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LHCb VELO Upgrade Strip Chip Option: Data Processing Algorithms

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### LHCb VELO Upgrade Strip Chip Option: Data Processing Algorithms

* AGH-Krakow

GiulioForcolin, Abdul Afandi,

Chris Parkes, Tomasz Szumlak*

- Part I: LCMS and/or MCMS?
- Part II: Effect of 6-bit ADC
- Part III: Zero suppression
- Part IV: Data Output Rate

Current VELO Data Processing

- Data digitized and processed on TELL1 board
- 10 bit ADC
- Processing algorithms currently used
- Pedestal Subtraction
- Common Mode Suppression (in units of 32 chan.)
- Strip Reordering
- Zero-suppression and Clusterisation

Planned Changes for Study

- Algorithms currently implemented on the TELL1 board for VELO will move to the SALT chip
- Reduce the ADC count from 10 bits to 6 bits
- Use a Common Mode Suppression (CMS) algorithm that uses all 128 channels in a chip
- Options are:
- MCMS, Mean Common Mode Suppression, find the average of a set of channels, and then subtracts this from the individual readings
- LCMS, Linear Common Mode Suppression, fit a line to a set of 128 readings and then subtract this line from the data
- To simplify the electronics, perform only Zero Suppression and no Clustering

Method

- Used algorithms based on the Vetra algorithms
- Performed algorithms in the order Pedestal Subtraction, MCMS, LCMS and Zero Suppression
- Used integers to simulate the limited number of bits available on the electronics; this was achieved by truncating all the numbers after the units.
- (i.e. equivalent to bit shifting rather than rounding)

Data Samples

No beam data from 2012 (Run 113600)

Beam collision data from mid/late 2011 (Run 98228, μav= 1.39)

Part I: LCMS and/or MCMS?

- Studies with 10-bit ADC
- Using data taken with no beam

Raw Data

- For the first step, data with no hits was studied

Pedestal Subtraction

- Pedestals calculated as the average of the readings of each channel
- Then subtracted from the data
- Due to integerization, the distribution was not centred around 0
- Fix by adding 0.5 to the average pedestal value, and 0.5 t during calculation,

Noise after MCMS and LCMS

- MCMS has a significant effect, simple algorithm
- LCMS is not a great improvement, (5% improvement in noise)
- It is a lot more processor intensive than MCMS
- Conclude – current data suggests only need to perform MCMS, and future proof should CM increase
- (caveat: new VELO will move closer to beam, but no sign of significant beam pickup )

Part II: Effect of 6-bit ADC

- Rescaling from 10-bit ADC to 6-bit ADC
- But making better use of dynamic range
- Using no beam data

Data Scaling

- Data was scaled to make it compatible with limiting ADC counts to 6 bits
- The aim was to allow a signal of two minimum ionising particle (equivalent to ~80 ADC counts above the noise in the current data)
- [Achieved by subtracting 480 counts from the raw data, and dividing what was left by 3]
- Algorithms were then carried out as before (with truncation to 6 bits)

Results with 6-bit ADC

Use of 6-bit ADC increases effective noise by 35%

Can counteract to measure noise by rescaling ADC

10 bit processing noise = 2.38 [10 bit ADC]

6 bit processing noise = 1.13 [6 bit ADC]

= 3.39 [10 bit ADC]

10 bit processing noise = 1.99 [10 bit ADC]

6 bit processing noise = 0.88 [6 bit ADC]

=2.64 [10 bit ADC]

[Achieved by subtracting 480 counts from the raw data, and dividing what was left by 2]

10 bit processing noise = 2.38 [10 bit ADC]

6 bit processing noise = 1.63 [6 bit ADC]

= 3.26 [10 bit ADC]

10 bit processing noise = 1.99 [10 bit ADC]

6 bit processing noise = 1.14 [6 bit ADC]

=2.28 [10 bit ADC]

- Rescaling the data by a factor of 2 increases effective noise by 15%, however it would not be possible read a 2 MIP signal using this scaling, would need 7-bit ADC.
- Using 8-bit ADC would remove the need for scaling and still save 2 bits

Part III: Zero suppression

- Using one simple threshold to zero-suppress
- Using late 2011 data (after radiation damage)

Zero Suppression (ZS)

- To simplify the design of the electronics it has been decided that they (if possible) should only perform Zero Suppression
- Clustering would then be performed on TELL40
- This means that no reordering needs to be performed at this stage
- i.e. even were it needed it could be done on the FPGA
- Zero Suppression performed by only reading out signals above a certain threshold
- E.g. look for reading 4σ above the noise level

Zero Suppression (ZS)

- Setting threshold of 4σ excludes almost all of the data for the no-beam data set
- Integerization causes there to be a difference in the number of readings between the two sets of data

Zero Suppression (ZS)

- Threshold set at 10 [10-bit ADC counts] or 3 [6-bit ADC counts]
- It appears a lot of “noise” is outputted even if the threshold is 4σ above noise level
- This “noise” is not present in no beam data, probably due to radiation damage second metal effect
- In 6-bit ADC integerization is significant (10 in 10-bit would be 3.3 in 6-bit) and can lead to significant increase in noise

10-bit ADC

6-bit ADC

VELO R Sensor

Chris Parkes

VELO R Sensor 2nd Metal Layers

Chris Parkes

Part IV: Data Output Rate

- Studies of data size given different protocols for data output

Data output rate

- Have 27-28 bits that are sent out once for each event
- Channel ID and ADC count sent out for each channel with a reading above the threshold
- Number of channels that can output data limited by payload size (above this saturate)

Data output Rate

3 Options to output data

- Output Bunch ID, Sensor ID and Chip ID for each bunch crossing while the Payload Size, is sent out only when at least 1 hit is present
- Output Bunch ID, Sensor ID, Chip ID and Payload Size for each bunch crossing
- Output Bunch ID, Sensor ID, Chip ID and Payload Size only when at least 1 hit is present

For each of these options can have a payload size of either 3 or 4 bits;

therefore the number of readings that can be outputted is limited to

- 8 or 16
- 7 or 15
- 8 or 16

Data output Rate

- Assume a bunch crossing every 25 ns
- Assume that there is a certain occupancy rate, P, for each channel to register a hit
- For a probability of P = 1%, expect 5.12x107 readings per chip per second
- For a probability of P = 2%, expect 1.02x108 readings per chip per second
- Use a Poisson distribution to calculate the probability that each chip records a certain number of hits
- Can therefore calculate the expected data output rate for each chip

Data Output Rate

For a single chip

Observations

- If payload size is limited to 3 bits, expect to lose up to 0.26% of readings if the P ≤ 2%, however the data rate is also reduced by ~1.5% compared with 4 bits
- For P = 1%, the data rate is reduced by ~15% when sending headers only when data is present (option 3), however for P = 2%, it is only reduced by ~3%
- Option 2 increases data rate by ~2%, however should be easier to implement.
- Conclude:
- 1) Need detailed knowledge of occupancy rate to make decision on whether send headers out is significant
- 2) 3-bit for payload size seems sufficient for 2% occupancy

Conclusions

- MCMS needed, but not LCMS
- 6-bit ADC, integerization causes 25% increase in measured noise
- Zero-suppression in 6-bits
- Needs further study. 2nd metal layer effect could cause a problem if at upgrade.
- Data rates: 3 bit payload size, rate at 2% occupancy ~ 0.27Gbytes s-1 per chip

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