AD8495 Break Detection (we need a neg output)

We are currently using an AD8495 with a K type thermocouple in our portable medical device.  The circuit works great...simple to use and easy to configure.  However, the recommended break detection configuration drives the output of the amplifier to the positive rail.  This works in many applications but unfortunately not in ours.  We want the output of the amplifier to go to GND when the thermocouple breaks or is open.  Any suggestions?

+Vs = 3.3vdc

-Vs = GND

Vref = GND

Sense is tied to the output

There is a 1M ohm resistor to GND at the thermocouple connector

Our circuit is exactly like figure 33 in the datasheet.

Thanks in advance!!

CJL

  • 0
    •  Analog Employees 
    on Apr 22, 2011 1:15 AM

    There is a simple way to have your break/open thermocouple detection rail to GND or negative.  The 1MOhm resistor in your setup as shown in Figure 33 would be connected to the (-) inverting input of the AD8495.  If you connect this 1MOhm to the (+) non-inverting input instead, the output will be driven to the negative rail in an open thermocouple condition.

    Glad to hear that the part works well for you!

  • Thank you for the quick reply Reemo.  Before I posted this question,  I tried moving the 1M ohm resistor to the +in non-inverting input and my output from the amplifier was severly effected... like it measured 3 deg C.  Now, I may have something wrong with my mod and I'll double check this.

    Thermocouples are polarized and the thermcouple we're using has a positive and negative.  is it possible this could be causing the amplifier to not operate normally with the 1M ohm resistor on the +in input?

  • I went back to my test fixture and you were right Reemo.  Moving the 1M ohm resistor to the +IN non-inverting input forces the output to GND or the negative rail in the event the thermocouple opens or shorts.  My test fixture was causing the output to stay at 3 deg C.  When I removed the test fixture and installed a thermocouple, the output goes low when the thermocouple is open as well as shorted.  This is exactly what I want.  Thanks for your help!!

  • 0
    •  Analog Employees 
    on Apr 29, 2019 10:38 AM
    This question has been closed by the EZ team and is assumed answered.