GOSSIPO-2 chip: a prototype of read-out pixel array featuring high resolution TDC-per-pixel architecture. Vladimir Gromov, Ruud Kluit, Harry van der Graaf. NIKHEF, Amsterdam, The Netherlands. April 18, 2008.

Outline  Drift time measurements in the GOSSIP detector.  Time-to-Digital Conversion with local oscillator.  GOSSIPO-2 chip: history, structure and features.  Characterization and performance of the pixel array of the chip.  Conclusions and plans. V.GromovRD51 Workshop, 4/18/2008 2

GOSSIP detector: principles of operation. Cluster3 Cathode (drift) plane Integrated Grid (InGrid) Cluster2 Cluster1 Slimmed Silicon Readout chip Input pixel 1mm, 400V 50um, 400V 50um 3-D track reconstruction Cluster’s drift time measurements - low capacitance on the pixel ( down to 10 fF). - narrow drift gap (1 mm). - fast charge collection time (20 ns). - low diffusion of the primary electrons (70 um/1.6 ns) X Y Z Gas On Slimmed Silicon Pixel (GOSSIP): a detector combining a thin gas layer as signal generator with a CMOS readout pixel array. High resolution TDC V.Gromov RD51 Workshop, 4/18/2008 3

Time-to-Digital Conversion based on local oscillator. Start Stop Local oscillator Pixel_1 Hit Pixel_2 Clock Bus Clock signal source Hit signal Clock signal (period is T slow ) Local oscillator: output signal (period is T fast ) Drift time Measured time Start Stop The number of clocks (N fast ) at the output of the local oscillator gives the value of the drift time as follows: Tdrift = T slow - N fast ● T fast -Time resolution is determined by the frequency (T fast ) and performance of the local oscillator circuit. -The local oscillator is active only within restricted space of time. - Only “slow” Clock signal is being distributed across the chip. Low power consumption Out V.Gromov RD51 Workshop, 4/18/2008 4

V.Gromov RD51 Workshop, 4/18/ GOSSIPO-2 chip: history, structure and main features. History GOSSIPO-1 (0.13um CMOS): analog front-end: fast, low-noise (ENC=70 e - ) threshold =350 e -, low-power (2 uW per channel) GOSSIPO-2 (0.13um CMOS) : read-out pixel array. - sensitive area: 0.88 mm 2 (16 pixels x 16 pixels) - pixel size: 55 um. - high resolution TDC-per-pixel architecture (bin=1.8 ns). -separate TDC block. -separate local oscillator circuit. -analog monitor block.

Performance of the local oscillator circuit. NAND EN OUT Delay = T fast /2 = Function (Temp, Vdd) EN T fast (1.8 ns) OUT Power supply voltage, Volts Temperature, ◦ C T fast,ns - 12% / 100mV 2% / 10 ◦ C Channel-to-channel statistical spread is 4% Effect of the power supply voltage. Effect of the temperature. 0ns…T slow (25 ns) 0…15 (4-bit TDC) -If the power supply changes within 50 mV -If the temperature changes within 30 ◦ C Accumulated error will be kept within 6% (1.8 ns or 1 bin of the 4bit TDC). Schematic of the local oscillator. V.Gromov RD51 Workshop, 4/18/2008 6

Performance of the TDC block. Start Stop Local oscillator Hit Out In 4bit“slow” clock Counter TDC Control Out In 4bit“fast” clock Counter Out Clock Trigger Read Reset Hit signal Signal at the input of the “fast” clock counter (period is T fast = 1.8 ns) Start Stop Signal at the input of the “slow” clock counter (period is T slow = 25 ns). Trigger signal Stop N slow N fast DATA _FORMAT = 4bit (N slow ) + 4bit (N fast ) Clock - Time resolution is ( T fast ) is 1.8 ns. - Differential non-linearity is about 0.3 ● 1.8 ns. - Dynamic range is 4bit (T slow = 25 ns) is 350 ns. Data Output code, converted as N slow ●25 ns + N fast ●1.8 ns delay of the Hit signal, ns Transition region is about 30 ps! Discontinuity occurs when the Hit signal goes over the leading edge of the slow clock (“slow”) signal. Will be solved. Vdd=1.3 V Vdd=1.2 V Vdd=1.1 V TDC structure. V.Gromov RD51 Workshop, 4/18/2008 7

GOSSIPO-2 chip: charge-sensitive preamplifier. I b =1 nA V b2 V b1 Vdd=1.2 V Output Input C par ≈ 30 fF C fb = 1 fF Features. Low parasitic capacitance at the input (30fF): - low power consumption (2 µW per channel). - pulse response rise-time is 20 ns - low noise (σ n =70 e - ENC) threshold =350 e - - In order to confine time jitter to the TDC bin size (1.8 ns) the signal must be larger then 4000 e -. -The DC feedback circuit is not tolerant for variation of the fabrication process. This results in a number of not operational channels (15 %-30 %). This will be solved in the future Time, ns Threshold =350 e - Signal size = 2400 e - Signal size = 800 e - time jitter / internal delay ≈ 8 ns time jitter / internal delay ≈ 3 ns Time jitter / internal delay = Rise_time ● 5σ n / Signal size C in ∆V OPAMP V.Gromov RD51 Workshop, 4/18/2008 8

GOSSIPO-2 chip: voltage comparator. Features. The comparator is an OPAMP based on current mirrors architecture. the internal delay is inverse proportional to the size of the input signal (as usual). the minimum value of the internal delay (∆t min ) is much larger than one bin of the TDC (1.8 ns). ∆t min depends on the value of the tail current (I tail ) and therefore ∆t min is temperature dependent ∆t min is power supply voltage dependent ∆t min takes different values due to channel-to- channel mismatch ∆t min =15 ns Threshold =350 e - I tail = 0.2 uA Signal size, electrons Comparator delay I tail = 0.4 uA I tail = 0.8 uA Currently we are developing a faster comparator having internal delay close to 1.8 ns (one bin of the TDC) even at low tail currents (0.4 µA). Instability and spread of the value of ∆t min will be negligible. V thr Output Input I tail Vdd=1.2 V ∆t min =10 ns ∆t min =7 ns V.Gromov RD51 Workshop, 4/18/2008 9

GOSSIPO-2 chip: read-out pixel array. Functionality. -Only “Slow” clock (40 MHz) is being distributed across the array. -All 256 pixel will be read out serially when the Read (Trigger) signal arrives. - “Clear_ON” mode: data on the pixel will be RESET locally if the Read signal is not available within 350ns after the hit signal. - “Clear_OFF” mode: data on the pixel will be KEPT UNCHANGED even if the Read signal is not available within 350ns after the hit signal. -4 bit DAC for threshold tuning. - Control registers for masking and test-pulse enabling. Pixel_1 In TDC block Out Read (Trigger) Reset Clock Data Preamp Comp 4bit Threshold DAC Common threshold Data Local threshold Input pad Test pulse Pixel_2Pixel_16 Pixel_256 Front-end V.Gromov RD51 Workshop, 4/18/

Test set-up for the GOSSIPO-2 read-out pixel array. PCB with GOSSIPO-2 PCB with FPGA & USB port PC with LabView FPGA & software: Upload configuration data (threshold DACs and mask registers). Readout chip data. Read(trigger) signal and test pulse. GOSSIPO-2 FPGA DC source Pulse generator Common threshold power supply voltage USB GPIB test pulse Read (trigger) Clock (“slow”) generator 40MHz. Clock (“slow”) generator 40MHz. Clock V.Gromov RD51 Workshop, 4/18/

Time resolution as a function of threshold value. - Low threshold operation (350 e - ) in combination with large signal size (larger than 4000 e - ) will allow for high time resolution (jitter less than 1bin of the TDC = 1.8 ns). Threshold scan. Read (Trigger) signal ∆V(Q in ) Test pulse Measured time Measured time (converted output code), ns 140 time jitter ≈ 4bin ● 1.8 ns = 7.2 ns Threshold voltage, mV Threshold voltage, mV Threshold voltage, mV noise time jitter ≈ 2bin ● 1.8 ns = 3.6 ns time jitter ≈ 1bin ● 1.8 ns = 1.8 ns Signal size = 1200 e - Signal size = 3000 e - Signal size = e - In TDC block Out Read (Trigger) Reset Clock Data Front-end Threshold Test pulse Pixel cell structure. V.Gromov RD51 Workshop, 4/18/

Time scan. Read (Trigger) signal ∆V(Q in >4000e - ) Test pulse Measured time In TDC block Out Read (Trigger) Reset Clock Data Front-end Threshold (350e - ) Test pulse Pixel cell structure. Delay of the Test pulse, ns transition region ≈ 1 ns Measured time (converted output code) [ns] Measured time (converted output code ) as a function of the delay of the Test pulse. Large signal (> 4000 e - ) and low threshold operation (350 e - ). delay Distortion occurs when the Hit signal goes over the leading edge of the slow clock (“slow”) signal. - The complete read-out chain demonstrates good time resolution (transition region is 1ns) when the threshold is low (350e - ) and the input signal is large (>4000e - ). V.Gromov RD51 Workshop, 4/18/

Threshold equalization in the pixel array. 4bit Threshold DAC Common threshold voltage Local threshold voltage Configuration memory Serial Link Generation of the pixel’s threshold voltage. Common threshold Before equalization. After equalization. Common threshold Preamp output Preamp output Offset Pixel_1 - Threshold spread after equalization per pixel: 150 e -. Pixel number Threshold offset, mV Offset Pixel_2 Offset Pixel_3 600 Threshold offset, mV Not operational pixels (15%). These pixels should be masked Precision = 160 mV/16(4bit DAC) = 10 mV (150e - ) 160mV V.Gromov RD51 Workshop, 4/18/

Channel-to-channel spread of the measured time. Pixel_1 Output Code_1 Pixel_2 Pixel_256 Read Reset Clock Test pulse Pixel array. Distribution of the value of the measured time (converted output code ). Threshold equalization has been done. Large signal (> 4000 e - ) and low threshold operation (350 e - ). Output Code_2 Output Code_256 Measured time (converted output code), ns Entries time interval ≈ 4bin ● 1.8 ns = 7.2 ns -Due to uneven delay inside the read-out circuit the pixels will give different values for the measured time (converted output code). These values are spread across 4 bins of the TDC (7.2ns). - In the future we will solve this problem by means of using a faster comparator (internal delay close to 1.8 ns). V.Gromov RD51 Workshop, 4/18/

Conclusions and plans.  A small chip sensor array has successfully been prototyped in the GOSSIP0-2 chip in 0.13 μm CMOS technology.  The TDC per pixel with local oscillator satisfies the design requirements: low power consumption, high time resolution (1.8 ns bin) and simplicity.  The front-end circuit of the pixel’s readout will benefit from the low detector parasitic capacitance and no need to compensate for the leakage current.  Low threshold (350 e - ) and fast peaking time (20 ns) enable for high quality drift time measurements (jitters 1.8 ns) at large input signals (>4000 e - ) and after accurate threshold equalization. Plans:  The DC feedback in the preamplifier will be revised (matching).  A faster comparator circuit is required in order to reduce internal delay, spread and jitter.  A small-area detector will be constructed on top of the GOSSIPO-2 chip. This detector will be tested on the beam in the end of V.Gromov RD51 Workshop, 4/18/

Additional slide. Protection against discharges. Metal layer LM Metal layer TD Polymide Oxide High Resistive Amorphous Silicon 20um 3um Layout of the input pad R fb C p-sub ≈ 10fF Q discharge Output A C p-grid ≈ 0.5fF R prot ≈ 1MΩ - protective resistor causes neither signal distortion nor noise increase as long as R prot ●C p-grid is less than 1ns. V.Gromov RD51 Workshop, 4/18/

Additional slide. Integrated Grids. V.Gromov RD51 Workshop, 4/18/

RD51 Workshop, 4/18/2008 V.Gromov19 Additional slide. Time scan. Delay of the Test pulse, ns Measured time (converted output code) [ns] Measured time (converted output code) [ns] Threshold equalization has been done. Low threshold operation (350 e - ). Signal = 3000 e - Signal = 1200 e -