1. CORRELATION BETWEEN LEAKAGE CURRENT AND NOISE
Talk by Edmund WIDL
(HEPHY Vienna)
presentation can be downloaded from: wwwhephy.oeaw.ac.at/u3w/e/ewidl/www/talks/NOISE.ppt

2. Equivalent Noise Charge (ENC)
INTRODUCTION
For detectors measuring absorbed energies it is useful to quantify the noise in terms of
Equivalent Noise Charge (ENC)
This ENC corresponds to the amount of charge that would have to traverse the detector to yield a signal as big as the actual noise:
noise = e . ENC
e calibration constant

3. AMPLIFIER NOISE main noise source: input transistor at the amplifier
contributions of further stages usually neglected
load capacitance plays an important role due to the integrating nature
In a simple approach, the amplifier noise can be described by the sum of:
a constant value (parallel noise) and
a part which scales with the load capacitance C (series noise)

4. ADDITIONAL NOISE SOURCES
equivalent network for a single strip & readout:
Ileak ... fraction of the detector leakage current seen by one strip
Rpoly ... polysilicon resistor
C ... strip capacitance
Rstrip ... line resistance of one strip
TP ... peaking time

5. numerical noise equations:
The total noise figure is the square sum of the individual contributions, since the individual sources are uncorrelated:

6. CORRELATION FOR OB2-MODULES
OB2-sensors:
Rpoly and Rstrip from the TrackerDB
C calculated via an empirical formula

7. OB2-modules:
2 electrically daisy-chained sensors
Rsensor of each sensor in series
C and Rpoly of each sensor parallel
APV: TP = 0.05 ms
This yields:

8. RESULTS
In order to correlate the noise from different sensors the relative noise of each sensor is used:
left side of the equation:
data from 57 OB2 modules
Ileak for each strip from the tracker database
noise(Ileak= 0) is taken as the noise from daisy-chained strips where the sum of Ileak is less than 4 nA (minimal error – less than 1 ‰)
right side of the equation: theoretical relative noise

9. following strips were not included:
strips #1, #2, #127 & #128 of each APV (are known to have higher noise)
strips with an Ileak-entry in the database higher than the total detector current at 450V (some of the QTC-measurements were done at Pisa where due to the setup a bad contact can result in an extremely high value)
unbonded strips
28167 strips remaining

10. leakage current distribution:
(in bin 0-10 nA)
good statistics only for up to 60 nA
no strips with more than 500 nA

11. leakage current vs. strip noise (deconv.):
good agreement

12. leakage current vs. strip noise (deconv):
One can see that for high currents several strips fulfil the theoretical correlation – others seem to show no correlation (might be due to a higher absolute noise level for some modules).

13. leakage current vs. strip noise (peak mode):

14. leakage current vs. strip noise (peak mode):
Apart from the fact, that the observed correlation is flatter than the theoretical correlation (see previous slide) the same holds true as for deconvolution mode.

15. MODULES WITH CM-PROBLEMS
Several modules showed drastically increased noise on single APVs, probably caused by single, extreme leaky strips.

16. Which modules are affected?
At UCSB (until middle of October) following modules were affected:

17. All affected strips, that showed a drastic increase of noise in the ready-made modules, behaved perfectly during the QTC-measurements, i.e., not even one of these strips had a leakage current above 6 nA.