1. Readout of the ARGONTUBE DISCUSSION ON :  CHARGE- versus TRANSIMPEDANCE-AMPLIFIER  NOISE PERFORMANCE  DYNAMIC RANGE  SPEED ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 1

2. ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 2 I(t) Amplitude prop. ∫I(t)dt CHARGE-AMPLIFIER: preamplifiershaping amplifier 100M  1pF I(t) V(t) prop. I(t) TRANSIMPEDANCE-AMPLIFIER: I to V converterpost amplifier ≤500k 

3. ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 3 In a LAr-TPC the shape of the current signal depends on:  the angle of the track  the drift speed  the diffusion of the charge the 2 extreme cases are:  tracks parallel to the pad plane  short,  -pulse like current signal  tracks at 90º to the pad plane  I(t) ≈ const. ≈ 1nA  v drift [mm/  s]  limited bandwidth of the transimpedance-amplifier is good for  smoothing the short current signals (dynamic range!)  achieving low noise

4. SPICE-simulation shows what is possible with low noise MOSFET op-amp : ( AD8656 with 2.7nV/√Hz, 3rd order frequency cut off, C det = 200pF, 10  10mm 2 pads ) ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 4 BANDWIDTHRISETIME  -pulse response w out,min signal-ratio for 0°/90° tracks (v drift = 2mm/  s) I noise w.r.t. input Q noise w.r.t. input 90 kHz 3.8  s3.2  s 1.50.12 nA rms2500 e‘s 136 kHz 2.5  s2.2  s 2.10.19 nA rms2800 e‘s 230 kHz 1.5  s1.6  s 3.20.33 nA rms3300 e‘s 460 kHz 0.8  s 6.10.87 nA rms4500 e‘s

5. ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 5 What performance do we need? from the Nuclear physics B 154 (2006) paper:  … the device (ARGONTUBE) will be operated with a reduced electric field in order to simulate drift distances up to 20m. Charge attenuation and amplification will be studied in detail …  … one expects an attenuation of ~ 150 over the distance of 20m …  … after a drift of 20m at 1kV/cm, the electron cloud diffusion reaches a size of ~ 3mm …  … seeking a solution which yields gains between 100 and 1000 in pure argon … This meens that we need a very big dynamic range and low noise: for horizontal tracks the signals get smaller from the top to the bottom by a factor of 150 due to the attenuation and by an additional factor (depending on the amplifier bandwidth) due to the diffusion

6. ARGONTUBE-MEETING IN BERN 26-06-2006, Kurt Borer, slide 6 Let‘s check the numbers for the amplifier with 136kHz BW and w out,min = 2.2  s A) DYNAMIC RANGE: set LSB of 12-bit ADC equal to noise ≈ 0.2nA rms  I max ≈ 800nA and Q max ≈ I max  w out,min ≈ 1.8pC  max. gain ≈ 1.8pC/9.6fC ≈ 190 note:max. gain for tracks close to and at about ±15° to the pad plane B) WORST CASE SIGNAL-TO-NOISE: simulate 20m drift by reducing drift speed to 0.5mm/  s assume: gain = 150 for compensating attenuation, electron cloud diffusion of 3mm  signal width = 6  s and signal amplitude ≈ 9.6fC/6  s = 1.6nA ( = 8 ADC counts )  signal-to-noise ≈ 8 ( marginal with external trigger, to small for self triggering! ) C) SPEED: with w min = 2.2  s the diffusion to 3mm can be well measured at reduced drift speed, and a sampling rate of 1Ms/s is adequate