CLICpix2 Design Status

Pierpaolo Valerio & Edinei Santin

CLICdp Vertex Meeting - September 24th 2015

Digital Design Part

Pierpaolo Valerio

Digital Signals Delay Maximum delay of data signal from the pixel matrix

measured from the edge of the input clock is: ~3.4 ns if VDD = 1.0V ~1.6 ns if VDD = 1.2V

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Corner choice This is true for the Worst corner, which is very pessimistic:

Slow/Slow process, 125 °C, -10% VDD If the delay is bigger than a clock period, it becomes

impossible to implement a fully synchronous readout In CLICpix1 it worked because the columns were “self-

managing” the readout algorithm. It can’t work in CLICpix2

The solution is to characterize the design at 1.2V (with a < 20% increase in digital power consumption) and then reduce the voltage during the tests as the chips won’t have to work at the worst corner

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Ongoing implementation

The end-of-column block is now implemented and it correctly meets the timing specifications

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Path 1: MET Setup Check with Pin u/out_reg/CP Endpoint: u/out_reg/D (^) checked with leading edge of 'clkin_divided'Beginpoint: datain_column (^) triggered by leading edge of 'clkout'Analysis View: av_maxOther End Arrival Time 0.325- Setup 0.080+ Phase Shift 3.125+ CPPR Adjustment 0.001- Uncertainty 0.150= Required Time 3.221- Arrival Time 2.132= Slack Time 1.089

Output Serializer The Double Data Rate serializer was implemented,

allowing for 640 Mbit/s data output

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Ethernet IDLE sequence

Status of the digital design

Other improvements: Large code cleanup All reset signals are now synchronous, to avoid possible

glitches Reorganization of control registers in more logical groups:

Readout control Global configuration

To do: Place and route the rest of the periphery Validate the timing analysis with extracted simulations

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Analog Design Part

Edinei Santin

Outline (cf. DR checklist)

Periphery buffers stability, DC gain, and offset voltage Test pulse circuitry simulation with switching overlapping

and varying pixels load Cascode current mirrors at periphery DACs outputs Bandgap IP integration (preliminary) Resolution adjustment for Ikrum and one of the test pulse

DACs

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Biasing/reference lines capacitive loading

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Signal

Cgg,devices [pF]

Crouting* [pF] Total

[pF]per pixel matrix per 2-column matrix

Vbpcsa 0.0134 219.55 1.36 88.40 307.95

Vcpcsa 0.0011 18.02 1.36 88.40 106.42

Vbpikrum 0.0797 1305.80 1.85 120.25 1426.05

Vfbk 0.0239 391.58 2.33 151.45 543.03

Vt1 0.0112 183.50 1.31 85.15 268.65

Vt2 0.0112 183.50 1.12 72.80 256.30

Vbncomp 0.0316 517.73 1.23 79.95 597.68

Vbpcomp 0.0225 368.64 1.46 94.90 463.54

Vth 0.0244 399.77 0.91 59.15 458.92

Vbpcaldac 0.0737 1207.50 1.51 98.15 1305.65

Vcpcaldac 0.0228 373.56 1.61 104.65 478.21

Vcncaldac 0.0071 116.33 1.67 108.55 224.88* Periphery routing excluded.

Biasing/reference lines leakage current

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SignalIleak*

per pixel (nom / max**) [pA] matrix [nA]

Vbpcsa 5.35 / 28.50 87.7 / 466.9

Vcpcsa 1.87 / 140.00 30.6 / 2293.8

Vbpikrum 4.35 / 247.50 71.3 / 4055.0

Vfbk 1.86 / 225.00 30.5 / 3686.4

Vt1 ~ 0 / 0 ~ 0 / 0

Vt2 ~ 0 / 0 ~ 0 / 0

Vbncomp 6.13 / 1312.50 100.4 / 21504.0

Vbpcomp 2.79 / 62.50 45.7 / 1024.0

Vth 2.93 / 1715.00 48.0 / 28098.6

Vbpcaldac 1.77 / 312.50 29.0 / 5120.0

Vcpcaldac 3.70 / 125.00 60.6 / 2048.0

Vcncaldac 3.03 / 1050.00 49.6 / 17203.2 * Strongly dependent on biasing.** ‘max’ means Ileak,den∙Agate for the worst (but unrealistic) bias condition (i.e. |Vg - Vd,s,b| = Vdd).

Biasing/reference lines model

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Approximated model for the biasing/reference lines including parasitic RC elements and leakage current The capacitance and leakage current vary from line to line since they depend on the devices connected

to the respective lines. The resistance does not vary much because the lines are equally sized (R route,2col ~ 181.6 Ω).

The periphery routing (Rs & Rup) has an important impact on the buffers response

Accounts for bias dependence

Buffer stability vs CL & Rs/Rup

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The resistance Rs has a strong influence on the stability of the buffers. It creates a left-half plane (LHP) zero at wz = 1/(RsCL) which improves the phase margin as the zero is pushed to lower frequencies.

Approximately the same holds for Rup, but to avoid different potential through the columns this resistance needs to be minimized (i.e. it cannot be unrestricted sized up). Hence, the main parameter to set wz becomes Rs.

Buffer stability vs CL & Rs at # Ileak

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The leakage current has a very minor impact on the phase margin, even considering that the nominal I leak is increased by a factor of 10 to account for bias dependence

Buffer DC gain & fu

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As expected, the DC gain is constant and ~ 64 dB. The unit-gain frequency, fu, has a minimum value of ~ 600 kHz and a maximum ~ 16 MHz for varying CL and Rs values.

Buffer offset voltage

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For nominal conditions, the buffer has a systematic offset voltage of ~ 1.8 mV and a sigma of ~ 1.2 mV

Buffer summary

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It was kept the same topology of the previous design, i.e., a two-stage Miller compensated amplifier

Same buffer sizing for all biasing/reference lines. Good phase margins (> 45°) achieved by controlling the Rs values via the periphery routing.

Considering a worst leakage current of 1 µA, the voltage drop due to Rs is ~ 1 mV. However, this can be seen as offset, since it affects all the columns equally.

Test pulses with tpsw/tpswn overlapping

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States 2 and 4 may be problematic if RonN1,2 and/or RonP1,2 are too small and td is sufficiently large

Interestingly, if Vt1,2 < Vdd/2, the critical state is 2, since in this case RonN << RonP. Conversely, if Vt1,2 >

Vdd/2, the critical state is 4, since now RonP << RonN. Hence, if really needed, we can choose the least

critical “transition” to inject the test pulses, and swap the magnitudes of Vt1 and Vt2 to achieve the

desired test pulse polarity.

Test pulses with varying td and Vt1,2

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Test pulses are simultaneously applied to 4096 pixels and also the RC model of the lines (Ru = 1.5Ω, Rup = 6Ω, Rs = 1kΩ, Cu = 1.3pF/128) is accounted for the loading of the two test pulse buffers

Slightly differences in the test pulses happen only for sufficiently large td and at the critical (worst) transitions (set by the Vt1,2 magnitudes)

For typical td values (one INV gate delay ~ 200 ps), the switching overlapping is not a problem at all

Vt1,2 < Vdd/2 transition 2 critical Vt1,2 ~ Vdd/2 no critical transition Vt1,2 > Vdd/2 transition 4 critical

Test pulses for varying pixels load

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Ideally, applying a ΔVt of 80 mV in the test capacitor, Ct = 10 fF, would inject an input charge of 5ke- Increasing the load (i.e. the number of pixels being driven by the buffers) distort the applied test

pulses. Up to ~ 512 pixels, however, the distortion is minimal. For relatively small number of pixels, the input charge injected by the test pulse circuit correlates

very well with an equivalent charge injected by a current pulse

DAC cascoded output

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Added NMOS/PMOS cascode current mirrors at DAC output to improve the output voltage, Vo, linearity

The cascode voltages, Vcn and Vcp, are provided by same periphery DACs that set the cascode voltages of the on-pixel calibration DACs (devices sized to have approx. the same ID/(W/L))

DAC buffered output linearity

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The buffered DAC output has a best-fit INL better than 0.5 LSB for a range from approximately 0.26 to 0.80 V, i.e., ~0.54 V

At the lower end the linearity is limited by the buffer, and at the higher end it is limited by the combination of the buffer and the current mirror

Segmented DAC cascoded output

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For the segmented DAC, cascode current mirrors are added to both the LSB and MSB DACs

The current mirrored by the MSB DAC is 9x larger than the one mirrored by the LSB DAC. The two currents are summed up and generate an output voltage, Vo, across an output resistor.

Segmented DAC buffered output linearity

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For the segmented DAC, the output linearity of the LSB DAC is of most importance For a set of transfer characteristics of the LSB DAC covering the most linear region

of the MSB DAC, the INL is larger than 0.5 LSB only for some LSB DAC codes. However, the INL may be improved using the middle range of LSB DAC codes where the LSB DAC is more linear.

best fit from 32 to 224 LSB DAC codes

Bandgap

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Bandgap IP block provided by Stefano Michelis (CERN) Layout area ~400 x 300 µm2

Bandgap voltage Vbg ~300 mV Power dissipation ~60 µW A slightly different bandgap (w/ diodes instead of DTNMOS) is under characterization, and

depending on the results Stefano will provide us that block for integration

Iref generation

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Rbg implemented with unsalicided N+ poly resistor (TC1 = 150 ppm/°C) Bandgap programmable by 3 bits plus 1 bit for the output multiplexer control Two circuits to generate Iref in case the bandgap does not work as expected

Vbg variation over PVT

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The worst relative variation of Vbg over a temperature range of -20 to 100 °C is ~1.5% Vbg is almost insensitive to Vdd variations but quite dependent on the process corners

(maximum variation of ~6% over corners)

Vbg variation over R2

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The programmable value of R2 can be used to adjust the absolute value of Vbg by about ±10% around the nominal value of Vbg = 300 mV

Iref variation over PVT

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The worst relative variation of Iref over a temperature range of -20 to 100 °C is ~1.5% Iref has a significant variation over process corners (about ±20%) which can be partially compensated by R2. The

variation of Iref over Vdd is negligible. For very low temperatures (< 0°C), “ff” process corner, and Vdd = 1.32V, the Iref generation performance is being

limited by the OTA used to set Vbg across Rbg. However, the chip is unlikely to operate at these low temperatures.

Periphery DACs dynamic ranges

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Signal DAC output (Iop) M:N / R Resulting range Nominal value DAC code

Vbpcsa 0 – 12.75 µA 3:1 0 – 4.25 µA 1.50 µA 90

Vbpcsaoff 0 – 12.75 µA 300:1 0 – 42.5 nA 15 nA 90

Vcpcsa 0 – 12.75 µA 1:9 / 10 kΩ 0.05 – 1.2 V 601.5 mV 133

Vbpikrum 0 – 12.75 µA 500:1 0 – 25.5 nA 8 nA 80

Vfbk 0 – 12.75 µA 1:9 / 10 kΩ 0.05 – 1.2 V 601.5 mV 133

Vt1 0 – 12.75 µA 1:9 / 10 kΩ 0.05 – 1.2 V 601.5 mV 133

Vbncomp 0 – 12.75 µA 1:1 0 – 12.75 µA 1.50 µA 30

Vbncompoff 0 – 12.75 µA 100:1 0 – 127.5 nA 15 nA 30

Vbpcomp 0 – 12.75 µA 1:1 0 – 12.75 µA 2.50 µA 50

Vbpcompoff 0 – 12.75 µA 100:1 0 – 127.5 nA 25 nA 50

Vth,Vt2 0 – 12.75 µA 1:1 / 1:9 / 10 kΩ 0.05 – 1.2 V 579.0 mV 0 / 138

Vbpcaldac 0 – 12.75 µA 64:1 0 – 200 nA 50 nA 64

Vcpcaldac 0 – 12.75 µA 1:9 / 10 kΩ 0.05 – 1.2 V 601.5 mV 133

Vcncaldac 0 – 12.75 µA 1:9 / 10 kΩ 0.05 – 1.2 V 601.5 mV 133

Vbpbuf1 0.8 – 12.0 µA 1:1 0.8 – 12.0 µA 6.4 µA 136

Vbpbuf2 0.8 – 12.0 µA 1:2 1.6 – 24.0 µA 12.8 µA

To-do list

Finalize the integration the bandgap IP block (waiting Stefano feedback)

Final chip assembly and verification Chip tape-out planned for end Oct./2015

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Thank you!

Backup Slides

Buffer stability vs CL & Rs at # conn. points

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Connecting the buffered lines close to the 1st (or end) double column demands slightly less Rs to achieve the same buffer phase margin, since the contribution of Rup adds up saliently

Rs connected at middle column

Rs connected at 1st column