Transcript Document

VFE/MGPA considerations for EE/VPT
EB/EE electrical spec. differences:
full-scale signal
noise
input capacitance dominated by:
APD capacitance (x2) for barrel
VPT -> MGPA interconnect for end-cap
MGPA suits barrel and endcap full-scale signal
requirements because input stage gain defined
by external feedback components
Detailed lab measurements so far concentrated on EB
application, but linearity and pulse shape matching
performance dominated by gain and diff. O/P
stages => should be same for EE
Some EE specific measurements exist for noise and
pulse shape, but not extensively studied so far
Mark Raymond (Nov.2004)
MGPA Target Specifications
Parameter
Barrel (APD)
End-Cap (VPT)
fullscale signal
60 pC
16 pC
noise level (ENC)
10000e (1.6 fC)
3500e (0.56 fC)
input capacitance
~ 200 pF (APD)
~ 50 pF (cable)
output signals
(to match ADC)
gain ranges
differential 1.8 V,  0.45 V around
Vcm = Vdd/2 = 1.25 V
1, 6, 12
 10 %
pulse shaping
40 ns CR-RC
nonlinearity
(each range)
<  0.1 % fullscale
pulse shape matching
(Vpk-25)/Vpk
<  1 % within and across gain
ranges
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MGPA Architecture
input stage
CF chosen for max. poss. gain
depending on barrel/end-cap
RF chosen for 40 ns decay
avoids pile-up
CFRF external components
=> 1 chip suits barrel & end-cap
CF//RF = 39pF//1k (barrel)
= 8p2//4k7 (endcap)
differential current O/P stages
external termination
2RICI = 40 nsec.
=> low pass filtering on all
noise sources within chip
3 gain channels 1:6:12
set by resistors (on-chip),
for linearity, feeding commongate stages
i
I2C and
offset
generator
RG1
CI RI
i
i
RI
VCM
DAC
CI RI
ext.
trig.
CCAL
input stage
charge amp.
I/P
RF
CF
RG2
RI
CI RI
RG3
gain stages
RFCF
RI
VCM
VCM
diff. O/P stages
VCM
2
Transistor Level Schematic
3
Noise Sources
input stage
vRf
Rf
Cf
common-gate
gain stage
source
iCG
follower
RG
vFET
CIN
iRG
ENC 
input stage
high Cf (low gain) to cope with large full-scale signals
=> corresponding low Rf for 40 ns time const.
=> Rf noise dominates over input FET
hand
calculation
barrel ENC
(CIN=200pF)
diff.
output
stage
end-cap ENC
(CIN = 50 pF)
Rf noise
4900 e
2700 e
I/P FET
1800 e
660 e
total
5220 e
2780 e
2.718
1.6  1019
2
2
2
kTC 2f RG C 2f RG2 icg
CTOT
kT v FET



2R f
8

4
gain stage contribution
can’t avoid for low gain range (RG big)
but this range only used for larger signals
so signal/noise still acceptable
these values calculated for
CF//RF = 39pF//1k (barrel)
CF//RF = 12pF//3k3 (endcap)
(vFET ~ 0.23 nV/(Hz)1/2 )
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Choice of 1st stage feedback components
CF
RF

fullscale
signal
[pC]
RF
noise
[e]
6p8
5k6
38.1
10.5
2048
8p2
4k7
38.5
12.6
2236
10p
3k9
39.0
15.4
2475
12p
3k3
39.6
18.5
2700
39p
1k
39.0
60
4900
values normalised to
60 pC barrel fullscale
ENC 
depends on fullscale signal requirement
(VPT response and dynamic range requirement)
barrel case
2.718
1.6  1019
2
2
2
kTC 2f RG C 2f RG2 icg
CTOT
kT v FET



2R f
8

4
5
Noise Measurement (VFE card)
ENC [rms electrons]
BARREL (CF//RF = 39pF//1k)
END-CAP (CF//RF = 8p2//4k7)
high gain chan.
mid gain chan.
high gain chan.
mid gain chan.
4000
10000
8000
6000
4000
2000
0
3000
7240+5.8/pF
7870+4.9/pF
2000
3040+4.5/pF
3270+4.5/pF
1000
0
0
100
200
0
100
200
added capacitance [pF]
0
20
40
60
0
20
40
60
added capacitance [pF]
weak dependence on input capacitance as expected
within spec. for high and mid-gain ranges:
barrel < 10000 e, end-cap < 3500 e
low gain range:
barrel: 27300 e ± 12% end-cap: 8200 e ± 11%
completely dominated by gain stage noise
but signals large => electronic noise not significant (< 0.2% contribution to overall barrel energy res’n.)
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End-cap signal simulation
CSA O/P
End-cap VPT interface
coax
without
coax
I(t)
Cdet
with
coax
MGPA
I(t) current source with 10 ns decay time
Cdet = 5 pF (2 pF + stray)
chip O/P
coax = RG 179 (thin 50 ohm) 75 cm long
some ringing observable at input stage O/P
without
coax
smoothed out at chip O/P
with
coax
previously presented at
design review (Jan.2003)
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Measurement
0.4
fast (5 ns) charge injection
10 ns exponential charge injection
2 pF
0.2
80 cm. thin
50 W coax
pulse shapes
at chip O/P
Volts
0.0
-0.2
MGPA
high gain range
~ ¾ fullscale signal
-0.4
-0.6
-0.8
200
250
300
350
nsec.
400
450
500
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Specific EE issues
input protection
on-chip protection not sufficent to withstand VPT breakdown
(RAL measurements already proved)
=> additional external protection diode
=> extra capacitance (~ few 10’s pF shouldn’t be a problem)
=> prot. diode rad-hardness issues
coax
MGPA
more remote opto-electric transducer
=> transmission line effects
=> grounding/shielding issues
HV filter card diagram - courtesy
Claire Shepherd-Themistocleous
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