This local copy of the Hawaii array user guide has been put together from
a number of items on Rockwell's web site.
The original information is at http://creation.risc.rockwell.com/779/mct_fpa/hindex.html
The original author is Craig Cabelli (firstname.lastname@example.org)
Copyright Rockwell International 1996. All Rights Reserved.
The HAWAII 1024 x 1024 readout is structured in four independent quadrants having four outputs. Six CMOS-level clocks, two 5V power supplies (one analog and one digital), on e fixed dc bias and one variable dc bias are required for basic operation. The multiplexer architecture has been optimized to minimize glow. The goal of < 0.1 e-/s has been achieved using correlated double sampling. Read noise of < 10 e- has also been achieved due to refinements in the readout design to suppress the pixel reset anomaly associated with earlier astronomy arrays such as NICMOS3.
Each quadrant contains two digital shift registers for addressing pixels in the array; a horizontal register and a vertical register. Each register requires two
clocks; one dual edge triggered clock and one level triggered clock. To obtain a raster scan output, the horizontal register is usually clocked in the fast direction
with the vertical register being clocked in the slow direction.
Pixel and Lsync are the two required clocks for the horizontal register. The Pixel input clock is a dual edge triggered clock which will increment the selected
column on both edges (odd columns selected on positive edges and even columns selected on negative edges). The Lsync clock is an active low input which will
set a '0' in the first latch and a '1' in the remaining latches of the shift register, thereby initializing the shift register to select the first column in the quadrant. Since
this is asynchronous to Pixel clock, Lsync should be pulsed low prior to initiation of the first Pixel clock edge. The horizontal register selects which column bus
will be connected to the output source follower amplifier.
Line and Fsync are the two required clocks for the vertical register. The Line input clock is a dual edge triggered clock which will increment the row selected on both edges (odd rows selected on positive edges and even rows on negative edges). The Fsync clock is an active low input clock which will set a '0' in the first latch and a '1' in the remaining latches of the shift register, thereby initializing the shift register to select the first row in the quadrant. since this is asynchronous to the Line clock, Fsync should be pulsed low prior to initiation of the first Line clock edge. The vertical register selects the row to be read and/or reset depending on the ResetB and Read inputs.
The two remaining clocks are ResetB
and Read. These two clocks are used to gate with the vertical
register outputs to form the line reset and read function of the
multiplexer. ResetB is an active low clock which will reset
all of the detectors in the selected row to the voltage Vrst
(supplied externally off chip). Usually the process of resetting
the detector array involves addressing the desired row to reset
using the vertical shift register, and pulsing the ResetB
line low. The Read clock is an active high clock which
will allow signals from the currently row to be transferred to
the column (vertical) buses. The column buses are input to horizontal
register controlled transmission gates. The output of the transmission
gates is the horizontal bus, which is input to the output source
follower amplifier. The horizontal bus can also be directly accessed
through the Bus pins on the chip carrier.
Correlated Double Sampling (CDS)
CDS is a clocking method by which the array is reset, sampled, allowed to integrate, and re-sampled with the difference between the 1st and 2nd samples being recorded. CDS is effective at reducing noise and eliminating detector offsets.
For long integration times, IR glow from the output source follower amplifiers will be evident in the image as high dark current areas in the corners of the array. The glow will be on the order of 1,000's of electrons/sec but can be reduced by minimizing the output source follower conduction during integration.
Turning off the source follower amplifiers
can be accomplished by ensuring that the gate of the PFET output
source follower is pulled high (+5V) when not in use. This occurs
when the Read input clock is pulled low, hence disconnecting
all of the column buses from the gate of the source follower.
The gate will be pulled up via the Biaspwr bias input.
Only 2 of the 14 biases, Vrst and Biasgate, will require voltage adjustment during operation of the hybrid. Vrst is the reset voltage that gets applied to the detectors during the reset operation. This voltage is applied through an NFET reset switch which has an associated voltage drop across it due to parasitics of the reset FET; hence, this will reduce the actual voltage to the detector by about 100mV - 150mV. Vrst is usually operated in the 0.5V to 1.0V range.
Biasgate is used to adjust the
speed and dynamic range of the unit cell source follower. A trade
off can be made between speed and dynamic range by adjusting Biasgate
from 3.3V to 3.8V. Lower voltages increase the speed at the expense
of dynamic range, while higher voltages increase the dynamic range
at the expense of speed. A typical Biasgate voltage of
3.5V is used for initial characterization of the hybrid.
Source and Bus Outputs
The Source and Bus pins
on the carrier are two simultaneous outputs available on the multiplexer.
Source is connected to the source of the output source
follower; by using a pullup resistor of 10Kohms to +5V, the multiplexer
can directly drive off-chip loads such as cables and preamp inputs.
Bus is connected to the gate of the output source follower.
A 200Kohms to +5V pullup resistor is required if the user would
like to use this output and provide their own off-chip driver.
NICMOS3 Functional Comparison
For those who are familiar with Rockwell's
NICMOS3 256 x 256 SWIR focal plane array, the transition
to the HAWAII 1024 x 1024 SWIR focal plane array should
be relatively easy; however, there have been some slight changes
to the basic architecture which should be noted:
In an attempt to reduce the effects of noise caused by resetting pixels in the array (reset anomaly), the HAWAII multiplexer has a line by line reset instead of a pixel by pixel reset. This means that at any time while accessing a row, the entire row will be reset when ResetB clock is pulsed low. In order to reset an entire frame using NICMOS3 it is necessary to address every pixel in a quadrant, however, in order to reset an entire frame using the HAWAII multiplexer it is necessary only to clock through the vertical register.
Both NICMOS3 and HAWAII require 6 input clocks to properly operate the array. Both require 2 clocks per shift register; however, for HAWAII, Lsync should be pulsed low before the first Pixel clock edge for the horizontal register and not during as is the case for NICMOS3. This also true for Fsync and Line inputs for the vertical register. The Line clock for HAWAII is dual edge triggered, not negative edge triggered as in NICMOS3.
The two remaining clock inputs are
ResetB and Read. While accessing any row in the
array, pulling the ResetB input low will simultaneously
reset all pixels in that row; this is in contrast to NICMOS3 which
requires the accessing of every pixel which is to be reset. By
pulling the Read input high, the currently selected unit
cell source follower is allowed to pass to the column bus. This
also means that anytime the Read clock is low, none of
the unit cell signals can be transferred to the output source
follower via the column buses; regardless of the state of
the horizontal or vertical register. This feature is very
useful in turning 'off' the output source follower when not in
use to decrease the effects of the output source follower glow.
NICMOS3 required an extra horizontal register Pixel clock
at the end of the row in order to ensure that the output source
follower was 'off'.
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