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Original question:

DAC121S101: DC Output Impedance

DAC121S101: DC output impedance measure

Francesco Buonomo2
Prodigy 210 points
Part Number: DAC121S101
Other Parts Discussed in Thread: DAC80501

I try to measure output impedance of DAC121S101, in my setup I have power supply on Vin and on the pin Vout I have SMU and DMM.

The two current that sink the SMU are 100uA and 10uA.

With input code #FFF the DMM measure:

V1(VA=5V and Iout=100uA)=4.9900V (in datasheet FSO[max]=4.992V)

V2(VA=5V and Iout=10uA)=4.9912V (in datasheet FSO[max]=4.995V)

So the two voltage are compliant with datasheet indication.

The Zout=(4.9912V-4.9900V)/90uA=13.3ohm (in datasheet Zout[max]=1.3ohm)

I repeat the same measure on different component...

Thank you for your support and best regard

Francesco B.

  • 0
    TI__Guru 53333 points

    Hi Francesco,

    The PDS does not give specific conditions for the ZOUT specification, which in my opinion is a minor oversight. The output impedance is really only valid when the DAC (or really the integrated output buffer) is in its linear operation ranges. 

    Let's consider the source of full-scale and zero-scale error: the output buffer is really just an op amp.  Like all op amps, they have some output limitations.  Some are specified with rail-to-rail output operation, but some have a common mode output/input limitations.  Really what you are seeing is the output buffer running into headroom limits with its supply.  With devices like the DAC121S101 were the reference and VDD are connected, it will by definition have 0V headroom at full-scale. 

    Devices like the DAC80501, which have a dedicated reference (internal) and separate VDD pin can get around this as the reference can be 5V and the supply can be 5.5V, so there is 500mV of headroom at full-scale. 

    If you are trying to confirm the ZOUT measurement, I suggest you do the same experiment when the DAC is at mid-scale (2.5V).  This means the device will have plenty of headroom (and "foot-room" if you try sinking current). Also, note that small things like series resistance of the PCB and meter can add error to this measurement.  In our lab, we usually use a kelvin style connection to sense the voltage on the pin so it does not share the same current path.

    Hope this helps,

    Paul