Robert Schurz & René PadillaFermi National Laboratory

A Transimpedance Amplifier Under Cryogenic Temperatures

CRYOGENIC LOSS MONITOR (CLM) A diagnostic device used

in Superconducting Radio Frequency (SRF) accelerators

Detects ionizing radiation Operates at 5 Kelvin CLM’s are critical because

they cover cold sections of the accelerator

Implemented in the SRF Test facility currently under construction at Fermilab

Comprised of:• Ionization chamber

filled with liquid helium gas that outputs a current due to ionization

• Transimpedance amplifier with a power supply that converts current to voltage

WHY CRYOGENICS? The signal to noise ratio (S/N) improved Decreases pickup noise Eliminates the long wires used to connect external equipment Balestra & Ghibaudo (2001) mentioned that Electronic

circuits that function in harsh environments (Fig. 3) can:• simplify design• increase efficiency, thermal conductivity, and

resistance• mitigate thermal electrical noise• reduce costs

NASA interests

Is it possible to design a transimpedance amplifier, using

commercial components, that operates at

cryogenic temperatures suitable to conditions in

superconducting accelerators?

CONSTRUCTION OF CIRCUIT Designed a series of five test circuits using

commercially available components :• A 1.4 MHz operational amplifier ICL7611• Two 0.1 µF capacitors• BNC connectors• Teflon wiring• G10 FR4 metal clad circuit board• 1 MΩ feedback resistor Rf • A 3.3 pF ceramic phase compensation

capacitor Cf

Conducted a series of tests looking for:• A linear DC response• Frequency response (1Vp-p) to determine the bandwidth at 3db point

Found bandwidth at 3db point (0.7V)

INITIAL RESULTS AT ROOM TEMPERATURE

Overshoot

ENCAPSULATION Circuit was

encapsulated to protect the components from the cooling liquid

Rubber paste is an unsuccessful sealant

Used epoxy instead Layer by layer

prevented damage The epoxy added

resistance which minimized any small overshoot still left

RESULTS AT LIQUID NITROGEN TEMPERATURES

(a) (b)Figure 1. DC response for a circuit with a 4.7pF capacitor and 1MΩ resistor encapsulated in epoxy at room (a) and liquid nitrogen temperatures (b).

SINGLE SUPPLY CIRCUIT The circuit had to be changed to a single

supply (-15V) for successful implementation The feedback circuit and resistor were

increased for optimal performance and increased sensitivity

Winding cables through a ferrite core reduced noise due to Eddy currents

0.2% error instead of 8% error for DC response

FINAL RESULTS

(a) (b)Figure 2. Frequency response before (a) and after (b) encapsulation in epoxy for the final single-supply circuit with a 6.6pF feedback capacitor and 10MΩ feedback resistor at room temperature.

(a) (b)Figure 3. DC (a) and Frequency (b) response for the final encapsulated single-supply circuit with a 6.6pF feedback capacitor and 10MΩ feedback resistor at liquid nitrogen temperature.

FINAL RESULTSTable 1. Percent error calculation for DC response at room (RT), liquid nitrogen (LnT), and predicted liquid helium temperatures (HeT) for the final encapsulated single-supply circuit with a 6.6pF feedback capacitor and 10MΩ feedback resistor. ideal V(o) %errorRT %errorLnT %errorHeT(predicted)

0.00-0.50 0.40 0.20 0.40-1.00 0.30 0.10 0.23-1.50 0.20 0.07 0.16-2.00 0.20 0.05 0.13-2.50 0.20 0.08 0.17-3.00 0.13 0.07 0.13-3.50 0.14 0.09 0.16-4.00 0.12 0.08 0.14-4.50 0.13 0.07 0.13-5.00 0.14 0.08 0.15-5.50 0.13 0.07 0.14-6.00 0.12 0.08 0.15-6.50 0.12 0.06 0.12-7.00 0.00 0.14 0.19

OTHER STUDIES Hayashi et al. (2009) constructed an I-V

converter for an atomic force microscope with a confirmed:• Transimpedance gain of 10^6 V/A• Bandwidth of 200 kHz at 299.9 k and 77.8

k• An improved S/N ratio

OTHER STUDIES A study by NASA tested critical

components from 293k to 83k (Patterson, Hammoud, & Dones, 2009)

• metal oxide 10 kΩ resistors had some temperature dependency as resistance increased by 14%

• non-polarized (NPO) ceramic capacitors had only minute changes in capacitance

Helped us choose components

SUMMARY We have successfully constructed a single

supply transimpedance amplifier with commercial components

Confirmed operation at liquid nitrogen temperatures with a bandwidth of 2.5 kHz

Future studies could: Use a different OpAmp Further eliminate noise with a low pass filter Find larger bandwidth

ACKNOWLEDGMENT

The study was made possible by the collaboration of IMSA and Fermilab efforts. The author would like to thank René Padilla and Arden Warner at Fermilab for their help in conducting and reviewing the investigation.

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Electronics. Boston: Kluwer Academic Publishers.

Hayashi, K., Saitoh, K., Shibayama, J., & Shirahama, K. (2009). A Current to Voltage Converter for

Cyrogenics Using a CMOS Operational Amplifier. Journal of Physics: Conference Series, 150

(1),1-4.

Lui, T., Gong, D., Hou, S., Liu, C., Su, D.-S., Teng, P.-K., Xiang, A.C., & Ye, J. (2012). Cryogenic

Digital Data Links for the Liquid Argon Time Projection Chamber. IOP Science: Journal of

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Patterson, R., Hammoud, A., & Dones, K. R. (2009). Evaluation of Advanced Cots Passive Devices for

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