AD5683 Headroom/Footroom Error

Hi,

About Figures 38 and 39 in the AD5683 data sheet,

I understand that a headroom error appears in the source load current state and
a footroom error appears in the sink load current state depending to the load current increase.

In the Figure 38, I accept it.
But in the Figure 39, it seems like a opposite direction of the load current that
a headroom error appears in the sink load current state (minus load current) and
a footroom error appears in the source load current state (plus load current).
Is my understanding right?

Best regards,

Nikkee

  • 0
    •  Analog Employees 
    on Jan 25, 2015 10:15 PM over 6 years ago

    Hi, Nikkee-san.

    Thank you for pointing this out. The issue is being addressed as of this writing. I will get back to you as soon as possible.

    Also, I will be moving this discussion to the Precision DACs Community.

    Regards,

    Mark

  • 0
    •  Analog Employees 
    on Jan 26, 2015 9:34 AM over 6 years ago

    Hi All,

    Figures 36 and 38 shows DAC Vout against load sinking/sourcing current, from teh load point of view (Y-axis)... not from the DAC point of view...

    Figure 38 specifies the output voltage from the DAC point of view (stated on the note label).

    Let me know if you have any other question, please.

    Regards,

    Miguel

  • Hi Miguel,

    In the Figure 36, the Vout of 0x0000(Blue) line is increasing from 0 mA to around 25 mA in the load current.
    I think it means a footroom error appears in a source load current state.
    In the Figure 39, the Vout of 0x0000(Blue) line is increasing from 0 mA to around 25 mA in the load current
    and the Vout of 0xFFFF(Light Blue) line is decreasing from 0 mA to around -25 mA in the load current.
    I think those mean a footroom error appears in a source load current state and
    a headroom error appears in a sink load current state respectively.
    Are those right?

    Best regards,
    Nikkee

  • 0
    •  Analog Employees 
    on Jan 29, 2015 1:25 PM over 6 years ago

    Hi Nikkee,

    Let me explain the problem, when the output transistors are saturated, there is a parasitic resistor between drain and source which physically limits the output voltage.

    Depending on the current that you are injecting/extracting from the output, the parasitic resistor generates  a different potential.


    Headroom or footroom only refers to the voltage used as a reference, heatroom (VDD), footroom (GND), but the consequences are the same...  as higher current you are sourcing/sinking, the more the output diverge from the "nominal" value.

    Regards,

    Miguel