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READOUT TECHNIQUES FOR DRIFT AND LOW FREQUENCY NOISE REJECTION IN INFRARED ARRAYS

Finger1, G, Dorn1, R.J1, Hoffman, A.W.2, Mehrgan, H.1, Meyer, M.1, Moorwood A.F.M.1 and Stegmeier, J.1
1) European Southern Observatory 2) Raytheon Infrared Operations

Abstract:

Three different methods are presented to subtract thermal drifts and low frequency noise from the signal of infrared arrays: first dead pixels with open Indium bumps, second the reference output as implemented on the Hawaii2 multiplexer and third dark pixels to emulate reference cells having a capacity connected to the gate of the unit cell FET. The third method is the most effective and yields a reduction in readout noise from 15.4 to 9.4 erms. A novel method will be described to extend this readout technique to the Aladdin 1Kx1K InSb array. infrared detector, readout, HgCdTe, InSb, reference cell, noise

Key words:

1.

INTRODUCTION

Contrary to CCD's the video output of infrared arrays is DC coupled. The time interval between the two samples of a double correlated clamp may be several thousand seconds instead of microseconds typical for CCD's. For this reason long exposures with IR arrays are extremely susceptible to drifts and low frequency noise pick-up down to the mHz regime.

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In an ongoing effort to reduce noise pickup and thermal drifts of the video signal of IR arrays we have tried different readout schemes. The best configuration tested so far is a symmetrical amplifier located as close as possible to the detector as shown in Figure 1. It consists of two linear CMOS operational amplifiers and is cooled to cryogenic temperatures as it is placed on the detector board next to the detector [1]. It constitutes the front part of a differential signal chain. The amplifier requires two input signals and generates two antisymmetric output signals Signalout+ and Signalout- for the differential input of the ADC located at a distance of a few meters outside the instrument. One input is reserved for the video signal and the second input is a reference which has two functions. First, it is used to shift the DC offset of the outputs. Second, the reference helps to cancel thermal drifts and low frequency noise pickup, since only the difference between reference and video input contributes to a signal change. The topic of this paper is to discuss different options for the reference input to achieve this goal in an optimum way.

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Figure 1. Cryogenic preamplifiers. Left side: schematics of symmetric amplifier with video and reference input. Right side: backside of detector board for Rockwell Hawaii2 2Kx2K HgCdTe array with 36 preamplifiers, 32 for video outputs and 4 for reference outputs. The cold finger is above the amplifiers and cools the detector via the central pins of the PGA package.

2.

DEAD PIXEL AS REFERENCE

A standard technique to reduce the readout noise of deep exposures with long integration times is to apply multiple nondestructive readouts. This technique suffers from thermal drifts of the detector, as can be seen in the


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plot of Figure 2. This shows the nondestructive readouts of the integration ramp of a dark exposure taken with an engineering grade Aladdin 1Kx1K InSb array. The raw signal is represented by triangles and exhibits an irregular time evolution reflecting thermal drifts. The cosmetic of the engineering grade array shown by the flat field on the right side of Figure 2. is degraded and has cracks and dead pixels in the upper right corner. These are unit cells having open Indium bumps. The gate of the unit cell source follower is not connected to the infrared diode but floating. However, it can be used as a reference to monitor the thermal drifts. The signal of dead pixels is represented by diamonds in Figure 2 and closely follows the irregular pattern of the video signal. For the Aladdin array the temperature drift of the DC level of the video signal was measured to be 1700 e/K at a temperature of 27 K. Subtracting the dead pixel signal from the video signal results in the compensated video signal which is represented by squares. For demonstrative purposes the measurement in Figure 2 has been carried out without active temperature stabilization. It yields a dark current of 2.7 10-2 e/s. Of course, temperature stabilization is the best way to eliminate drifts. But even with an active detector temperature control loop extremely low dark currents are masked by thermal drifts. Stabilizing the detector temperature it was necessary to apply this monitoring technique to measure the dark current of 4 10-3 e/s . This is the lowest value reported for InSb [1].

Figure 2. Left: Integration ramp of dark exposure with multiple nondestructive readouts. Triangles: raw detector output. Diamonds: dead pixel output. Squares: compensated detector signal by subtracting the signal of dead pixels from detector signal. Right: cosmetics of Aladdin 1Kx1K InSb engineering grade array. Dead pixels for monitoring thermal drift are taken in upper right corner


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3.

HAWAII2 MULTIPLEXER REFERENCE

The first infrared array providing an on chip reference cell is the Hawaii2 2Kx2K HgCdTe array from Rockwell. Unfortunately, the reference output is implemented as an additional output for each quadrant and requires four channels in addition to the 32 channels for the video signal if the subtraction is to be done in the real time processor of the data acquisition system. The reference output gets activated as the 129th pixel at the end of each row which comprises 128 pixels if 32 outputs are used. Since the reference output is not available while the detector pixels are being read a cryogenic clamp is needed on the detector board. First the multiplexer has to be clocked to the reference pixel. Then the reference output is clamped and fed as reference input into the symmetric preamplifier while the same row is clocked again to read the detector pixels. On our detector board we have implemented four additional channels for digitizing the reference output as well as a cryogenic clamp to make an analog subtraction of the reference. The temperature dependence of the video signal and the reference outputs of the Hawaii2 array are shown in Figure 3. The rate at which the DC level of the video and reference outputs drifts is 327 e/K and 338 e/K.

Figure3. Temperature drift of Hawaii2 array. Triangles: video signal shift 327 e/K. Diamonds: reference signal shift 338 e/K.

Hence, the reference output is well suited to track the temperature drifts of the video signal as demonstrated by the 46 hour integration shown in


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Figure 4. It shows the deviation of the uncorrected (squares) and the corrected (triangles) nondestructive readouts from a least squares fit to the measured integration ramp of the detector signal. The standard deviation of the uncorrected readouts is 9.0 erms. The standard deviation of the readouts from which the reference output has been subtracted is 4.1 erms. The reference output on the Hawaii 2 multiplexer does not work in the unbuffered mode, which bypasses the second stage on-chip source follower having the current source of the internal bus switched off. This is the standard way we operate the Hawaii2 array. The gate of the Hawaii2 reference output source follower is tied to Vreset. If the internal current source is switched on by changing the bias power from 5V to 3.4 V the reference output follows Vreset even if the second stage source follower is still bypassed (Vdrain = 5V). However, good thermal monitoring of drifts is only achieved in the buffered mode (Vdrain = 0V). First measurements of thermal drifts did not take this fact into account and were misleading.

Figure 4. Deviation of uncorrected (squares) and corrected (triangles) nondestructive readouts from least squares fit to measured integration ramp. Standard deviation uncorrected : 9.0 erms. Standard deviation corrected with Hawaii 2 reference output : 4.1 erms.

The efficiency of the reference output to suppress low frequency noise and pickup has also been investigated. The difference of two subsequent 5 second dark exposures is shown in Figure 5. The left image is the raw unsubtracted difference and shows some low frequency stripes. The high frequency stripes are due to 50 Hz pickup. The different orientation of the stripes in the four different quadrants is due to the readout topology of the


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multiplexer. The stripes are parallel to the direction of the fast shift register. The histogram of the readout noise is shown by the dotted line in Figure 6. The mean readout noise is 19.4 erms. It is partly increased by quantization noise of the ADC since the conversion factor had to be increased from 3.7 e/ADU to 9.7 e/ADU to cope with the higher DC level when switching form unbuffered to buffered mode of operation. The difference image in the middle of Figure 5 has been corrected with the reference output. The intensity of the stripes is slightly reduced and the histogram of the readout noise is shown by the dash dotted line in Figure 6. The mean noise is 16.7 erms. The time to read out a row of 128 pixels is ts=586µs. The squared transfer function of a double correlated clamp is 2-2cos(2fts). For a pickup frequency of 50Hz the sampling interval ts between reading the pixel and reading the reference will be 586µs in the worst case with the transfer function being 0.185, which is the suppression factor for 50Hz pickup [2].

Figure5. Difference images of double correlated clamps corrected with the reference output of a Hawaii 2 multiplexer. Left image: uncorrected. Middle image: corrected with single readout of reference output after reading row. Right image: corrected with two readouts of reference output before and after reading row and using linear interpolation.

In order to explore the possibilities to further reduce ts, the reference output was read out twice, once by clocking to the end of the row and reading the reference before reading the row and a second time after reading the row. The reference is sampled 16 times and the average is taken to minimize the noise increase induced by the subtraction. For each pixel the reference signal is linearly interpolated from the two readings of the reference output. The result is shown in the right image of Figure 5 and in the noise histogram represented by the solid line in Figure 6. The mean readout noise is 16.2 erms, a marginal improvement. The Hawaii2 reference output only partially suppresses low frequency noise components.


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Figure 6. Noise histogram of a double correlated clamp corrected with the reference output of a Hawaii 2 multiplexer. Dotted line: uncorrected, mean noise 19.4 erms. Dash-dotted line: corrected with single readout of reference output after reading row, mean noise 16.7 erms. Solid line: corrected with two readouts of reference output before and after reading row using linear interpolation, mean noise 16.2 erms.

4.

DARK PIXEL AS REFERENCE

Since the effectiveness of an on-chip reference having the gate of its source follower tied to a fixed voltage is insufficient, we tried to use reference cells with the gate connected to a capacity. As such a reference is not available on the Hawaii2 multiplexer, dark pixels have been used instead. They are located in the lower left quadrant on the left edge and are shaded in the left image of Figure 7. The pixels on the left edge of channel number 1 are used to correct the complete array. The noise histogram of the uncorrected image is shown by the dotted line in Figure 8. The mean noise is 15.4 erms. Subtraction of dark pixels, as shown by the middle image in Figure 7 and the dash dotted line in Figure 8, is very effective in removing low frequency noise components and reduces the read noise to 10.4 erms. Linear interpolation of two readings of the dark pixels before and after reading the row further improves the removal of pickup components and yields almost perfect images. The mean readout noise is reduced to 9.4 erms as displayed by the solid line in Figure 8. Most of the low frequency noise sources Vi are capacitively coupled into the integrating node capacity C


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which is assumed to be large in comparison to the coupling capacities Ci. In this case the coupling constant of noise source Vi is C/Ci.and the capacity connected to the gate of the reference cell FET should be equal to C for best noise rejection.

Figure 7. Difference images of double correlated clamps corrected with dark pixels indicated by the shaded area in the lower left quadrant of the left image which is uncorrected. Middle image: corrected with single readout of dark pixels. Right image: corrected with two readouts of dark pixels before and after reading row using linear interpolation.

Figur 8. Noise histogram of double correlated clamp corrected with reference output of Hawaii 2 multiplexer. Dotted line: uncorrected, mean noise 15.4 erms. Dash-dotted line: corrected with single readout of reference output after reading row, mean noise 10.4 erms. Solid line: corrected with two readouts of reference output before and after reading row using linear interpolation, mean noise 9.4 erms.


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5.

COLUMN CLAMP OF ALADDIN MULTIPLEXER AS REFERENCE

Considering the attractive feature of reference cells not only to monitor thermal drifts but also to remove all noise pickup components and get clean and low noise images, we searched for ways to implement an on-chip reference for the CRIRES mosaic focal plane, which consists of four Aladdin 1Kx1K InSb arrays. The column clamp circuit of this array seems to be well matched to function as a reference cell [3]. Its geometry and layout is identical to the unit cell source follower connected to the detector pixel. If VddCl is tied to VddUc and VggCl to VdetCom the clamp circuit emulates a saturated pixel during row transitions when the row enable switch disconnects all unit cells from the column bus. We have both simulated and measured whether the gain is reduced by keeping Vggcl at Vdetcom during readout of the detector pixels and confirmed that the gain is not affected. The effectiveness in suppressing noise by utilizing the clamp FET as a reference cell still remains to be tested with the Aladdin array. It is planned to evaluate the performance of this technique by both connecting Vdetcom to the clamp gate and to connect a capacity to the gate reset together with detector pixels but left floating while reading out the array and the reference.
VddUc Res et D S Ren VdetCom R ow Enable Column Bus SF D UNIT CELL

Detect or UNIT CELL

G D iode

VddCl--->VddUc VggCl--->VdetCom

Cen Output Source Follower Cen Load R es is t or

Vslew VssCm

Vidle

Figure 9. Column clamp circuit used as reference cell by replacing VddCl with VddUc and VggCl by VdetCom. Reference is sampled during row transitions when row enable switch is open.


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CONCLUSIONS
Reference cells are indispensable on IR arrays to eliminate thermal drifts during long integrations and to obtain images untainted by low frequency noise pickup. Dead pixels not hybridized to the unit cell can be used to eliminate thermal drifts. The reference cell of the Hawaii2 multiplexer reduces thermal drifts but does not reject low frequency pickup very well. Dark pixels have been used to demonstrate that connecting a capacity to the gate of a reference FET achieves clean images free of noise pickup. By employing this technique, the readout noise of a double correlated clamp on a Hawaii2 array could be reduced from 15.4 erms to 9.4 erms. In next generation multiplexers such as the Hawaii-2RG, reference cells will be implemented with capacities. A method to use the clamp circuit of the Aladdin 1Kx1K InSb array as reference pixel is under development.

REFERENCES
[1] G. Finger, H. Mehrgan, M. Meyer, A.F.M. Moorwood, G. Nicolini, and J. Stegmeier,"Performance of large format HgCdTe and InSb arrays for low background applications", Proceedings SPIE 4008, pp. 1280-1297, 2000. [2] J. R. Janesic, "Scientific Charge Coupled Devices",SPIE Press, pp 564, 2001. [3] A. W. Hoffman, Raytheon Infrared Operations, private communications, 2001.