AD74413R - Current Input, Loop Powered Mode

  • In Voltage output mode of operation, a positive current indicates that the AD74413R is sinking the current. so please correct if in below statement
    +25mA sink current path is I/OP_x --> Rsense --> VIOUTN_x --> into AD74413R through AVSS

  • How the ADC measures 10V range with 2.5V as reference

  • Why ADC equation for Voltage input mode has VMIN term but not in ADC eqn for Current output mode.

  • Current Input, Loop Powered Mode
    "tie the VIOUTN_x pin to ground via the on-chip 200 k? resistor by enabling the CH_200K_TO_GND bit in the ADC_CONFIGx registers." please elaborate on this statement present in AD74413R datasheet,
    • More basically why the name as Current Input??, since in this mode AD74413R provides a current limited voltage to the I/OP_x screw terminal, similar to Current Output mode .
  • 0
    •  Analog Employees 
    on Apr 22, 2021 10:45 AM

    Hi, 

    We will check on this and get back to you. 

    Thanks,

    Jellenie

  • 0
    •  Analog Employees 
    on Apr 22, 2021 4:51 PM

    Hi, 

    1. You are correct, when sinking current , the current flows from I/OP through Rsense through VIOUTN to AVSS

    2. There is a CGA onboard the device that scales the input voltage range to match the ADC input range. 

    3.  In current output mode, we expect a positive voltage between I/OP and I/ON, so the default ADC range is 0 - 10V. In this case, Vmin = 0V. (The ADC equations in the channel function section of the datasheet are written with the default ADC settings for that function in mind). 

    If you intend on changing the default range setting for a function, you should consider the Vmin voltage.

    4. Current Input, Loop powered is normally used for unpowered sensors. The current limit to the sensor is provided by the AD74413R. the sensor will determine the current sourced by the Ad74413R, but the current limit is provided by the AD74413R DAC. The ADC is configured to measure the current response from the sensor. The 200k to ground connection is required for best performance of the function.

    Thanks,

    Bríde

  • Hi Bride,

    A follow up question to point 1.,

    How can this be called as voltage output mode when current is being sink into the device. What is the status of MOSFET (Is it off? or on?) controlled by CASCODE_x pin, and if there a current path through this MOSFET.
    Ref. Figure.40 in AD74413R datasheet (Voltage output mode configuration)
    Thanks, 
    Umesh
  • Hi Bride, 

    Can you please answer this...

    Thanks,

    Umesh

  • 0
    •  Analog Employees 
    on Apr 26, 2021 11:03 AM in reply to USS

    Hi Umesh,

    In Voltage Output mode, the job of the AD74413R is to set the voltage on the I/OP terminal. The load connected to I/OP will determine the magnitude and direction of the current. 

    Once the output stage is enabled (by selected Vout, for example), the FET will be turned on. If the AD74413R is sourcing current, it will flow through the FET to the I/OP terminal. If the AD74413R is required to sink current, it will flow through the BAV99 to the VIOUTN pin, to AVSS. 

    The FET is optional to limit on-chip power dissipation when in Current Output mode. It is not needed for Voltage output mode, so you have the option to leave it out of your design. 

    Thanks,

    Bríde