Application Note
Temperature compensation and tests of the ELT BB3
Detector package
I.Prochazka, J.Kodet, J.Blazej, CTU Prague, March 2011
Goal: Design and tests of the temperature compensation circuits of the ELT detector package,
verification of correct operation of the resulting version within specified temperature range.
General
 The ELT BB3 detector package is operational within a temperature range specified for its
components, it means -55 to +85 degrees Celsius as a minimum
 The detection delay change of the device is influenced by two main contributors: detection
delay of the fast comparator (ADCMP553) and of the detection chip itself.
 The detection delay of the comparator is specified as +0.2 ps /K, it was verified by our
laboratory measurements to be 0.2 ps /K .
 The dominant effect in the delay dependence is the K14 detection chip. Its detection delay is
depending on the bias above its breakdown voltage , see Graz Report 2010 and Figure 1. The
SPAD will be operated 1.5 V above its breakdown voltage, it means 30.8 V bias at room
temperature +25 C. From Figure 1 one can conclude, that the detection delay changes for 12
ps for 100 mV change of the SPAD overvoltage. The SPAD breakdown voltage is depending
+30mV / K. The detector circuit has an additional voltage / temperature drift of +2 mV /K
 From the item above follows, that the SPAD bias power supply must have the temperature
coefficient of +32 mV / K.
Figure 1 ELT BB3 detector package detection delay versus SPAD bias voltage at +25 C.
SPAD bias temperature control
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The SPAD bias is stabilized using “723” type circuit using internal voltage reference with
very low temperature drift. The desired bias / temperature dependency is obtained by the
connection of the temperature sensor in parallel to the voltage adjustment divider, see Figure
2.
The temperature sensor AD590 provides a current equal to 1 μA/K. In the voltage divider
the R1 resistor defines the voltage dependence on temperature and the resistor R2 is used to
adjust the output voltage.
The detector internal temperature was monitored using the built in temperature sensor via the
Test board and voltmeter. The detector package was warmed up using hot air blowing device.
Figure 2 The scheme of the voltage divider with temperature dependent element
Figure 3 The measure dependence of the SPAD bias on the detector assembly temperature as
recorded by internal temperature sensor.
Detection delay versus detector temperature
The detection delay dependence was measured in the following setup: the laser pulse 42 ps long at
778 nm wavelength was generated by Hamamatsu picosecond light pulser, the laser pulse was
divided into two parts, the strong signal was directed to the fast linear photodiode followed by the
fixed threshold discriminator. The second part of the light pulse was directed onto the input aperture
of the ELt detector package. The time interval between the Start and ELT output signals was
measured using the NPET timing system with sub-picosecond resolution and stability. The detector
internal temperature was monitored using the built in temperature sensor via the Test board and
voltmeter. The detector package was warmed up using hot air blowing device. The repetition rate
was 300 Hz, the data yield was 6%.
..
Figure 4 Block scheme of the detection delay change monitoring. The optional voltmeter (lower
left) is used to monitor or ELT internal (PCB) temperature or SPAD chip bias.
Figure 5 Optical part of the experimental setup (left to right ) laser pulser, focusing optics, beam
splitter, fast linear photodiode, Start discriminator, (to the right) optical attenuator and ELT detector
BB3.
Figure 6 Measured detection delay change with temperature, the linear fit with a slop +0.6 ps/K is
added
Figure 7 Measured detection delay change with temperature after Power ON warm-up, the linear fit
with a slope +0.8 ps/ hr added. Note the drift < 1 ps just 5 minutes after the Power ON.
Detection delay long term stability
The entire setup described above was operated for 3 days (weekend) in standard lab conditions, the
temperature change exceeded 4 K peak-peak. The detector internal temperature was monitored using
the built in temperature sensor via the Test board and voltmeter. The detector package was warmed
up using hot air blowing device. The repetition rate was 300 Hz, the data yield was 6%.
Figure 7 a,b ELT BB3 detector package as monitored by internal sensor (top), the measured delay
(bottom). Kindly note the temperature variations > 4K p-p and the detection delay changes +/- 1 ps.
Figure 8 Detector stability in the sense of Tdev recorded for various experiment repetition rates in
the range of 10 to 300 Hz laser rep.rate, the data yield was typically 8 %. Note – no saturation of
Tdev for fraction of days, temperature changes +/- 2 K !!!
Summary – Conclusion
 The SPAD bias power supply was designed and tested to have temperature
dependence of +32 mV/K to minimize the detection delay temperature
dependence
 The detector delay temperature drift was measured to be 0.6 ps / K, this
corresponds to 3.9 ps over one orbit (6.5 K change), ESA requirement is 20ps
with a goal of 10 ps.
 The detection delay is stable within 1 ps as soon as 5 minutes after Power ON
 The detector package long term stability is significantly better than 1 ps for time
intervals of 3 days
 The detection delay stability Tdev is better than 0.5 ps for time intervals
exceeding 50 000 seconds
Prague, March 29, 2011