EMCCDs and Digital CDS Techniques

17 dec 2003 ESO conference 1

EMCCDsEMCCDs&&

Digital CDS techniquesDigital CDS techniques

Jean – Luc GACH Observatoire de Marseille


«« 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€


Talk structureTalk structure

Needs for low noise CCDsEMCCDs theoryEMCCDs resultsEMCCDs controller considerationsDigital CDS theoryDigital CDS results

Note : EMCCD = {E2V’s L3CCD, TI’s impactron}


EMCCDs EMCCDs devicesdevicesAvalaible EMCCDs

53581kx1kTITX2850.512.57.4656x496TITC2531001116512x512E2VCCD97101116512x512E2VCCD87???1124128x128E2VCCD60101120x30576x288E2VCCD65

PriceK€

SpeedMHz

Pitch µm

sizeMFDevice


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



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)



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)



Examples of performance: FP observations


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%)


EMCCDsEMCCDs theorytheory

EMCCD structure



Multiplication process


EMCCDsEMCCDs theorytheoryPer-stage multiplication probability

(from Robbins & Hadwen)



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



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…



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


EMCCDsEMCCDs theorytheoryExcess noise factor for a 536 stage amplification register

(from Robbins & Hadwen)



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)



Generalized SNR formula:

If G is sufficient, readout noise contribution is ~ 0



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



Speed issues



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



-

-

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)


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!


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



Photon counting image (40ms exposure, 11Mpixels/s, EEV CCD65 @ 183K)



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


L3CCD resultsL3CCD resultsCCD 87 @ 223K 10Mpixels/s smaller gain (1k)

(lower output amplifier noise)DARK

6311 counts

Horizontal cut


L3CCDL3CCD resultsresultsCCD 87 @ 223K 10Mpixels/s

7482 counts

Horizontal cut

825 +/- 28 photons light



V phase image & storage increased by 3 volts

100 000 counts !

Horizontal cut


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


L3CCD resultsL3CCD resultsConfirmed by ING tests with overscan

(Courtesy of S. Tulloch)



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



Need slow rise times, but short pulses (but short pulses will also degrade CTE)

From Eastman Kodak. MPP classical CCD



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


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


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



Phases are RF signals (0 to 40 MHz). Need coax feed & strict impedance matching or in-house electronics.


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


Controller considerationsController considerationsVertical phases : need slew rate control or shape control.Corot design (courtesy of C.Guillaume):

Or full digital design



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)


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


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


Digital CDS systemsDigital CDS systems

Developped in 1999 @ Marseille Observatory & Haute Provence Observatory

Actual status : prototype working


Readout noise of a CCD+CDS system


Real time Digital filter (DSP)


Digital CDS systemsDigital CDS systemsSystem structure :

Replaces straightforward an « old » analog CDS


Black level

Pixel level

Uncorrelated samples

These samples are strongly correlated !

DARK CONDITION

CDS makes the mean of the signal (1st order filtering)


Black reference

Pixel value

Sample

Digital CDS Digital CDS systemssystems


Perfect CDS noise floor


Sample


Digital CDS systemsDigital CDS systems

Frequency response


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)


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)


Thank you for your attention !Thank you for your attention !

EMCCDs and Digital CDS Techniques

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