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AMC7834: Exceeding bipolar DAC pin voltage

Part Number: AMC7834

Hi guys,

I wanted to know what could happen if the pin voltage of that DAC will exceed. Could or will it affect also the second device hanging on the same DAC-pin pair? Could it also affect the second DAC which is in the AMC? 

We are a bit afraid that if at one DAC-pin a higher voltage (then specified) could have an impact on the other chanels of the DAC(s). 

Are there any recommendations to protect the AMC at that pins or any application notes?

Thanks,

Markus

  • Hi Markus,

    The only effect it would have on the adjacent channel would be heating effects.  When the output is forced to a voltage that is not the output voltage of the DAC's output amplifier, the device will sink or source current until it hits the short-circuit limit.  After the limit, it should clamp to the supply rail or the external voltage.  If the external load is driving the pin beyond the supply rail, it will likely cause the devices ESD diodes to turn on.  These diode can sink/source more current than the short-circuit limit. You will likely be violating the abs. max. limits of the device in this condition, which could damage the device.

    Thanks,

    Paul

  • Hi Paul,

    thanks for your quick answer! 

    What do u mean with "After the limit, it should clamp to the supply rail or ..."? Do u mean AVDD oder AVSS? I guess AVSS which is negative in our case. The external voltage will be positive. 

    Our scenario could force pretty much current/volage into the pin. So the ESD diode would be for sure thermally destroyed. 

    Any experience if the ESD-diode is likely to make a short or an open if there is way too much Ptot?

    Second question came up - what is roughly the input impedance of that ADC-pin? High/low...? 

    Thinking about a protection circuit after the ADC-pin so it would be good to know a bit more about that pin...

    BR,
    Markus

  • The DAC output is just the same as an amplifier output, so once it hits it's short-circuit current limit, the external load will dictate what voltage the output becomes.  If you connect a current source, the amplifier will look like it's output impedance increases once it is at it's current limit, so it will slew to the supply rail (VCC/VSS) depending on the direction of the current.  If you connect a voltage source to the DAC output, it will will slew to the voltage that source is forcing.

    It is hard to say what behavior the ESD diode will take once it is damaged.  If you suspect that this load could exceed the abs. max values, I recommend you implement additional external protection circuitry.

    The ADC input is high-impedance.  Usually, the pin is a mux input that is disconnected.  When the ADC is sampling a channel, the MUX connects to the pin and the input must charge a sample capacitor, so during that acquisition time, it could be considered a low-impendence input, but that is very momentary.