HART modem AD5700

Question

Hi 
 I'm trying to work with AD5700-1 according to CN-0382 circuit. I used internal oscillator and internal reference. When I connect 3.3V (feed from AD5421) to AD5700-1, I can't get any voltage or output on CLKOUT pin and voltage of REF pin is 1.7 V instead of 1.5V. Would you please help me to realize and solve the problem? 
 
 Best Regards, 
 Pyng

MarkThoren · Accepted Answer

Hi Pyng, 
 Can you clarify - are you actually using a CN0382, or a circuit that you built based on the CN0382? 
 If it is the actual CN0382, are you connecting 3.3V from an external source, or are you applying power to J1? 
 The circuit is designed to be operated in a 4-20 mA loop, where a power supply is connected across the CN0382 J1, and the CN0382 allows from 4 to 20 mA of current to flow, depending on the sensor input. 
 Note that the J1 negative terminal is NOT at DGND potential, so to facilitate probing with a benchtop oscilloscope, make sure you use a floating power supply. The circuit note is not clear on what voltage range is acceptable - the AD5421 can handle up to 52V, but 24V would be a good place to start. Also, put a milliammeter in series with the power supply, this will allow you to monitor the output signal. 
 With a floating power supply connected, then you should be able to connect a scope ground clip to DGND and probe various points on the board. 
 Hopefully these steps will lead to an explanation for what you are seeing. 
 Also, a photograph is often very helpful in troubleshooting. 
 -Mark)

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HART modem AD5700