AD5422 Output tied together with other AD5422

I am using a DAC (AD5422) in an application where there is the possibility of a customer misusing this by tying all the analog outputs together, where the outputs are at different potentials, and possibly in different modes (-5V to +5VDC and 4-20mA).  I have use the datasheet op amp example to tie the Voltage output and current outputs together.  My supply voltage to the device is 18-36VDC, so the 4-20mA voltage of one unit could possibly be trying to drive 36V (trying to drive to 20mA), while another unit could be configured as a voltage unit trying to drive to -5V.  The -5V supply is pretty weak, so it will likely current limit and the 36V would win out.  There is an 18 ohm resistor on the Volt/Current output.  I have reverse polarity protection diodes going to the +input and -5V supply, and a 36V TVS on the analog output.

In the worst case scenario, with 2 or more of these trying to fight each other, would any there be anything destructive happening to the devices?  If so, what would the failure mode be? Included in a worst case scenario would be two units next to each other with a short length of cable (very low impedance), where both outputs are virtually shorted together.  

Thanks in advance for your response.

  • 0
    •  Analog Employees 
    on Nov 3, 2016 5:22 PM

    Hi Photoman,

    My apologies for the delay,

    So if we assume that you have the following setup:

    2 x AD5422 (Lets call them AD5422_1 and AD5422_2).

    I'll omit the protection circuitry to simplify the setup.

    AD5422_1

    0mA to 24mA Output, Set to output 24mA

    AD5422_2

    -5V to +5V Range, Set to output -5V.

    If the outputs of both were connected, AD5422_1 would try to supply 24mA to AD5422_2. In voltage output mode, the short-circuit current is typically 20mA. IOUT will try to sink 24mA into VOUT but will fail to do so. As the AD5422_1 tries to drive the current, the voltage on the short will increase to VDD - 2.5V, an IOUT fault will likely be reported by AD5422_1.

    If we now apply 20mA, the VOUT pin will typically be able to sink 20mA and its possible that FAULT may not be reported.

    In summary, you are correct, the IOUT pin will overdrive the VOUT pin. In the cases we've discussed, you are within the absolute maximums allowed by the datasheet (IOUT to GND and VOUT to GND) , so no damage should occur. However, depending on the level of current being sourced, a FAULT may not be reported by the AD5422_1 unit.

    Kind Regards,

    David