2008-12-09 Offset dependency

Previous year, I wrote about the Fiske steps routine, a routine which automatically searches for an optimum sensor setting.

For this routine to work, the setting and reading back of voltages and currents has to be very precise. Thus, a 'measure offsets' routine was also developed, but it turns out this routine is going to be a bit more complicated than expected.

Here's an image of the output of the Fiske steps routine:
fiske example.png

We had a hard time today getting the routine to work nicely. The physics student told us that it has something to do with the fact that the FFO voltage depends in the first place on the FFO bias current, but also in a much lesser way on the FFO control line.

Some background about this. The sensor behaves as a resistor, thus when setting a particular FFO bias current, the resulting FFO voltage has a direct relation. This relation becomes somewhat less simple when the FFO control line is set. This control line has two influences: running a current creates a magnetic field, and because the copper line at the back of the sensor substrate acts as a resistor, a little heat is given off.

The heating makes the offset somewhat different, and should be taken into account. Also, there's timing involved; setting the FFO CL to zero after a high setting, the FFO voltage can be seen slowly dropping from near zero to zero.