Impact of AC-coupling on the SSD performance.
Report given at ITS meeting at CERN on 5.12.2001.
Vladimir Gromov()
Department of electronics,
NIKHEF, Amsterdam, the Netherlands.
. due to use of double-sided structure, the front-end electronics of SSD on both sides operate at different potentials (detector bias 55V)
. the front-end chips of the detector modules are readout in series, both the P- and N side of each detector module to one ADC channel with 10MHZ rate.
. AC-coupling is the only way to provide the signal transmission. However it results in signal distortion inasmuch as the signal gets differentiated according to
F(p)=p/(1+p), where =C 1.2k
. the coupling capacitor value should be kept as low as possible for safety and space reasons. On the other hand the smaller the capacitor is the greater signal distortion becomes. That leads to the detector performance deterioration.
What is the problem with AC-coupling?
AC-coupling in front of analog buffer changes the shape of the signal coming out of the front-end chip (HAL25).
Distortion of the hit charge could leads to wrong position information,
Capacitor is bigger.
AC-coupling causes extra inefficiency.
Method is Monte-Karlo simulations.
Items taken into account:
1. number of channels (strips) to be read out at every TRIGGER coming in 6*128=768
2. number of hits for a TRIGGER is Poisson statistics with average of occupancy*768
3. charge deposit to the detector by a hit is distributed according to Landau with most probable value (MIP) of 22000e (41.8)
4. threshold is set at 5*400e(ENC)=2000e (3.7).
5. noise contribution to a signal variation is not taken into account.
6. pitch (strip-to-strip distance) is 100um.
7. .deposit charge is eaten by one strip if an hit interaction point is within 30um from center of the strip (digital zone).
8. deposit charge is shared between two neighbouring strips if an hit interaction point is within the area from 30um to 70um from center of a strip ( analog zone).
9. AC coupling is simulated with response of differentiation circuit
where t - current time, - circuit time constant, - delay time, (t)=1, if t>0, (t)=0, if t<0.
10. the signal is sampled with 90ns delay in respect to the leading edge of it.
11. position of the hit is determined by central gravity method.
Example.
Results: Position resolution distortion due to AC-coupling.
Runs=100. Occupancy=5%.
Results: Position resolution distortion due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
Results: Amplitude distribution distortion due to AC-coupling.
Runs=100. Occupancy=5%.
Results: Amplitude distribution due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
Results: Base line fluctuation due to AC-coupling.
Runs=100. Occupancy=5%.
Results: Base line fluctuation due to AC-coupling.
Runs=100. Occupancy=5%, 7.5%, 10%.
Conclusion.
1. Hit position information is almost not distorted by AC-coupling. Position resolution remains almost the same when the coupling capacitors are in range down to 150pf (=150ns) used even under relatively high occupancies of 10%.
2. Amplitude information has been heavily distorted when AC circuit with small time constant is used (below 1ms, capacitor is 1000nF). No considerable amplitude distortion can be noticed even under 10% occupancy if capacitor of 1000nF is used.
3. AC-coupling causes base line fluctuation and hence extra noise.
If the coupling capacitor is 500nF (=500us) the total noise increases by 13% in the worse case when occupancy is 10%.
Results on AC-coupled ALABUF testing.
14.10.2002.
Vladimir Gromov.
NIKHEF, Amsterdam, the Netherlands.
Objectives of the testing.
By doing measurements with a real set of signals I am going to confirm Monte-Karlo simulation earlier carried out. I will prove that:
a). additional noise (base line fluctuations) occurs due to ac-coupling in front of the ALABUF chip.
b). the smaller nominal of the coupling capacitor the bigger the additional noise is.
c) the smallest acceptable (additional noise is negligible in comparison with expected electronic noise of HAL25 chip) value of the capacitor is below 100nF.
Introduction.
According to Monte-Karlo simulations ac-coupling causes signal distortion and base line fluctuations while fast (10MHz) analog signal read is going on. The fluctuations slightly modulate the red-out signal giving a deviation from the initial value. Such a deviation can be interpreted as an additional noise and assigned with statistical parameters (standard deviation and mean value). Taking into consideration expected electronic noise of HAL25 chip, we are able to find out operation conditions under which the additional noise contribution becomes negligible.
The effect we are looking at is very small (0.02*MIP) it makes us to avoid any side distortions capable to hide the effect. Namely settling of the AWG signal must be better 1% within 100ns and the ALABUF output signal must fit to the full dynamic range of 12-bit ADC (-1V….+1V). That is why an attenuators and the second ac-coupling used between the ALABUF and the ADC card. The second ac-coupling chain does almost nothing to the signal shape as long as its time constant is very large (=10uF 1k=10us).
Fig. 1. Experimental set-up used for the testing.
- Calibration (Cin=10uF).
To start with the real measurements an accurate calibration has been carried out first. To do so I generated two set of number to cover 5V range . Then I loaded the sets into Arbitrary Waveform Generator device, which generated stimulus signals for the ac-coupled ALABUF chip (see Fig.1, Fig.2). The ALABUF chip is self-biased circuit operating at +1.25V on the input pads. Therefore it cannot be coupled to an external generator but via capacitors. For the calibration purpose I used capacitors as large as 10uF. In this case signal distortion is surely are below level of interest.
Fig. 2. Calibration. Example of AGW output signals.
Due to signal attenuator (0.9) behind of the ALABUF chip its dynamic range seems a little bit narrower than the actual one (1.05V) (see Fig.3, Fig.4).
Fig. 3. Calibration. Positive output of the ALABUF chip.
Fig. 4. Calibration. Negative output of the ALABUF chip.
Difference between input and output data lies in range (3mV) is a result of imperfection of the AWG generator. This imperfection restricts sensitivity of the experimental set-up to the effect of base line fluctuation. Shift of the values corresponding to even samples (zeros) is 4mV. It occurs due settling process following the AWG pulse. The greater the pulse is the greater its residual becomes giving shifting values instead of a fixed level (see Fig 5, Fig.6).
Fig. 5. Difference between generated data and the ALABUF chip output data. Positive output.
Fig. 6. Difference between generated data and the ALABUF chip output data. Negative output.
- The measurements
MathCad facilities have been used for the data generation. For each pattern there were 768 numbers generated according to charge left in the strip of the detector. Landau distribution has been taken into account as well as 5% occupancy on the strips (see “Study of AC-coupling impact on SSD performance by means of Monte-Karlo simulation”). When running at 10MHz frequency the numbers convert into a set of 100ns pulses (see Fig.7). In total there were 39 patterns coming with 250us gap in between (see Fig.8).
Fig. 7. An example of the signals to be analyzed.
Fig. 8. An example of the signals to be analyzed.
Amplitude distribution of the ALABUF output signals given in Fig.9. A substantial portion of the small signals is caused by charge division mechanism built-in into event generator. The amplitude distribution becomes Landau distribution like when signals from adjacent channels (strips) are summed up. The most significant information on the plot is that MIP is 180 mV therefore expected electronic noise of HAL25 is
el=180mV 400e/22000e= 3.3mV
Fig. 9. Amplitude distribution of the ALABUF output signals.
. 3. Results of the measurements with Cin=10uF ( 10uF1k 10ms).
As it was mentioned for the tests and calibrations there were large capacitors (Cin=10uF) used to couple the ALABUF chip to AWG generator. It this case difference between generated data and the ALABUF chip output data caused by the AWG generator imperfection and resolution of the ADC card. As we can see standard deviation of the difference distribution is =1.3mV whereas that of expected electronic noise is el=3.3mV (see Fig.10). It means that the experimental set-up is “sensitive” enough to observe effect of base line fluctuation we are going to see with smaller coupling capacitors. For the negative output the resolution is slightly worse ( =1.9mV) hence it is more difficult to observe the effect after all.
Fig. 10. Cin=10uF. Difference between generated data and the ALABUF chip output data. Positive output.
Fig. 11. Cin=10uF. Difference between generated data and the ALABUF chip output data. Negative output.
. 4. Results of the measurements with Cin=100nF ( 100nF1k 100us).
When capacitors in front of the ALABUF chip are Cin=100nF, the base line fluctuation determine difference between generated data and the ALABUF chip output data. Distribution of the differences becomes wider ( = 2.1mV for the positive output and =2.4mV for the negative one) (see Fig.12, Fig.13).
Fig. 12. Cin=100nF. Difference between generated data and the ALABUF chip output data. Positive output.
Fig. 13. Cin=100nF. Difference between generated data and the ALABUF chip output data. Negative output.
Conclusion.
The measurements carried out with a real set of signals show that additional noise (base line fluctuations) occurs due to ac-coupling in front of the ALABUF chip.
The additional noise becomes “visible” by the experimental set-up when coupling capacitor in front of the ALABUF chip is smaller than 100nF. It is hiding behind finite resolution of the experimental set-up (=1.3mV) if the coupling capacitor is much larger than 100nF.
By reconstructing of the amplitude distributions of the ALABUF output signals I determined the MIP value=180mV and calculated expected electronic noise of HAL25 (el=MIP400e/22000e=3.3mV).
When the coupling capacitor is 100nF, standard deviation of the additional noise is =2.1mV (see Fig.12). Ratio between expected electronic noise and the additional noise is 2.1mV/3.3mV = 0.63. That is in reasonable agreement with simulation results 346e/400e = 0.86 (see “Study of AC-coupling impact on SSD performance by means of Monte-Karlo simulation”). The discrepancy is most probably caused by imperfection at the stage of fitting the data with Gaussian functions.
∑ = (2bl + 2el)0.5 ∑ = 1.2∙el
Report on analogue buffer chip (ALABUF) development in 0.25u CMOS technology for the ALICE Silicon Strip Detector (SSD).
28 March 2002.
V. Gromov (), R. Kluit.
ET NIKHEF, Amsterdam.
Abstract.
For the purpose of driving of analog signals from the on-detector front-end electronics of the ALICE SSD to the off-detector ADC, an analog buffer chip (ALABUF) has been designed. The design is performed in 0.25 CMOS technology.
Inputs of the design as well as the design goals specification to be met are described along with circuit optimization procedures and detail chip description.
Results on testing of the chips taken from the experimental batch are presented and compared to the simulations.
.
Fig.20. Measurements of linearity and dynamic range of the ALABUF chips.
Fig.24. Measurements. Differential transient response of the ALABUF chip.
Talk at the werkbespreking.
ALABUF chip for the ALICE SSD detector. Design and test.
Vladimir Gromov. ET.
9.04.2003.
Content.
- General information.
- Function of the ALABUF chip.
- Principal and schematic diagram.
- Main specifications of the chip.
- Detector simulator approach to test AC-coupling in front of the chip.
- The experimental set-up.
- The test results discussion.
- Conclusion.
Conclusion.
To meet the needs of the read-out of the ALICE SSD a new analog buffer chip ALABUF has been designed in 0.25u CMOS technology.
In order to test the new-designed chip we have developed a new method. The method is based on simulation of the operation of the detector. The output of this simulation is a file that describes signal process coming out the detector. The file is an input for the software controlled Arbitrary Waveform Generator. The generator produces “detector-like” real time signals for the inputs of the tested chip. The ADC card digitizes and saves signal process at the output of the chip in a file. By taking note of difference between the input and the output files we analyze signal distortion caused by chip. Thank to statistical nature of the analysis tiny effects become visible.
I suggest that a many of other applications can make use of this method.
1. 1)ALABUF chip for the ALICE SSD detector. Design and test. Is the subject of my present talk.
2) Here are the issues I would like to cover today.
3) Right after a few general words I am going to tell over the function of the ALABUF chip in the detector read-out.
4) Then we will have a closer look at the inside of the chip as well as its performance and its main specifications.
5) Further I will disclose the substance of the approach we have developed to test the chip.
6) We will examine the experimental set-up and will briefly discuss the test results.
7) Finally, of course, I will draw some conclusions out this development.
2 . 1)Since two years ago, VLSI group has been taking part in project ALICE.
2)The group is responsible for development and mass production of two chips in 0.25u CMOS technology for the Double-Sided Silicon strip detector.
3) Chip I am going to tell is an analog buffer. It is designed for taking analog signals from the front-end chips on the both sides of the Silicon strip detector, amplifying and sending them over 25m twin-pair cable to an off-detector ADC.
4)The read-out rate will be 10MHZ.
5)The Front-end chips on P-side and N-side of the detector will operating at different potentials.
6)So as to couple the chips to the rest of the world we had to break DC-path and insert capacitors in front of the analog buffer chip.
7)However the capacitive or AC-coupling is never for free, it bring a lot of troubles at the same time. What are they?
8)Passing over the capacitor each signal leave behind a tail of negative polarity, which actually is the capacitor discharge.
9)The tails overlap each other forming a fluctuating substrate for next coming signals.
10)By making the capacitor larger we make the effect less significant on the one hand.
11)But on the other hand then the capacitor will store a huge charge as long as it holds 55Volts. In the case of accidental break down the will likely destroy all the electronics around here.
12)Therefore we fall into dilemma. From safety point of view we would prefer a small capacitor although to minimize signal distortion the capacitor must be large.
13)You may often come across the same problem in your application, therefore it makes sense to give you a perspective on the approach we have developed to resolve it.
3 . 1)First, however, I would like you to have a look at the ALABUF chip itself.
2)The analog buffer called ALABUF consists of a feed-backed operational amplifier and a pair of analog multiplexers to be able switch inputs to the bus on any side of the detector.
3)So as to save power when the chip is out of use a Disenabling option has been implemented.
4)Schematic of the amplifier is on the plot.
5)It has a fully differential configuration (differential input and differential output).
6)The two-stage structure with Miller capacitors guarantees high open loop gain factor (50dB) and good stability (phase Margin is 83.5 degrees).
7)Resistive feedback sets differential gain (5.9).
8)The common mode feedback takes care that outputs and inputs are kept at the middle of the power supply range.
4 . 1)The amplifier stays linear until the output swing does not exceed 1.85V (VDD=2.5V).
2)The Step response signal smoothly settles to the dc level in the course of 20ns.
5 . 1) The ALABUF chip houses 2 channels of the analog buffers on area as large as 2mm by 2mm.
6 . 1)Main specifications of the analog buffer have been put together in the Table. All of them comply with the requirements.
7 . 1). As soon as the electronics has been designed and preliminary tests are in line with the simulation you may start to think over a functionality test, when the ALABUF operates as a link of the read-out chain being set under real conditions.
2). The most common way to provide a real condition tests and to judge the functionality is to go the real beam of particles. However this way involves a lot of facilities, time and manpower.
3). At the same time we could think of a signal source capable of simulating processes taking place in the detector.
4). What, in fact, make the detector on the beam a special signal source, as long as for the electronics it is just a signal source?
5). First we know that the signals are disorderly spread in the time domain.
6). Second the value of the signal varies in a very wide range.
7). Third a charge will be shared between two neighbouring strips in a special manner.
8 . 1). If we put all these distribution and dependences into the MathCAD software we will generate a file that contains description of pulse coming out of the “Detector”.
2). The Arbitrary waveform generator converts the file into real time signals and sends them to the AC-coupled ALABUF chip.
3). The ADC card does digitizing of the chip output signals.
4). We estimate signal distortion by taking difference between signals coming in and out of the chip.
9 . 1). This picture illustrates how the signal patterns look like.
2). Each pattern virtually corresponds to one trigger event.
3). It takes 76.8us to read-out all the strips.
4). After 250us next trigger is coming and the read-out starts again.