Development of a

Low Noise Preamplifierfor the LEM read-out

A. BadertscherM. LaffranchiG. Natterer

P. OtiougovaA. Rubbia

Low Noise PreamplifierCircuit

• Preamplifier circuit inspired from Ciro Boiano et al. INFN 1):– Modern junction

FET‘s are used: BF862

– 4 matched FET‘s in parallel

– Different feedback paths

1) C. Boiano, R. Bassini, A. Pullia,A. Pagano: Wide Dynamic-Range Fast Preamplifier for Pulse shape Analysis of Signals From High-Capacitance Detectors. IEEE Transact. on Nucl. Science, Vol. 51, No. 5 Oct. 2004

Low Noise PreamplifierPCB Layout

This PCB is designed for the first test measurements.

For working with the future DAQ board it will be a SIP board

4 BF862

LM6161

5cm

2.5c

m

Low Noise Preamplifier Noise Sources

• Ui: Input noise voltage of the OPAmp

• Ii: Input noise current of the OPAmp

• URf = Rf IRf: Thermal noise of the feedback resistor

2 2 2

0

1||

'noise

out i i Rf ff

U U C I I Ri C

C0=200pF (LEM + Cable)

Cf=1pF

Rf=5M, 2.5G

Ui=0.4nV Hz-1/2

Ii=1fA Hz-1/2

T=300K

_ ( ) 4 ; /thermal noiseu rms kTR V HzC‘=Cf /5

Low Noise Preamplifier Modified Circuit :

• Modifications:-The base of Q7 has no

resistor in series, but a capacitor to GND

-The Gate voltage of the protecting FET Q5 has been changed from -12V to -6V

Characteristics:

-Bandwidth: 9MHz

-Amplitude Outp: +4V, -5V

-Input Noise:

5*10-18 C Hz-1/2 @0pF

2.1*10-17 C Hz-1/2 @200pF

Low Noise PreamplifierNoise Measurements (without protection circuit):

Bandwidth: 200Hz @500kHz

S=4.4mV/fC

S=5.2mV/fC

S=4.9mV/fC

S=4.5mV/fC

Low Noise Preamplifier Measured noise caracteristics (rms, without protection circuit)

No. of

FET‘s

Csource S Unoise Unoise Qnoise Qnoise

BF862 BW=200Hz

@500kHz

BW=1MHz

@500kHz

BW=1MHz

@500kHz

BW=1MHz

@500kHz

- pF mV/fC Vrms Vrms fC # of e-

3 0 5.2 5 350 0.07 420

3 82 4.9 9 640 0.13 800

3 202 4.7 14 990 0.21 1300

1 0 4.4 2 140 0.03 200

Low Noise Preamplifier Picture of the first prototype

Gain@300kHz: 5mV/fC

2.5cm

Low Noise Preamplifier PCB 2.5cm x 5cm

Gain@300kHz: 5mV/fC

Low Noise Preamplifier Discussion

• Noise - The measurements show an increasing output noise with increasing source capacitance. This effect demonstrates the dominance of the input voltage noise of the BF862 FET at 200pF detector capacitance over the thermal and current noise as predicted.

-With a very low source capacitance the current noise dominates and only one FET should be used.- The measurements with a FFT spectrum analyser shows white noise characteristics between 100kHz and 10MHz.

• Bandwidth- The amplifier has a wide bandwidth of about 10MHz. This allows us to adapt the pulse shaping frequency in a wide range and the sampling frequency to our needs .

• Output voltage range- The output voltage range is high between +4V, -5V. 4V equals to 910fC or 5.7*106 electrons.

Low Noise Preamplifier Protecting Circuit

• The first tests with this amplifier on the LEM shows a very high vulnerability of the FET-gates, even protected with a gate source diode of a FET in antiparallel.

• Now we have added a surge arrestor with 2 protection diodes (BAS34 or BAV199). The principle is shown in the next sheet. This diodes have a relativ low reverse current (1pA).

• The surge arrestor has a nominal breakdown voltage of 90V and has been successfully tested at 77K.

• The protection circuit has been tested by Polina.• A new single in line (SIP) PCB was designed consisting

the BAV199 protection diodes.

Low Noise Preamplifier Protecting Circuit Schematic

LEM electrode

GND

-1V

100

Part of the premaplifier

100M

HV

Cc1

T=90K

The spark energy will be absorbed first by the surge arrester and second by the diodes, only a small amount will appear at the preamplifier. The current will be limited by the surge arrestor voltage and the resistor R. The additional noise due of the resistor is:

Un= (4kTRf)1/2 U‘n=0,7nV Hz -1/2 @R=100, T=90KU‘n=1.8nV Hz -1/2 @R=100, T=300K

Uf

Uf

RCc2

GNDsurge arrestor

Input JFET‘s

Low Noise Preamplifier New PCB layout

• Dimensions: 20mm x 48mm• All components are on the same surface

Low Noise Preamplifierhow to proceed

• Design the input protection circuit layout near the LEM

• Design of the pulse-shaper amplifier