1. Ongoing R&D in Orsay/Saclay on ps time measurement: status of the USB-powered 2-channel 3.2GS/s 12-bit digitizer D.Breton & J.Maalmi (LAL Orsay), E.Delagnes (CEA/IRFU)
Séminaire DRT/LIST 08/09/08 SACLAY.

2. History of the Orsay/Saclay SCA Developments
Story begins in 1992 with the first prototype of the SCA for the ATLAS LARG calorimeter. The final numbers of this rad-hard circuit:
40 MHz sampling
13.6 bits Dynamic Range with simultaneous write/read
80000 chips produced in 2002 and mounted on the detector.
Since then, 3 new models of fast samplers have been designed (>30000 chips in use).
Design philosophy:
1. Maximize dynamic range and minimize signal distorsion.
2. Minimize need for calibrations and off-chip data corrections.
3. Minimize costs (both for development & production):
Use of inexpensive CMOS technologies (0.8µm then 0.35µm);
Use of packaged chips (cheap QFP).

3. Main Common design options (1):
4-switch memory cells:
Voltage-mode writing.
Floating voltage-mode reading with read amplifier.
Gain and pedestal spread insensitive to capacitor mismatches.
Sequencing of S1-S2 switch opening.
Sampling time very well defined and independent of signal amplitude.
Use of analog input buffer (voltage follower):
Keeps the real input impedance very high to avoid signal distortion
Penalty in power consumption and bandwidth.
N capas
Vout=A / (1+A) * Q/Cs
=V1 * A/(1+A) 3
Bottom Read BUS 4
Top Read Bus Cs 2 v
V1=V
Q=Cs.V1
N capacitors
Write Bus
Return Bus 1

4. Main Common design options (2):
Relatively high value of storage capacitance (200fF to 1pF):
minimize both kt/C and readout noise.
Use of differential channels:
Easier interface with modern commercial ADCs.
Low signal distortion
Noise rejection
Use of internally servo-controlled Delay Lines (DLL) to define the time steps:
No need for timing calibration.
Stability with temperature.
While on-chip phase detector and charge pump, fast setup time for the servo-control is possible: sampling DLL

5. The Sampling Matrix Structure: main features
Short DLL:
smaller jitter.
small timing non-linearity
1 servo control of Delay / Col => high stability.
Analog bus split in divisions (lines) => shorter analog bus
More uniform bandwidth.
Less analog delay along the bus.

6. The SAM (Swift Analog Memory) chip
2 differential channels
256 cells/channel
BW > 500 MHz
Sampling Freq 400MHz->3.2GHz
High Readout Speed > 16 MHz
Smart Read pointer (integrates a 1/Fs step TDC)
Few external signals
Many modes configurable by a serial link.
power on
AMS 0.35 µm => low cost for medium size prod
NIM A, Volume 567, Issue 1, p , 2006
6000 ASICs manufactured, tested and
delivered in Q2 2007

7. The USB_WaveCatcher prototype board
Power consumption < 2.5W
Reference
Clock: 200MHz => 3.2GS/s
Pulsers for reflectometry applications
1 GHz BW
amplifier.
µ USB
Trigger
input
2 analog
inputs. DC
coupled
Trigger
output
Trigger
discriminators
Dual 12-bit ADC
SAM Chip

8. Examples of acquisitions: no off-line correction
Channel 0
Channel 1
Channel 0
Channel 1
75 mV amplitude, 1ns FWHM pulse. 3.2GS/s
2ns FWHM consecutive pulses, separated by 22ns
, (300mV & 170mV amplitude). 3.2 GS/s
-3dB
530 MHz
Bode plot for
300mV pp Sinus

9. Fixed pattern jitter
DNL => modulo 16 pattern = 6.6 ps rms => ~0.5ps after correction.
After correction
DNL
After correction
INL
INL => modulo 16 pattern + slow pattern = 16 ps rms => ~1ps after correction.
Position correlated => can be corrected by software
Advantage of servo-controlled structure: very small dependence to time and temperature

10. Random jitter With random trigger: jitter floor ~ 2.2 ps rms
A little more jitter on “transition” samples (3.5 ps).
Understood: due to the clock jitter which can be seen only on the last cell of the DLLs
=> Mean jitter ~ 2.5 ps rms

11. Towards the cosmic telescope @ SLAC
We started designing a version of the board compatible with the MCPPMT equipped part of the SLAC cosmic telescope
It will house channels of analog memory and an USB 480Mbits/s interface
We will also develop the acquisition software with LabWindows/CVI
The time target is fall of this year

12. R&D on a ps TDC in IBM 130nm technology
We are collaborating to the design of a new TDC in the IBM 130nm technology
This is a collaboration between Orsay, Saclay, and the University of Chicago (with the help of Gary Varner).
The goal is to reach the ps precision thanks to the addition to an usual DLL based TDC of analog memories sampling at very high frequency (20GS/s).
Input clock frequency should be MHz
The design of the first prototype is well on tracks
It will include a complete measurement channel
Main elements are designed now, except the fast discriminator
In the near future, we aim at building a 16-channel chip with integrated output buffers.
These chips will be used at the output of fast PMT’s
We already have a SiPM test bench here at LAL driven by the “instrumentation” group
This bench will soon be extended to MCPPMT’s

13. Conclusion
We built a USB board to push the SAM chip towards its limits.
Timing measurements showed a timing resolution of ~16 ps rms without time INL correction, and a few ps after correction.
The board will be tested with MCPPMT’s.
There are still some points to understand (strange modulo 4 effect appears on time INL after power cycling: seems to come from FPGA …)
=> but the next version of the board is ready to leave.
Tests gave us new guidelines for future chips to improve timing performances.
We are now convinced that a single chip can’t be optimum for all applications (long depth vs time precision).
Next circuit will be submitted in July: 32 channels, 5GS/s sampling freq, larger BW (700MHz ?), same techno (0.35µm), larger depth (512 pts/ch)
Then an upgraded version of SAM will be designed
target = precision time measurement
We started designing a board for the cosmic telescope at SLAC
A new ps TDC using ultra-fast analog memories is under design.