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ADS1261: ADS1261 multiplexed excitation current

Dennis McGlumphy
Expert 1390 points
Part Number: ADS1261
Other Parts Discussed in Thread: TMUX1308

Dear Sirs

I will be sampling five bridges using the ADS1261 and two 8-channel differential multiplexers.  One multiplexer will be used to switch and measure the differential bridge outputs  The other will be used to  switching and measure the bridge supply voltage.  The second multiplexer will also switch the bridge excitation current.  The excitation current for the bridges will be 1mA derived from the ADS1261 current source.  My question is , given the multiplexer is a break-before-make device, what will the effect be on the ADS1261 current source when the current source gets unloaded during switching?  Thank you for your time.  

  • 0
    TI__Guru 53666 points

    Hi Dennis,

    If the IDACs are on but unconnected (floating), they will tend to pull the voltage at the IDAC output pin to the supply rail (AVDD). The IDACs require some compliance voltage to operate, which is AVDD-1.1V on the ADS1261. Therefore, the output current might begin to droop as the IDAC voltage approaches the supply.

    When the mux is switched and the IDAC now has a path to ground, the circuit will correct in some amount of time. This amount of time depends on any filtering (RC) seen by the IDAC output and the existing voltage at the IDAC pin. I would imagine that if the mux is off for a very small amount of time you will not see any large change. In looking at the TMUX1308 for example, the switching time is in the 10s of ns range. I doubt this will cause any issues.

    However, you can always use the ADS1261's programmable delay to ensure that there is a small window where the ADC waits for the IDAC to settle before beginning conversions.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Expert 1390 points

    Thank you for your timely response.  The transition time of the Multiplexer that I'm using is 40ns typical and 60ns max.  I typically use the programmable delay.  My sampling rate is relatively slow so I allow plenty of settling time.  Thank you for your help.