KETEK VITUS Silicon Drift Detector ( )3

Steffen Pahlke KETEK VITUS Silicon Drift Detector

APPLICATION NOTE

KETEK VITUS Silicon Drift Detector (SDD) using Thermistor as Temperature Sensor

To readout the chip temperature of KETEK SDD modules a temperature sensor is mounted on the same ceramic as the SDD chip. Diodes operated in forward direction with a constant current show a linear increase of the voltage drop with decreasing temperature. The drawback of temperature diodes is the emission of light during operation. The light can be absorbed in the SDD chip and lead to an increased noise.

170 -35°C with TD -55°C with TD -35°C with Thermistor 160 -55°C with Thermistor

150

140

FWHM @ 5.9 keV (eV) keV 5.9 @ FWHM 130

120 0 10 20 30 40 Digital Peaking Time (µs)

Typical energy resolution comparison of a KETEK VITUS H30 SDD with Diode or Thermistor as temperature sensor Thermistors as temperature sensors do not show this phenomenon. Plot 1 shows the improvement in full width at half maximum (FWHM) for a 40 mm² detector operated at two different temperatures. Especially at long peaking times there is a clear improvement in FWHM for Thermistor modules. The Thermistor is a temperature dependent resistor. KETEK uses an NTC-Resistor. The temperature can be calculated from the resistance using the following equation: 1 = .1 129241 ⋅10 −3 + .2 341077 ⋅10 −4 ⋅ln( R) + .8 775468 ⋅10 −8 ⋅()ln( R) 3 T

A calibration is obsolete.

Rev. 0 KETEK GmbH, Hofer Str. 3, D-81737 München, Germany Information furnished by KETEK is believed to be accurate and reliable. Tel: +49-(0)89-67346770 However, no responsibility is assumed by KETEK for its use, nor for any infringements of patents or other rights of third parties that may result from its Fax: +49-(0)89-67346777 use. Specifications subject to change without notice. No license is granted by www.ketek.net implication or otherwise under any patent or patent rights of KETEK. ©2011 KETEK GmbH. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.

KETEK Application Note Thermistor

The following plot shows the resistance vs. the temperature of the Thermistor. At +25°C the resistance is 10k Ω.

1E7

1000000

100000

10000 Resistance [Ohm] Resistance

1000

100 -100 -50 0 50 100 150 Temperature (°C)

Typical output resistance of the thermistor dependant on the temperature The following table shows the Thermistor reading at different temperatures: Resistance ( Ω) Temperature (°C) 10000 25 20000 10 32000 0 55000 -10 97000 -20 177000 -30 243000 -35 336000 -40 471000 -45 669000 -50 962000 -55 1403000 -60

The characteristic of the thermistor can be nearly linearised using the following equivalent circuit. R V and R P have to be chosen as follows: Rp

Rt Equivalent Circuit for linearization of the thermistor response curve Rt is representing the thermistor inside the SDD module.

The above values for R V and R P are leading to a quasi linear characteristic of the thermistor response curve which is equal to the diode used before at least in the range between 0 and -60°C. This can be achieved by a linearization of the total current.

The following diagrams show the linearization with R V=540k Ω and R P=180k Ω and a compliance voltage of 2.5V with a series resistor of 2.2M Ω leading to a diode current of 1.13µA.

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KETEK Application Note Thermistor

1600000

1400000

Thermistor 1200000 TDiode Equivalent Circuit

1000000 ] Ω

800000 Resistance [ Resistance 600000

400000

200000

0 -60 -50 -40 -30 -20 -10 0 10 20 30 Temperature [°C]

Resistance comparison Thermistor, Diode and equivalent circuit with Thermistor, R V=540k Ω and R P=180k Ω

800000

750000

Thermistor 700000 TDiode Equivalent Circuit

650000 ] Ω

600000 Resistance [ Resistance 550000

500000

450000

400000 -60 -50 -40 -30 -20 -10 0 10 20 30 Temperature [°C]

Typical resistance comparison Thermistor, Diode and equivalent circuit with Thermistor, R V=540k Ω and R P=180k Ω zoomed The corresponding resistance of the equivalent circuit can be calculated as follows:

RP ⋅ Rt Rres = RV + RP + Rt

With R V=540k Ω and R P=180k Ω, the above table can be completed as follows: Resistance ( Ω) Resulting Resistance ( Ω) Temperature (°C) 10000 549474 25 20000 558000 10 32000 567170 0 55000 582128 -10 97000 603032 -20 177000 629244 -30 243000 643404 -35 336000 657209 -40 471000 670230 -45 669000 681837 -50 962000 691629 -55 1403000 699533 -60

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KETEK Application Note Thermistor

To read out the temperature sensor, the following schematic is recommended. D1 is representing both the diode and the thermistor equivalent circuit. Note, that the dimensioning is dependant on the diode current. R4 represents the current limitation and should be dimensioned as follows: U V1 R4 = = I I(D )1 For example: The voltage V1 is +5V and the current I(D1) should be 1µA. This leads to R4=5MΩ. The compliance voltage V1 should be dimensioned at least above the total voltage drop over the serial circuit of R4 and D1. R1 and R2 are forming a voltage divider representing the offset voltage. They can be used to set the voltage at the output of the amplifier to zero at a certain zero point temperature.R1 and R3 determine the gain of the circuit. It can be calculated by the given equation: R3 A = 1+ (R1 R )2

Typical Read Out Circuit for temperature sensors with linearization, offset correction and gain. Dependant on the application and the necessary accuracy it is recommended to use 100ppm resistors with 0.1% tolerance or better. The regulation for the Peltier can be either a PI/PID loop in case of the diode and the linearised thermistor circuit or a PID loop in case of using the thermistor without linearization. The corresponding step functions are shown in the following diagrams:

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KETEK Application Note Thermistor

900

700mA 800 600mA

700 500mA

600

500 400mA

400 Voltage [mV]

300 300mA

200 200mA

100 100mA

0 0 50 100 150 200 250 300 350 400 Time [s]

Typical thermistor voltage drop step response function without linearization for different Peltier currents using a Keithley 6221 current source with 1µA at 10V compliance voltage

0 100mA

-10

-20 200mA

Temperature [°C] Temperature -30 300mA

-40 400mA

500mA -50 700mA

-60 0 50 100 150 200 250 300 350 400 Time [s]

Typical corresponding temperature step response function for different Peltier currents using a Keithley 6221 current source with 1µA at 10V compliance voltage

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KETEK Application Note Thermistor

Specifications

Parameter Conditions/Comments Min Typ Max Tolerance Unit Temp.-Sensor Dependant on compliance voltage 0.1 1 5 ±0.1 µA Resistance @25°C 10000 ±0.2°C Ω

Absolute Maximum Ratings

Parameter Rating Stresses above those listed under “Absolute CURRENTS Maximum Ratings” may cause permanent Temp.-Sensor 50µA damage to the device. This is a stress rating only; functional operation of the device at these High currents may heat the resistor, leading to or any other conditions above those indicated a worsened cooling performance of the module. in this application note is not implied. Exposure This can lead to performance degradation of to absolute maximum rating conditions for the SDD. Additionally the temperature reading extended periods may affect device reliability. will show increased values compared to the real values.

ESD Caution

ESD (Electrostatic Discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.

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KETEK Application Note Thermistor

Revision History

11/06 – Revision 0: Initial Version

Contact

KETEK Headquarter Sales Office

KETEK GmbH Hofer Str. 3 81737 München

Email [email protected] Phone +49 (0) 89 673467 70 Fax +49 (0) 89 673467 77 www.ketek.net

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