Designing fast analog memories
for precise time measurement
D.Breton (LAL Orsay)
D. Breton – ps Workshop– Chicago– April 2011
Introduction
• The technologies have evolved at an amazing rate in the last decade,
and during this period our electronics engineer everyday work has been
transformed consequently
• Sometimes we have to build systems housing only few channels, but
more often a lot, thus pushing us to look for the best solution to perform
as precise as possible signal measurements at the lowest cost and
power consumption, in order to be able to build large scale systems
• Our consecutive developments thus made us use high-end commercial
ADCs and TDCs as well as developing high performance analog
memories
• Now we deal with GS/s, which actually permits getting closer to ps
=> we can build high-end TDCs using waveform digitization …
D. Breton – ps Workshop– Chicago– April 2011
About TDCs …
•
Existing electronics for time measurement is mostly based on Time
to Digital Converters (TDC).
A TDC converts the arrival time of a binary signal into digital value.
It is characterized by :
–
–
–
–
•
Its time step and its main clock frequency
Its effective resolution (which can be very different from time step)
Its dead-time and its mean maximum hit rate
Its number of channels
Very few high-end products on the market, mostly dedicated
to LHC
–
HPTDC from CERN => 25ps & 40MHz
–
TDC-GPX from ACAM => 8 channels, 80ps & 40MHz
–
There is an important demand for time of flight
measurement in the medical community, and now for ps
precision in high energy physics
D. Breton – ps Workshop– Chicago– April 2011
State of the art for TDCs
•
TDC with voltage ramp:
=> best solution for precision
– Time resolution: ~ 10 ps
– Usually used with a Wilkinson ADC for power and
simplicity reasons
=> limited by dead time which can be a problem for
high rate experiments
•
TDC with digital counters and Delay Line Loops
(DLL):
=> advantage: produces directly the encoded
digital value but limited by delay line step
- Time resolution of today’s most advanced ASICs: ~
25 ps
BUT a TDC needs a binary input signal
 analog input signal has to be translated to digital with a discriminator
 overall timing resolution is given by the quadratic sum of the discrimator and TDC
timing resolutions
D. Breton – ps Workshop– Chicago– April 2011
New TDC developments
•
There are very few new developments in the high precision
TDC area
–
–
•
University of Alberta worked on using the HPTDC at 80MHz and
achieved 15ps of resolution using custom CFDs
CERN envisaged a new DLL-based TDC targetting 15 ps of
resolution
Anyhow, the standard DLL-based option is limited by the time
propagation step in the DLL and the fact that one cannot
interpolate this information
–
–
–
–
Usual trick for improving the precision: interleaving DLLs fed
with phase shifted signals
Only way to further increase the precision is to run for smaller
technologies
But single delay step will hardly go below a few 10 ps
And if one wants to keep reasonable clock frequencies in the
chips (used for rough time counters), DLLs will remain long =>
bad for precision
D. Breton – ps Workshop– Chicago– April 2011
ADC and TDC
• The best way to precisely measure the arrival time of a signal is to
digitize it with a very fast and precise ADC.
• Indeed, once data is digitized, one can perform a digital treatment of
data to precisely extract time information
• Waveform contains all information (if properly digitized)
• Depending on the information requested (amplitude, charge, time, FFT, …),
different types of algorithms can be used
• Goal is to find the both simplest and most effective algorithms which can be
integrated within companion FPGAs
 Waveform sampling can be used for designing high performance
TDCs
 It was shown that 5-10GS/s sampling rate together with a good SNR
(>10bits) was leading way to the ps level
• Signal to noise ratio is always an issue, even for a TDC
D. Breton – ps Workshop– Chicago– April 2011
Extracting the time from the signal: CFD
• The easiest way to extract the time information from a digitized signal is to perform a digital
Constant Fraction Discrimination (CFD)
– Indeed, a simple threshold method introduces Time Walk which depends on the signal
amplitude
A1
V
V
A2
A3
k x A1
Fixed threshold
relative threshold :
constant fraction
of the peak!
k x A2
k x A3
t
t
Δt ~ 0
Δt : time walk
 in order to remove the time walk, threshold has to be set as a constant fraction of the signal
amplitude
 Algorithm can be as simple as looking for the closest sample to the peak and performing a
linear interpolation between samples around the threshold => easy to implement within a
FPGA.
 More complex algorithms based on multi-sample digital filtering may improve the resolution
D. Breton – ps Workshop– Chicago– April 2011
Jitter induced by electronics noise
Simplified approach
Noise
slope = 2ЛAf3db
Zoom
Time
tr ~ 1/(3 f3db)
Jitter
Time
Jitter [ps] ~ Noise[mV] / Signal Slope [mV/ps] ~ tr / SNR
Ex: the slope of a 100mV - 500MHz sinewave gets a jitter of ~2ps rms from a noise of 0.6mV rms
Conclusions:
 The higher the SNR, the better for the measurement
 A higher bandwidth favours a higher precision (goes with its square root).
 But: for a given signal, it is necessary to adapt the bandwidth of the
measurement system to that of the signal in order to keep the noise-correlated
jitter as low as possible
 Designs become tricky for ultra fast signals with a bandwidth > 1GHz …
D. Breton – ps Workshop– Chicago– April 2011
ADC-based TDC
• The best digitizer would be a low-power flash ADC running at a
rate >> GS/s over a lot of bits (12)
– Because every single sample would have followed the same path:
• Sampled by the physical same clock (thus with an excellent jitter performance)
• Digitized by the same elements => thus avoiding any dispersion due to layout
non-uniformities
1. This doesn’t exist
=> very fast ADCs are based on a bank of parallel ADCs
=> they require an internal complex calibration
=> they consume quite a lot of power
2. The output dataflow of these circuits makes them very difficult to use
• The most powerful products on the market:
•
•
•
•
8bits => 3GS/s, 1,9 W => 24Gbits/s,
10 bits => 3GS/s, 3,6 W => 30Gbits/s
12 bits => 3,6GS/s, 4,1 W => 43,2Gbits/s
14 bits => 400MS/s, 2,5 W => 5,6Gbits/s
D. Breton – ps Workshop– Chicago– April 2011
An actual 12 bits and 3.6GS/s …
BGA
292 pins
Output links:
24 x1,8Gbits/s
1.8 GHz !
Need of a VERY high-end FPGA
 power, cost, board design complexity, …
 and what about radiation if any ?
D. Breton – ps Workshop– Chicago– April 2011
Why Analog Memories ?
• Analog memories actually look like perfect candidates for high
precision time measurements at high scale:
– Like ADCs they catch the signal waveform (this can also be very
useful for debug)
– There is no need for precise discriminators
– TDC is built-in (position in the memory gives the time)
– Only the useful information is digitized (vs ADCs) => low power
– Any type of digital processing can be used
– Only a few samples/hit can be read => this may limit the dead time
– Simultaneous write/read operation is feasible, which may further
reduces the dead time if necessary
– Main difficulty is less sampling frequency than signal bandwidth
• But their design is tricky if one wants to reach the necessary level
of performance.
D. Breton – ps Workshop– Chicago– April 2011
Basic principles of circular analog memory
• A write pulse is running along a folded delay line.
• Sampling stops upon trigger.
• Readout can target an area of interest:
– Starting from Trigger cell (marked during signal
recording) - programmable offset (linked to latency).
• Total readout can however be necessary.
• Read Cell index necessary.
• Dead time due to readout has to remain as small as
possible (<100ns / sample).
D. Breton – ps Workshop– Chicago– April 2011
Main design options
• The choices of architectures are different depending on the
recording options.
• Main options:
– Pure waveform recording (can cover a long time period)
=> can be used for rough time measurement.
– Precise time measurements.
• It looks like both are not achievable with the same designs
– The ps level relies on the quality of the sampling then in the capacity
to easily correct the remaining fixed sampling errors
– Of course the better original sampling, the easier
– The calibration has to remain reasonable, as well as necessary
correction of data
D. Breton – ps Workshop– Chicago– April 2011
Different types of implementation
SAM
D. Breton – ps Workshop– Chicago– April 2011
Targetting the ps …
• As said before, a perfect digitizer would be for instance a 10GS/s
12-bit flash ADC.
– But 10GS/s sampling doesn’t give enough time to work on the
sampled signal (100ps for sampling + transfer )
– Moreover, the 120Gbits/s output data rate could make anybody
nauseous …
 the idea is to keep the high sampling rate and to increase the time
for processing the samples by parallelizing the operation via the
Sampling Delay Line
 There will be as many sample and hold cells as delays in the
line
 This increases the time allowed for sampling the signal (write
pulse can cover many consecutive delay cells)
D. Breton – ps Workshop– Chicago– April 2011
Basic scheme
Pros: very simple
and easy to
implement scheme
Cons: time
propagation is not
servo controlled =>
individual delays
have to be
calibrated and may
vary with time and
temperature
D. Breton – ps Workshop– Chicago– April 2011
Time Non_Linearities
• Dispersion of single delays => time DNL
• Cumulative effect => time INL. Gets worse with delay line length.
• Systematic & fixed effect => non equidistant samples => Time Base Distortion
If we can measure it => we can correct it !
But calibration and even more correction have to remain “reasonable”.
Δt[cell]
Real signal
Fake signal
After interpolation
In a Matrix
system, DNL is
mainly due to
signal splitting
into lines =>
modulo 16 pattern
if 16 lines
Remark:
same type of
problem
occurs
with interleaved
ADCs
D. Breton – ps Workshop– Chicago– April 2011
Improved scheme
Here, time propagation is
servo controlled
=> individual delays are
much more precise and
should not vary with time.
However, if the Delay
Line is too long, integral
non linearity may be
important
D. Breton – ps Workshop– Chicago– April 2011
How to use it ?
Here, output rate =
input rate => this
actually is an ADC !
Actually, most ADCs
integrated in highend oscilloscopes’
front-end work that
way !
Fclock/Nb of delays
D. Breton – ps Workshop– Chicago– April 2011
Dual-stage analog memory
Here, output rate =
input rate only for
the first stage
Difficulty is to copy
the sampled signal
very properly
between the two
stages
Short first stage is
fixing the time
precision
performance but
transfer may
damage the
information
D. Breton – ps Workshop– Chicago– April 2011
Towards a ps TDC …
• In order to build a real TDC targetting the ps level, adding an analog
memory to a usual DLL TDC permits relieving the walk constraint on the
discriminator and improving the time precision by an order of magnitude
• Here the Delay Line is servo-controlled and can be as short as the
signal to measure => very good time resolution can be envisaged
Critical path
for time
measurement
D. Breton – ps Workshop– Chicago– April 2011
Remarks
• The technology choice depends on the design.
– It might be good not to chose too complex or expensive technology if this is
not necessary
– Following the trends might be risky
– There are many cheap and good “old” technologies
• An important element is the channel occupancy
– The architecture has to be adapted to the required channel hit rate
– Waveform sampling means that a certain amount of cells has to be
readout for each hit reconstruction
=> Intrinsic deadtime
– If the readout scheme becomes too difficult to implement, reducing
the pixel size and increasing the number of channels might be a
solution
– But then the overall number of channels will follow
=> try to increase the number of channels within one chip
D. Breton – ps Workshop– Chicago– April 2011
Remarks (2)
• The analog memory could also be used as a first level
derandomizer
– Either individual cells or banks of cells could be frozen waiting for
readout while sampling goes on
– This implies a simultaneous write/read operation in the memory
 potential source of crosstalk, noise, distorsion …
 critical design
 but if the derandomizer depth is sufficient, the memory could
get close to zero deadtime
• When the ps is the goal, the clock distribution becomes a
problem by itself, especially for high scale systems
• The board design also becomes really difficult, mixing high
bandwidth analog front-end and fast digital logics
D. Breton – ps Workshop– Chicago– April 2011
Memory-associated ADC
• Sampled analog waveform has to be digitized
– ADC can be internal to the chip, or external
– External:
• The simplest solution: multiplexing all the samples towards a single ADC
– Cheap but induces a relatively high dead-time
• The most effective in terms of dead-time: driving many ADCs in parallel
(can be multi-channel ADCs)
– Internal:
• The simplest solution: the Wilkinson ADC
• Can be parrallelized (ramp and counter can be shared)
• Can be partly inside (ramp and comparator) and outside the chip
(counter located in the companion FPGA)
• Counter can be embedded in each cell
• Smart implementations highly increasing the speed can be used (WILKY)
• The main problem with internal ADC: extracting rapidly the digital data
=> this could really become problematic for dense circuits
D. Breton – ps Workshop– Chicago– April 2011
Advanced integrated ADC design: WILKY
New ADC development based on boosted Wilkinson architecture.
 Use of DLL-based digital TDC techniques to measure the time in a
ramp ADC (patented in 2005)
 Equivalent to a Wilkinson ADC with a 3.2 GHz clock.
 4-channel prototype validated in 2006 in CMOS 0.35µm technology
 Easily expendable to 64 channels or more

Raw Time: counter
Fine Time: DLL
D. Breton – ps Workshop– Chicago– April 2011
Summary
• Waveform sampling can be used for designing high performance
TDCs
– ADCs would do the job nicely but at least 99% of data would go to
the bin at owner’s expense! (power, FPGA, …)
• Analog memories actually look like perfect candidates for high
precision time measurement at high scale and reasonable trigger
rates:
– Design has to be adapted for such a requirement (short servocontrolled sampling delay lines)
– Not only high sampling precision but high SNR is mandatory
• 1GHz of bandwidth currently looks like a technical boundary if one
wants to keep a low signal distorsion
– But this should already permit reaching the ps level
D. Breton – ps Workshop– Chicago– April 2011