MIPS 160um Voffset Adjustment Test

Principal: dmkelly
Deputy: jstansberry
Data Monkey(s): With an 800 lb data gorilla as my deputy?
Priority: Necessary
Downlink Priority: Normal
Analysis Time: 4h
Last Updated:

Objective

Determine the optimum values for the MIPS 160um array offset DACs.

Description

To maximize the dynamic range of the 160um detector, we want to start the data ramps fairly close to the lower rail of the A-D converter. At the same time, we do not want to throw away any data because of a data ramp hitting the bottom rail of the A-D. In ground-based testing, we determined optimal DAC settings for the offset voltage DACs to meet these criteria. In this test, we will confirm those settings.

Data Collected

mips_sur_C0F2N5 (x3) 

mips_mobs_phot
CESCANPOS '104,"MIPS"'
CEGESTIM '"AUTO",63,"BOTHOFF","BOTHOFF","BOTHOFF",10,10,"MIPS"'
CEGERSTCON '3,127,4,"MIPS"'
CESCANCON '"CHOP","REV",0,2048,2048,0,511,"MIPS"'
# Voffset1=Voffset2=0xC8
CEMIPSUR '0x0,0x0,0,"NO_COADD",2,5,"MIPS"'
CELDHTGIF '26,0xBE,"MIPS"'
CELDHTGIF '28,0xBE,"MIPS"'
CEMIPSUR '0x0,0x0,1,"NO_COADD",2,5,"MIPS"'
CELDHTGIF '26,0xD2,"MIPS"'
CELDHTGIF '28,0xD2,"MIPS"'
CEMIPSUR '0x0,0x0,2,"NO_COADD",2,5,"MIPS"'
CELDHTGIF '26,0xC8,"MIPS"'
CELDHTGIF '28,0xC8,"MIPS"'
mips_backto_mobs

Data Reformatting Requirements

Array Data Desired:

All Arrays

Data Reformatting Option:

Special Instructions:
Each set of 5 DCEs should go in a separate file.

Task Dependencies

Calibration Dependencies

Output and Deliverable Products

Median and minimum ramp starting points for each offset voltage.

Data Analysis

Step by step analysis:
1) Create a file list.  Use imstat in IRAF or some other tool to determine
   median and minimum values for the first sample in each of the 15 DCEs.
2) Compare these numbers with the values listed below from ground testing.
   Make sure that the minimum starting ramp is greater than 2000 DN in all
   cases.  Check the stability of the median and minimum ramp starting
   points and see how they compare to the ground data.
3) Determine the relationship between offset voltage and DAC count.
   Ground tests indicate a gain of 130 DN/DAC count.
4) If necessary, determine a new offset voltage.  Changing the offset
   voltage could invalidate the IOC linearity measurement, so the case
   for changing the offset voltage needs to be strong.  A change of
   smaller than 4 DAC counts (520 DN) is inadvisable.

Software Requirements

The only analysis tool that is needed is something that allows you to calculate median and minimum values for a single Ge frame and that can be used to calculate the minimum value for a data cube.

Actions Following Analysis

Determine appropriate offset voltages and apply them for use in future campaigns by making changes to irs-mips_pconst_ier.

Failure Modes and Responses

If the starting points of the data ramps are not on-scale or if they do not change as expected in response to offset voltage changes, a careful analysis will be needed to understand the anomalous array behavior. This would be a MIPS timeline stopper. If the starting points are shifted by no more than a few thousand DN relative to ground testing, an anomaly investigation should be performed, but new offsets can be patched and the MIPS IOC timeline can continue. If the test fails to execute, other data could be examined to see if the ramp starting points are reasonable, and we could continue with the IOC timeline.

Additional Notes

Additional Notes:

Summary of offset voltage data from ground testing:

1 DAC count = 20 mV = 130 DN.  Voffset1=Voffset2=0xC8=3.98V

Date    T_Ge    CE    Itime    median      minimum   Environment/notes
02-108  1.83     2       3     4010+/-15  2510+/-20  ATLO
02-108  1.83     2      10     4000+/-15  2490+/-10  ATLO
01-286  1.49     1       3     4110+/-30     0+/- 0  Brutus
01-227  1.53     1       3     4030+/-30  1010+/-40  Brutus
01-227  1.53     1       3     4050+/-30  1010+/-20  quick resets, first ramp
01-227  1.53     1       3     4040+/-40  1030+/-30  quick resets, second ramp
01-220  1.62     1       3     4120+/-20  1230+/-20  Brutus
01-131  1.83     1      10     4100+/-20  2630+/-20  CTA
01-129  1.79     2      10     4070+/-30  2590+/-30  CTA
00-353 1.73K     1       3     4350+/-30  2890+/-50  CTA
00-353 1.73K     1      10     4350+/-40  2880+/-50  CTA
00-124 1.25K     2       3     4300+/-100 2300+/-150 LBTC
00-124 1.25K     2      10     4300+/-100 2400+/-200 LBTC

I do not recall what changed between 00-353 and 01-129 that would change
the offset voltage.  The low minimum values observed during Brutus testing
were all located on the readout that eventually failed.  Other than these
two effects, the stability was quite good.  Integration time, CE, array
temperature, and test environment have minimal impact on the ramp starting
points.  It is a bit uncertain whether external cable temperatures affect
the ramp starting points; medians indicate that such effects are not large.
The expected values for IOC are thus:  median=4050+/-50, minimum=2600+/-100

This cushion of 2600 DN above the bottom rail of the A/D converter is
a good one that should prevent hitting the bottom rail during negative
crosstalk spikes (up to 2000 DN) and post-saturation recovery (up to
1000 DN).  An offset smaller than 2000 DN would be unacceptable.