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AD8218 / Vref Pin / Sourcing Power

Question asked by MindFlayer on Oct 10, 2013
Latest reply on Oct 25, 2013 by MindFlayer


I am using the AD8218 as shown in the schematic and the PCB view of the Vref track. SL1 and SL1 are shorted. SL9 and SL5 are open. The system runs on 5V not 3.3V as shown. TP3 and TP1 are mounting holes for the shunt. The power supply is an 260kHz SMPS deriving power from TP3. C5 is populated. R2 keeps the AD i/p at same potential in the absence of a shunt.


The FVR / DAC o/p is set at ~2V. When I connect the AD to the 48V / Shunt the voltage rises to ~ 3V!


I have used the AD chip in other perfectly working circuits and also with FVR. Previous working circuits had the vref to AD buffered via an opamp.


Tests performed:

  1. Connected a different product pcb with the same uC to one with an AD8218. Same results. 2.010V with AD not connected to 48V, 4.2V when connected.
  2. Connected different product pcb with a PIC18F45K22 to AD8218. Same result. 2.03V with AD not connected to 48V, 3.052V when connected.
  3. I lifted the FVR pin off the PCB and the voltages were
    1. AD8218 was connected to 48V
      1. On pin was 2.022V
      2. On PCB 5.37V
      3. Put my fingers across the probe when noting voltages on PCB 5.35V
    2. Removed the 48V to the AD and the voltages were
      1. On pin was 2.019V
      2. On PCB 1.325V
      3. Put my fingers across the probe when noting voltages on PCB 0.57V
  4. Connected a 1k between the FVR track the voltage drop was 26mV, current was 26uA, with a 100 ohm the voltage drop was 2mV and current 20uA


In conclusion there is no doubt that AD8218 when connected to 48V sources current / power. It is an input pin why would it do that?