NMOSbased high gain amplifier for MAPS

1
NMOS-based high gain amplifier for MAPS
Andrei Dorokhov Institut Pluridisciplinaire
Hubert Curien (IPHC) Strasbourg, France
VI th INTERNATIONAL MEETING ON FRONT END
ELECTRONICS FOR HIGH ENERGY, NUCLEAR, MEDICAL
AND SPACE APPLICATIONS Perugia, Italy 17- 20 May
2006  
e-mail address Andrei.Dorokhov_at_IReS.in2p3.fr slid
es are available at http//www-lepsi.in2p3.fr/do
rokhov/talks/Andrei_Dorokhov_FEE2006.ppt
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Contents

• Amplifiers for MAPS requirements, constraints
  and limitations
• Introduction to an improved amplifier schematics
• Implementation of the amplifier in test
  structures of Mimosa15 chip
• Tests with Fe55 and results
• Summary and future plans

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Amplifiers for MAPS
Amplification is needed to decrease noise
contribution from switching networks, like
clamping or sampling.

• PMOS transistors not allowed inside pixel -gt
  signal decrease due to parasitic NWELL
• but using PMOS transistor as a load would be the
  preferred choice to increase in-pixel amplifier
  gain

load
bias
gate
reset
out
bias
in
in
out
vb
cascode
signal current
in
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Amplifiers for MAPS
small signal Gain Vout/Vin gm1 /( gm2 gmb2
gds1 gds2)
M2
bias
As an example from simulation to be presented
later gm147 mS gm24 mS gmb20.9 mS gds18
nS gds20.5 mS
out

• gds1 and gds2 ltlt gm1, gm2 , gmb2
• so one need to increase gm1 and decrease gm2
  and gmb2
• with decreasing gm2 we decrease DC current, and
  hence gm1 so there is a limiting contradiction
  for the gain/bandwidth of this schematic

Id
in
M1
signal current
Due to gm2 there is unwanted dependency of Id on
Uout , so can we reduce dependency of Id on Uout
without changing gm2 ?
?
5
Improved load for the common source transistor
-gt decouple the gate of the load transistor from
the power supply with one additional NMOS
transistor, used as a diode

• due to the floating gate and parasitic
  gate-to-source capacitive coupling the AC voltage
  at the gate will follow to the output AC voltage
  -gt
• AC current and hence the load for the common
  source transistor decreases
• load for DC is almost unchanged as DC voltage
  drop on additional NMOS transistor is small

M3
M2
gate
out
bias
in
M1
signal current
Gain Vout/Vin gm1 /( gm2 gmb2 gds1 gds2)
The AC gain should increase, while the DC
operational point should not change!
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New idea works - simulation with Spectre
New schematic
AC gain is 8dB larger!
Standard schematic
gmb2 gds1 gds2 become significant and limit
the amplification, and also the gate is not
perfectly decoupled -gt still some fraction of gm2
exist
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Test structures with new amplifier implemented in
Mimosa15 chip
improved load with power on switch
low frequency-pass feedback
correlated double sampling circuit
common source transistor with power on switch
NWELL diode
NWELL size is 4.25 mm x 3.4 mm, pixel pitch size
30 mm x 30 mm, pixel matrix 4 columns x 15 rows
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Tests with Fe55 source, measurements

• 5.9 keV gammas create about 1640 e-h pairs in
  silicon
• pixel amplitude is determined as the voltage
  difference between two successive time frames
  (correlated double sampling)
• the time between two successive frames is 500 ms
  ( integration time)
• pixel readout time is 500 ns
• measurements are performed at stabilized
  temperature of 20O
• common mode and pedestals are subtracted

Single pixel amplitudes distribution,
superimposed for all 60 pixels, no amplitude cut
is applied
If gamma goes directly to the NWELL volume ( very
small ltlt1 fraction of all gammas), all delivered
charge will be collected by single pixel and this
value used to determine conversion gain (mV/e)
for the amplifiers
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Tests with Fe55 source, calibration
Each pixels is calibrated individually, example
of calibration peak for pixel07

• Events selection for the calibration peak
  amplitude distribution
• S/N (seed pixel) gt 5
• S (seed pixel) gt S (each of 8 pixels around seed
  pixel)
• S/N lt 10 for each of 8 pixels around seed
• S/N lt 5 for each of readout pixels in the ring
  5x5 around 3x3

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Tests with Fe55 source, calibration
Distribution of calibration peaks for all pixels

• Conversion gain is about 74 mV/e, including
  attenuation in source follower
• Calibration peak variation is about 2 , this
  includes charge-to-voltage conversion in NWELL
  diode, amplifier gain, source follower gain
  variations
• Output voltage variation before CDS, due to
  process parameters variation (NWELL, amplifier
  transistors, source followers) is about 20 mV

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Tests with Fe55 source, noise
Noise amplitude distribution for each pixel is
fit to Gaussian, and sigma is taken as noise
value for each pixel of 60
Noise value distribution for all pixels, average
noise is about 7.5 e or 540 mV at the column
output
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Tests with Fe55 source, charge collection
Seed pixel amplitude distribution for all pixels,
the most probable value for the collected charge
in the seed pixel is about 300 e, or 18 of
total charge

• Events selection for the seed pixel
• Border pixels are excluded
• S/N (seed pixel) gt 5
• S (seed pixel) gt S (each of 8 pixels around seed
  pixel)
• average S (8 pixels around seed pixel) gt
  average S (13 pixels around 3x3 cluster pixel)

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Tests with Fe55 source, charge collection in
cluster of 3x3
Charge collection in 3x3 pixels cluster, the most
probable value for the collected charge is about
950 e, or 58 of total charge
Events selection same as for the seed amplitude
distribution
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Summary

• new resistive AC load, which uses only NMOS
  transistors, is proposed
• NMOS based amplifier using new type of load and
  feedback is designed and simulated
• the gain increases by factor of 2 in comparison
  to the gain of existing amplifier schematics,
  which use only NMOS transistors
• in comparison to old schematic, the same gain
  can be achieved with smaller power consumption
• the designed amplifier implemented in MAPS using
  AMS0.35 OPTO process and tested with Fe55 source
• the tested MAPS has the following measured
  properties
• low noise, 7.5 e (after CDS), and hence higher
  signal-to-noise ratio
• conversion gain is about 74 mV/e
• gain variation due to process variation is about
  2
• charge collection in seed pixel is 18
• charge collection in the cluster 3x3 is 58
• the amplifier can be also used in schematics,
  where one need to save the space, cause it does
  not contain PMOS transistors (and hence PWELLs)

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Future development plans

• larger matrix and smaller pixel pitch size
• amplifier optimization to use it in combination
  with clamping readout circuitry
• 1 and 2 -gt chip submission in June2006 in
  collaboration with DAPNIA/SEDI (CEA/Saclay)
• test NMOS based high gain amplifier without
  feedback, different biasing -gt more test
  structures

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Acknowledgements
This development work would not be possible
without support of many people from CMOS sensors
group at IPHC W. Dulinski and M. Winter for
encouraging me to improve the amplifier for MAPS
and for the fruitful discussions, special thanks
to W. Dulinski for making the layout for my four
different amplifier designs of which actually
only two worked (well) CAD specialists - C.
Colledani, F. Guilloux, S. Heini, A. Himmi, Ch.
Hu, O. Robert, I. Valin, for their help with
Cadence, Data acquisition, test and measurements
specialists - G. Claus,M. Goffe, K. Jaaskelainen
and M. Szelezniak for providing me test setup and
acquisition software