HAWAII-2 Detector Characterization

1.- Crosstalk
2.- Noise
3.-"Image Lag" effect
4.- Bias Tilt
5.- Bias Stability study
6.- Dark Current
7.- Amplifier Glow
8.- QE transfer
9.- Linearity
10.- Future Tests
 
 
 

1.- Crosstalk

Like most HAWAII-2 users, we had connected the 10K output load resistors to VDDA right at the detector.  This meant that the RW=18 ohm (constantan) wire to VDDA was shared by all 4 channels.   For output FET transconductance, g, load resistance, RL ,  crosstalk =  RW/RL/(1+gRL).    Since there is a thermal penalty for using very low resistance wiring we solved the problem by moving the load resistors to the warm end of the signal wire to eliminate the common path.  We  supplied separate bias voltages in place of VDDA since the closed loop output impedance of the bias buffers could have produced more than the one part in 100,000 crosstalk seen in the signal chain.
We now have negative crosstalk of one part in 15,000,  which we think is due to 18 ohm wiring resistance in series with BiasPower. ... to be confirmed by rewiring the dewar with an intermediate resistance wire.

For finding up the crosstalk source  a series of resistors were added to each of the biases, finding that the main source was the common load  resistors connected to a common VDDA, as was already explained.   The 10K loads were moved from the fanout board to the video cards where they were biased by separate buffers.  This change the crosstalk from positive to negative, and reduced it from about one part in 1000 to one part in 15,000, at which point it is no longer visible in the noise of a single frame, at least not until the banding effects are controlled.

The mask consists in a one hole per quadrant. In the following images, the brighter spot corresponds to the actual hole, while the other are pure crosstalk from the other quadrants

figure 1.1 When a 30 ohms series resistor was added to VDDA
Crosstalk ratio: 1/100

crosstalk 1/100



figure 1.2: Normal images, 2 ohms wire resistance on VDDA:
Crosstalk ratio: 1/1000

crosstalk 1/1000

 figure 1.3: After the load resistors were moved to the video card (and separate buffers were used for each quadrant):
Crosstalk ratio: 1/15000

crosstalk 1/15000


 

2.- Noise Issues

Detector Specs:
noise: < 15 e-
Gain: 3 - 6 uV/e-
 

Measure electronic noise: ~0.8 adu (~15 uV ==> ~ 2 - 5 e-)
(See here the noise considerations for the preamplifier)

Banding effect? ...  ....it seems be dissapear ... what it was?
fan noise
 we found that the fan produces a magnetically induced noise mainly over the most  adjacent channel to the fan We confirmed that it is not an electric noise since we saw the same effect when using an external power supply for the fan. This noise was found to be about 6 adu peak to peak (figure 2.1, lower right quadrant)

60 Hz
Very strong 60Hz noise was found. This noise was fixed in the lab by connecting all the grounds together (controller ground to chasis, power supply ground to chasis, dewar to controller ground). See figure 2.2
 
 

figure 2.1: Fan noise (lower right quadrant) (just controller):

fan noise



fig 2.2: 60 Hz noise (enginering array)

60 hz


figure 2.3: science grade array in the lab, after all the noise was removed. The only visible effects are the bias tilt . This  dark was taken with 0 secs idle time.


 

3.- "Image Lag"

These effects are also seen at CTIO in their 2K arrays and in the HAWAII-1K arrays at ESO and Palomar.   The "image lag" is a small zero point shift seen in response to the integrated signal in the line.  The shift appears in all quadrants, not just the one containing the source signal, suggesting it might be a long time constant bias voltage fluctuation?. However,  if it is electronic-related, why this effect was not seen on the multiplexer (without HgCdTe detector), at  even higher ADU values?

figure 3.1:

image_lag


4.- Bias Tilt

Two effects can be seen in the folowing images: at the very beginning a negative value can be seen as a black border for every amplifier. After that has passed, a high value which dominates starts suddenly, and it starts decreasing untill it dissapears (white bands). This last effect is what we called "image tilt".
The bias "tilt" changes according to the waveform used. The first image is what appers with the reset-read line by line. The second is the reset-frame, read-frame.
This effect is seen on  CTIO too. The images shown are "0" exposure time (no idle time, just readout exposure time, whch means  about 3.18 secs exp time).
When the detector has some "idle" time (no clocking) this effect is minimized. For an idle time of about 3 seconds it dissapears almost completelly. In figure 4.1.2 can be seen that after 700 pixels (which equivals to more than 2 seconds of read time) the effect  dissapears too, and the only effect that can be seen is the first one, of the black bands (fig 4.1.3).
This behaivour also changes  according to the waveform scheme used

- initial black bands: can this  be a transient in the biases?. There are two interesting clues: in the first case (reset-read line to line) the transient appears independent on the exposure time; this means that it is present when there was and when there was not an idle time. This may discard the idea that it is caused by a "transient" in the biases, since even when the biases where not relaxed (no idle time) it is there. A reconfirmation on this would be that in the second case (reset-read frame to frame) this transient is not present, even when you have an idle time diferent from 0 (which implies a relaxing in the biases)

- tilt: As we can see, in the second case the effect it is highly minimized. What is the diference?. In the first case, we see that the effect lasts less than 3 seconds since the reset clock stoped (when there is no idle time, it takes about 700 hundreds lines to dissapear, which is about 2.5 seconds read time, and if we have 2.5 seconds of idle time the effect does not appears ...). In the second case, when you start reading the exposed frame, even when there was no idle time, already passed 3 seconds since the reset clock stoped, and this may explain why it does not appears so strongly. Is this suggesting that this is some reset-related issue?

4.1 Reset-Read line by line waveforms

fig 4.1.1: 0 secs idle time

tilt line to line reset

figure 4.1.2: 0 secs idle time

tilt
 

 fig 4.1.3: 3 secs idle time

tilt 7 secs



 4.2 Reset  - Read frame by frame waveforms
fig 4.2.1: 0 secs idle time
 
 

tilt frame reset




 fig 4.2.2: 0 secs idle time
tilt
 

5.- Bias Stability

We measured the voltage drop across a 200 ohm resistance placed in series with each bias voltage and ground of the engineering grade HAWAII-2 RG, when reading out a pattern with one large spot per quadrant, located so that each spot is read sequentially.   Signal induced currents were too small to be registered above noise on the scope.   This could be repeated using a video channel as a synchronous "scope probe".

A 10 K load resistor to a 10V p-p square wave was also connected to determine the sensitivity of the signal to a 200 mV modulation of the bias voltage.
 
200 ohm in series with... DC Current
     (mA)
    sensitivity 
(ADU/200mV)
noise bands 
 (ADU p-p)
Crosstalk ch2 to ch1
(rejection ratio)
signal memory
(rejection ratio)
 Ground = DSUB = CellDrain = Drain
VDDA:load resistor [1] 0.77[2] 3700 3.6 118
VDDA: substrate bias
BIASPOWER 0.02 15060 1 840 1600
BIASGATE 0 [4] 1-0.6 860 1520
VRESET 1.6 [3] 370 1 1020 1400
VDDD 0.01 157 0.7 760 1400

Notes

[1]  When VDDA crosstalk (etc)  was first measured, the 10K output load resistors were connected to a common VDDA.   The crosstalk was thus due to the common series resistance.   The 10K loads were moved from the fanout board to the video cards where they were biased by separate buffers.  This change the crosstalk from positive to negative, and reduced it from about one part in 1000 to one part in 15,000, at which point it is no longer visible in the noise of a single frame.

[2]  Expected total video load current  = 4 * (VDDA - video_DC)/10Kohm = 4*(5-3.5)/10K = 0.6 mA so true VDDA current to mux ~0.17 mA.

[3]  Why does VRESET source any current other than photocurrent ?   Is there some leakage poath in parallel wiht the detector ?

[4]  This measurement was invalid since we failed to notice that the reset frame was driving into the lower rail.   The sensitivity to BIASGATE fluctuations should be similar to BIASPOWER.

Video gain = 10
Video ADC: 10V unipolar 16 bit ... 152 uV/ADU

More measurements need to be performed to complete the previous table
 

6.- Dark Current

Negative dark currents appears frequently ... is this since a bias drift during the exposure?. Negative values seems to be fairly randomic (two images with the same exposure time taken one right after the other can give completelly diferent values, for example -60 and -300) ... why can this be?
Positive dark current value measured however, are inside expected range ...
Why about a method for measuring the diff to "dead" pixels on the corners ...?

T: ~ 79 K
G: ~5.47 e-/adu
read method: CDS, using one non-destructive read
ranging time measured: from 100 to 1000 secs
Dark current measured: ~0.048 adu/s ==> ~0.26 e-/s
 
 

7.- Amplifier Glow

figure 7.1: 1000 secs exposure time, without clocking the array -idle time- ...

amp glow

8.- QE transfer

readout time: 3.18 secs (3 usecs/pixel ==> ~330 KHz)

Values measured on the telescope:
gain = 5.467 e-/adu.
CDS noise: 13.76 e-

Out the telescope the shown (figure 8.1)  curve was measured. Counts varied from about 1000 to 20000 (about same range in which  linearity was measured). The coeficients obtained for the diferent channels were between 5.39 and 5.5 (e-/adu), which corresponds with the value measured on the telescope.

figure 8.1: example mean/variance curve obtained for channel 3 (slope adjusted by legendre, order 2).
variance
 
 

9.- Linearity

We need to talk about the graphic you sent me (from Chris?)

System Non-linearity was measured to be less than 1% (!!?). The curve shown below was done considering a gain of 5.4 e-/adu. ADU ranges varied then from about 1000 to  18000 counts.
The electronics (controller) non-linearity (without detector) by the other hand, was measured to be about 0.025%

fig 9.1: System Non-linearity
non-linearity
 

10.- Future Tests

        DSP waveforms:
           We need to play more with read modes (reset frame then read; reset-read by row; read-reset-read) and the effect over the various pheomena mentioned (bias tilt, image lag, amplifier glow, etc). Beside this, we want to implement Regions of Interest for using only parts of the detector

       Detector:
       Finish the crosstalk investigation: rewire BiasPower for decreasing the cable resistance
       Understand exposure zero point paradox.
       Finish measuring sensitivity of each bias (and clock?) to noise.
       Measure noise on each bias and improve where necessary.
       Measure noise vs bandwidth.
       Determine 1/f corner frequency.
       Investigate more preciselly what   leads to negative dark current (bias drift?)
       Determine cause of "image lag"; fix if possible.
       Determine cause of "image tilt"; fix if possible
       Test amplifier glow control strategy (play with DSP waveforms)
       Measure & mitigate glow from row and column select switches.
       Measure the departure from root N noise reduction for multiple sampling.  The above tests should give us a clue to the cause.
       Measure performance versus temperature.