emccds and digital cds techniques
TRANSCRIPT
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17 dec 2003 ESO conference 1
EMCCDsEMCCDs&&
Digital CDS techniquesDigital CDS techniques
Jean – Luc GACH Observatoire de Marseille
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«« L3CCD projectL3CCD project » » Participants & FundingParticipants & Funding
Started in mid 2002Active Participants:
- P. Balard (Marseille)- O. Boissin (Marseille)- O. Daigle (Montreal)- J.L. Gach (Marseille)- C. Guillaume (Haute Provence Observatory)
Funding- Lab funding (CNRS/UP): 31.1 k€ 35 %- CNRS/INSU 7.7k€ in 2004 9 %- ESO 18 k€
12 k€ in 2004 33 %- Canada (LAE) 20 k€ (30 kCAN$) 23 %
TOTAL 2004 88.8 k€
Estimated manpower cost 2002-2004: 630 k€
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Talk structureTalk structure
Needs for low noise CCDsEMCCDs theoryEMCCDs resultsEMCCDs controller considerationsDigital CDS theoryDigital CDS results
Note : EMCCD = {E2V’s L3CCD, TI’s impactron}
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EMCCDs EMCCDs devicesdevicesAvalaible EMCCDs
53581kx1kTITX2850.512.57.4656x496TITC2531001116512x512E2VCCD97101116512x512E2VCCD87???1124128x128E2VCCD60101120x30576x288E2VCCD65
PriceK€
SpeedMHz
Pitch µm
sizeMFDevice
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Needs for low noiseNeeds for low noise CCDsCCDs
Simplified classical CCD noise formula:
Usually in low selectivity instruments, noise is dominated by photon noise (background contribution)
In high selectivity instruments, noise is dominated by detector noise
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Needs for low noiseNeeds for low noise CCDsCCDs
Low noise applications:- Bright objects but short exposures
Time resolving applications (speckle, fast phenomenons…)Wavefront sensing
- Faint objects & high selectivityHigh spatial and/or spectral resolution spectroscopy
- Faint objects, high selectivity & short exposuresScanning instruments (FP, FTS)
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Needs for low noiseNeeds for low noise CCDsCCDs
Possible solutions :- Bright objects but short exposures
EMCCDs
- Faint objects & high selectivityDigital CDSEMCCDs
- Faint objects, high selectivity & short exposuresphoton counting EMCCDs
3rd generation GaAs intensified Photon counting systems (IPCS)
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Needs for low noiseNeeds for low noise CCDsCCDs
Examples of performance: FP observations
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Needs for low noiseNeeds for low noise CCDsCCDsNGC 2403 – Integral field radial velocities
3 H obs., CFHT 3.6 m 1 H obs., OMM 1.6m
+MOSFP instrument + CIGALE instrument
+Thin CCD (QE=90%) +GaAs IPCS (QE=25%)
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EMCCDsEMCCDs theorytheory
EMCCD structure
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EMCCDsEMCCDs theorytheory
Multiplication process
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EMCCDsEMCCDs theorytheoryPer-stage multiplication probability
(from Robbins & Hadwen)
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EMCCDsEMCCDs theorytheory
Electron multiplication is a stochastic process !Output probability distribution:
(from Basden & al.)Mean gain :
Total gain
Nb of input electrons
Nb of stages
Per-stage gain
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EMCCDsEMCCDs theorytheory
Probability distribution example with G=6650:
0 5000 10000 15000 20000 25000 30000-0,00002
0,00000
0,00002
0,00004
0,00006
0,00008
0,00010
0,00012
0,00014
0,00016
1 électron 2 électrons 3 électrons 4 électrons
Pro
babi
lité
Electrons
One output level will give several input possibilities…
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EMCCDsEMCCDs theorytheory
The stochastic process adds noise to the classical shot noise (convolution)In EMCCDs, output noise is:
When G and N are big, F2 =2A high gains :
(Excess noise factor F)2
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EMCCDsEMCCDs theorytheoryExcess noise factor for a 536 stage amplification register
(from Robbins & Hadwen)
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EMCCDsEMCCDs theorytheory
Excess noise could also be interpreted as a QE loss :
Equivalent to a noiseless CCD with 50% less QEBut QE is NOT a major issue at low fluxes (see NGC 2403 observations presented before)
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EMCCDsEMCCDs theorytheory
Generalized SNR formula:
If G is sufficient, readout noise contribution is ~ 0
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EMCCDsEMCCDs theorytheory
Reducing the noise factor : - incrase g (is it possible ? See later)- photon counting
If output signal > threshold level, we consider that we have one (and only one) photon in the pixel.And what if several photons are on the same pixel during exposure time ?Solution : avoid this by a fast readout
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EMCCDsEMCCDs theorytheory
Speed issues
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EMCCDsEMCCDs theorytheory
0 5000 10000 15000 20000 25000 300000,00000
0,00002
0,00004
0,00006
0,00008
0,00010
0,00012
0,00014
0,00016
0,00018
0,00020
Pro
babi
lité
Electrons
E2V CCD 65Gain ~ 6000One input electron
Missed events = QE loss
Cut level
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EMCCDsEMCCDs theorytheory
-
-
50 100 1000 10000 300000,1
0,2
0,3
0,4
0,5
0,6
0,70,80,9
1R
QE
rela
tif
Gain
Seuil de 500 e-
Seuil de 150 e-500 electron cut150 electron cut
At least 5σ cut is necessary…10σ is ideal (from previous IPCS data)
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EMCCDsEMCCDs theorytheoryPossible working modes for an EMCCD:
- No amplification (G=1)the CCD is a normal CCD
- Low to medium amplification (1<G<1000)The CCD has no readout noise but has excess noise
- High amplification (G>1000)The CCD is used in photon counting mode. No readout noise, no excess noisepossible non linearity at high fluxes
It’s possible to switch between modes at any time!
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L3CCD resultsL3CCD results
Various results of amplified mode operation in the literatureIn this talk : Photon counting results-Photon counting is the most demanding application in sensitivity(raises defects of detector)- Photon counting is also speed demanding
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L3CCD resultsL3CCD results
Photon counting image (40ms exposure, 11Mpixels/s, EEV CCD65 @ 183K)
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L3CCD resultsL3CCD results
293 K dark 183 K dark
??? Dark is not dark ???
CCD 65 @ 11Mpix/s Gain ~ 10k
Unwanted events will be reduced at lower gain, but due to high output amplifier noise QE loss is
important
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L3CCD resultsL3CCD resultsCCD 87 @ 223K 10Mpixels/s smaller gain (1k)
(lower output amplifier noise)DARK
6311 counts
Horizontal cut
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L3CCDL3CCD resultsresultsCCD 87 @ 223K 10Mpixels/s
7482 counts
Horizontal cut
825 +/- 28 photons light
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L3CCD resultsL3CCD results
V phase image & storage increased by 3 volts
100 000 counts !
Horizontal cut
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L3CCD resultsL3CCD results2 sources of unwanted events : serial and parallel registersIncreasing gain will increase unwanted events (serial register spurious charge). Therefore increasing g (per stage gain) to improve noise factor is impossible (for the moment…)
unwanted events in the parallel registers are generated by clock levels (spurious charge or clock induced charge).
Mean level can be reduced actually to 0.02 event/pixel/frame
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L3CCD resultsL3CCD resultsConfirmed by ING tests with overscan
(Courtesy of S. Tulloch)
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L3CCD resultsL3CCD results
CIC is charge multiplication in registers !Due to buried channel structure and hole migration in MPP devices
Al Electrode
SiO2
N Si
P substrate
Depleted buried channelCharge
Surface darkcurrent recombination
P substrate
Charge
Clocking
Moving holes to channelstop
Clock generatedelectron
Impact ionisation
P well
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L3CCD resultsL3CCD results
Need slow rise times, but short pulses (but short pulses will also degrade CTE)
From Eastman Kodak. MPP classical CCD
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L3CCD resultsL3CCD results
To avoid CIC :- Drive the CCD in non MPP mode- use slow slew rate clocks but short pulses. For HV use sine wave clock- run the CCD as slow as possible- Use other phase geometry (TI chips may have different properties) especially in first amplification stages (smaller pixels).- Use the lowest gain possible
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Controller considerationsController considerations
In most EMCCDs applications we want:- high readout speed- low CIC- low readout noise (also to minimize gain & CIC)
High speed = 10 (E2V) to 35 (TI) Mpixels/s !!
-> a classical controller won’t meet this constraints
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Controller considerationsController considerationsCCD amplifier :
need 20 to 40MHz bandwidth, low noise
C. Guillaume (Haute Provence Observatory) design gives 17 electron noise at 40MHz bandwidth with 1800V/us slew rates
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Controller considerationsController considerations
Phases are RF signals (0 to 40 MHz). Need coax feed & strict impedance matching or in-house electronics.
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Controller considerationsController considerationsHV Phase design :- sine wave drive- 48V peak for E2V chips (less in the near future ?)- Amplitude controlled (gain variation)- HV phase capacitance 50 to 100pF
75 pF @ 20 MHz = 106 Ohms load106 Ohms @ 50V peak drive = 470 mA peak drive !470 mA @ 50V sine drive = 8.3 W @ 20 MHz (reactive
power – at driver level only)
on chip dissipation : CVf 2 =0.075W + phases resistance
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Controller considerationsController considerationsVertical phases : need slew rate control or shape control.Corot design (courtesy of C.Guillaume):
Or full digital design
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Controller considerationsController considerations
ADC : 16bits impossible, 14bits @ 40MS/s, differential input.CDS : Digital CDS Sequencer : must drive sine and square waves in phase -> Direct Digital Synthesis (DDS) designInterface : Hi-speed digital (RS422 or LVDS)
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Controller considerationsController considerationsMicrocontroller
SequencerDDS
Driver
Shaper Driver
Low noise wideband (40MHz) amp
14 bits40 MSPS A/D
RF powerDriver
DigitalCDS
Rs422orLVDS
drive
HV phase
Horizontalphases
Vertical phases
Video
Bias gen BIASSerialcom.
Syncs
Amplitude (gain) control
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Controller considerationsController considerationsActual status of L3CCD optimised controller @ Marseille observatory
- Amplifier validated- HV phase & Horiz phase validated- Sequencer validated- Software & acquisition validatedRemains Vertical phase design (first design not
validated)
Expected fully working L3 optimised controller : 1st quarter 2004 with CCD65, 2nd with CCD 87
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Digital CDS systemsDigital CDS systems
Developped in 1999 @ Marseille Observatory & Haute Provence Observatory
Actual status : prototype working
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Readout noise of a CCD+CDS system
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Real time Digital filter (DSP)
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Digital CDS systemsDigital CDS systemsSystem structure :
Replaces straightforward an « old » analog CDS
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Black level
Pixel level
Uncorrelated samples
These samples are strongly correlated !
DARK CONDITION
CDS makes the mean of the signal (1st order filtering)
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Black reference
Pixel value
Sample
Digital CDS Digital CDS systemssystems
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Perfect CDS noise floor
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Sample
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Digital CDS systemsDigital CDS systems
Frequency response
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ResultsResults
- One prototype working- 1.7 electrons noise @ 50kpix/s obtained with a « 3 electron noise » CCD (EEV 42-20 read at 5e- with CDS)- 100 ppm linearity (1000 ppm with CDS)
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Future possible developmentsFuture possible developments
– Use of lower intrinsic noise CCDs – Use of more complex digital filters with
increase of DSP power– Coupling with NDRO or multiple skipper amps
(Janesick, Atwood)
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Thank you for your attention !Thank you for your attention !