I don't understand this at all, but I noticed that changing the polarity of the bias for DC channel as well as the offset of each quadrants changes the response of ESD measured by OPLEV.
I first set DC bias to 125000 counts, set the offsets of all quadrants to 51776, injected into each quadrants, one by one at 3.2Hz, and measured the transfer coefficients from the excitation to the OPLEV. No filter in the injection path, so this is the TF from ESD digital output to the oplev. Injection amplitude is either 40000 or 20000 counts.
Then I flipped the DC bias and offsets, but without flipping the signal polarity, and repeated the measurements at 3.19 Hz. The TF polarity flipped as expected, but the TF amplitude increased overall by a factor of 1.2-ish if you look at PIT, or 1.6-ish if you look at YAW.
In the attached, left two panels show PIT response, right YAW. In in each panel, to the right is non-flipped data set, to the left is flipped.
This is highly repeatable, the measurement error seems to be 5% or less. The results don't depend on the excitation amplitude (this is expected).
From the fact that the TF polarity flipped though the excitation polarity stayed the same, this cannot be just electronics coupling into OPLEV.
Keita, Do you suppose it is simply that the force is proportional to (Vsig-Vbias)^2 ? RW
My point is that it should be proportional to (vsig-vbias)^2, but it doesn't appear to be.
Expected behavior is that when you flip the sign of vbias without flipping vsig, you will have exactly the same amplitude (which is not the case in my measurement) but with exactly the opposite polarity (which is the case in my measurement).
Since I'm not measuring vbias (I just trust the DAC counts), it could be that there's an offset in ESD driver, but the offset should be huge.
Charge?