That's Odd

Finally getting around to looking at some of the data taken with the science array last week, trying to get a better idea of what drives the odd/even effect.

Bob used a trick to get a long series of consecutive non-destructive scans.  Using a 1-1-4-4 SUR opstable with a script with 50 sequential 'go' commands, we get 200 consecutive scans saved to disk.  Bob manually masked out the reset pulse by dialing down the level shifter after the first reset scan, resulting in 196 consecutive scans saved to disk without a reset.

This lets us map the debias curve and experiment with relinearization, and to look at the even- and odd-pixel subpopulations independently.

Starting from our 'nominal' operating point, and using a ctdelay of 110 to give ~3s scan times (to match earlier data from the detector lab), we get the following plot at T = 7K (temp not quite stable):

Mean values for all, pixels, all odd rows, and all even rows v. time.

T=7K, VRowOn clocked 0V/-3.9V.

Odds and evens differ by about 2kADU, or about 70K electrons.  The slope of each line is about 1.1ADU/s, or ~38 electrons/sec...this is way too high to be a thermal dark current at this temperature, but there may be a light leak or something warm inside the dewar.  It might also be a statistical artifact from a few bombing bright leakers, since the bimodal distribution of the pixels pushes me to use the mean rather than the median here.  We can tape off the array socket on the next cold cycle as a check on this.

Finding the slope in odds and evens to be the same is encouraging...if the odd/even effect is a pure offset it should subtract out in sky data, yes?  But sadly, no.

The data above were collected with VRowOn clocked opposite the rest pulse.  Bob tried setting both rails of the VRowOn level shifter to -3.9V, netting a DC level.  A similar dataset collected this way is presented here:

Mean values for all, pixels, all odd rows, and all even rows v. time.

T=7K, VRowOn @-3.9VDC.

Here the offset between even and odd is much smaller, but the slopes differ by about 25%.  The shallowest slope (the odd rows) is little changed from the first set.

Glow from the row mux was pretty evident in this data, and Bob reduced it by shifting VpRow/VpCol from -0.8VDC to -1.5VDC.  Retake data:

Mean values for all, pixels, all odd rows, and all even rows v. time.

T=7K, VRowOn =-3.9V. VpRow/VpCol=-1.5V.

Here again the odd rows are little changed, but the change in VpRow/VpCol has shifted the even rows away.  We now have both a gain and an offset discrepancy.

Bob repeated the test at different temperatures.  We consistently see more dark current in the even rows than in the odd rows, and the discrepancy seems to increase at higher temps (to 40% by 8.5K).  Here is an Arrhenius-style plot of log DkI v. inverse temperature (using the array average pixel):

Dark current v. inverse temperature (whole-array average)

Note the departure from the thermal activation line below 8K...leak or glow?

All this looks to me like a different net bias is applied to the even and odd rows, and that the difference may be affected by clock/bias details and possibly by temperature.