AD698 problem

It looks as if you are over-driving the demodulation electronics, which can't take more than 3.5V rms. As the data sheet says, if you put more than that across the LVDT drive coil, you've got to use a voltage divider to reduce the voltage across the B-inputs to less than 3.5V rms.

Generally, diffused bipolar transistors avalanche if you put much more than 5V across the base-emitter junction. If it comes from a low-impedance source, you will blow up the transistors, while a high-impedance source will merely degrade the transistors by progressively reducing their forward gain.

Granting that your circuit diagram only shows provision for voltage divider across the B-inputs, you probably blew up that input at an early stage, and it isn't surprising that the demodulator hasn't been working.

Il 08/08/2011 01:27, Bill Sloman ha scritto:

Initially I assumed the maximum 7 V reported by the datasheet was pk-pk, not rms.

Yep, because if I will need more voltage I've already have the voltage divider there.

I'm soldering a new PCB. I will verify if the behavior will be the same.

Marco