Take a Memo, or Two

So from the previous post we see that the odd/even effect is not a simple offset, but includes a gain variation between even and odd rows.  In one case, when the VRowOn line was clocked opposite the reset pulse, the gain was pretty close for all pixels.

Looking into this a bit further, it turns out that the MIRI folks have addressed a similar issue at least twice, but the prescription they provide differs a bit.

In a MIRI internal document provided by Mike Ressler, a change to the clocking code is described that minimized the odd/even effect and mitigates the 'droop' problem observed at JPL (basically a long-lived transient signal accompanying the transition from destructive to non-destructive reads).  Raytheon's suggested clocking pattern involves clocking the ØRowOn line exactly opposite the ØRstOn line, effectively disabling the row enable during application of the reset pulse.  Makes sense, because if a row is enabled by the circuitry at the time of reset, the paths available for the charge to redistribute are different than for a row which is not enabled.  This would hopefully cancel any artifact introduced by the decision to reset row-pairs simultaneously.

The change outlined in the MIRI internal document is to clock ØRowOn whether it's a destructive frame or not.  Mike concluded from testing that this did the trick, and attributes the improvement to clocking the array in a stable manner so that the reset pulse is the only difference between destructive and non-destructive reads.

However, in another document ( http://ircamera.as.arizona.edu/MIRI/miridetectorperf.pdf , SPIE Vol. 7021  70210O-12), Mike says the solution is to never clock ØRowOn:

"The MIRI detectors are reset by connecting a pair of rows of pixels to a reset voltage. This occurs 512 times during a frame readout. It had been recommended that the rows be disconnected from the column bus while the reset pulse was applied. Given that the readouts do not like abrupt changes at this temperature, we decided to change the clocking so as not to perform this disconnection. The result was spectacular. The droop disappeared entirely and now shows textbook behavior. We have not observed any change in the data quality or power dissipation, so, at least for the MIRI detectors, this disconnection is not desirable." 

Might just be a 'loose' description, but taken literally it's a different approach.

Because the eBox simply masks the reset bit during non-destructive reads, our scheme was identical to what's laid out in the MIRI-internal document as best practice.  Alternatively, if we set both rails of the ØRowOn line to the 'enable' voltage, we achieve the result in the SPIE paper.

Turns out that the devil is in the details.....