where you show 1st through 4th powerup.  It is interesting that when using the ADS8320 you do not have any issues.  The TLC4541has a very different sampling mode compared to the ADS8320, so this is still sounding like a MUX issue to me.  Please compare Figure 28 in the ADS8320 datasheet to Figure 16 in the TLC4541 document.  The Tsmpl time for the ADS8320 is functionally equivalent to the Ts time in the TLC4541.  The TLC4541 samples the input at the end of the conversion cycle and always provides sample N-1 conversion results on the output.  The ADS8320 samples at the start of the conversion cycle for a fixed number of SCLKs.  So, if your MUX switching is the same for the TLC4541 as it was for the ADS8320, that could be the cause of your issues.

  • 0 in reply to Tom Hendrick
    Prodigy 80 points

    The blue trace in the 1st image is recorded power up sequence. that is why I called "intermittently": some power up overlapped (X=Y; Z=Y), some power up everything is OK (X=X,Y=Y,Z=Z)

    ADS8320EB outputs previous value (start converting and outputting parallel); So only 22 SCLK needed (250kHz @ 0.1ms interval reading is OK)

    TLC4541IDGK outputs previous value then conversion; 46 SCLK needed (Min 500kHz)

  • 0 in reply to Chu Lee
    TI__Guru**** 178501 points

    Can you provide a screen shot of the ADC input wrt the FS, /CS and SCLK?

  • 0 in reply to Tom Hendrick
    Prodigy 80 points

    think your right, maybe the MUX latch up issue. here is scope shot. the /CS grounded (not used for control ADC), only use FS control the ADC. 

    thanks

  • 0 in reply to Chu Lee
    Prodigy 80 points

    any idea the ADC VDD noise (serial 100 ohm with 5V which is stable)?

  • +1 in reply to Chu Lee
    TI__Guru**** 178501 points

    That is too periodic to be just 'noise' - it looks like coupling from your SCLK or possibly a switching power supply somewhere in the circuit.