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Hybrid stability studies
Multi – chip hybrid stability problem when more then
~ 2 chips powered up -> common mode oscillation
Strasbourg experiments => sensitivity to decoupling
on V125 line
Instability not observed on single chip test setups but known
sensitivity of preamp to resistance in V125 supply
identified (during design phase) and solved on chip by
splitting V125 with separated bond pads
Outline
Explanation/simulation of phenomenon
Possible solutions
Single chip measurements of implemented solutions
Conclusions
Mark Raymond
[email protected]
Jan, 2001
CMS Tracker Electronics
1
7.1 mm
APV25s1 layout
8.1 mm
Jan, 2001
CMS Tracker Electronics
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Preamp schematic – 1 channel
V125
source
follower
input
FET
inverter
stage
V125
inverter stage originally included to allow maximum
use of dynamic range available at shaper output
required for p-strip readout
inverter FET operating point determined by input FET
since both device sources connected to V125
use simplified schematic
Jan, 2001
CMS Tracker Electronics
3
Common mode stability concerns identified
during design stage
track resistances
on-chip track resistance leads to positive feedback path
around preamp -> common mode instability which gets
worse with increasing number of channels
Jan, 2001
CMS Tracker Electronics
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On-chip solution implemented
Common
V125 supply
on hybrid
Split V125 supply on chip with separate bond pads
effectively removes on-chip coupling
Problem solved? Thought so but .....
Jan, 2001
CMS Tracker Electronics
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Multi-chip schematic
If sufficient resistance present in common V125 path
then problem can re-surface (more chips => more channels)
(can still occur for a single chip if resistance big enough)
Jan, 2001
CMS Tracker Electronics
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Simulation
Approach
use single channel model, put in calculated resistances
for tracks but multiply values by 128
Look at dependence of transient response on external
resistor in V125 supply – one chip only here
Figure 5
-0.515
-0.520
volts
-0.525
0 mOhms
-0.530
5 mOhms
10 mOhms
-0.535
-0.540
-0.545
0
20
40
60
80
-6
100x10
time
Preamp output shows some increase in gain for
resistances up to 10 mW equivalent
(note origins of waveforms offset for clarity)
Jan, 2001
CMS Tracker Electronics
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Increasing common resistance still further
Figure 6
-0.2
35 mOhms
volts
-0.4
30 mOhms
-0.6
-0.8
-1.0
-1.2
0
20
40
60
80
-6
100x10
time
Instability sets in for equivalent resistance >30 mW
frequency similar to that observed on hybrid (~30 kHz)
Interim Conclusion
instability effect can be simulated
strong evidence that V125 supply resistance to blame
simulation may help to find solution
Jan, 2001
CMS Tracker Electronics
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Solutions to problem
More than one – some more attractive than others
reduce power plane resistance on hybrid
probably end up pushing problem elsewhere
Maintain V125 separation on hybrid, through connector,
through mother cable, ......
where to stop?
others .........
Preferred solution
Concentrate here on option that appears to be most
robust
Jan, 2001
CMS Tracker Electronics
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Proposed solution
V125 supplies preamp input FETs only
V125 supply to inverter stage derived from V250 via resistor
probably better to supply each chip from individual
resistor (~100W) to ensure current shared equally
Input and inverter FET common sources completely decoupled
by power supplies
Jan, 2001
CMS Tracker Electronics
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How does it work?
Previous preamp power scheme
V125
Vg
V125
Current in inverter stages determined by operating point
of input devices (scaled by relative widths)
V250
New scheme
Vg+Vt
= ~V125
Vg
V125
Voltage on gates of inverter transistors same as before
All 128 inverter FETs operate in parallel to maintain their
common sources at a threshold above gate voltages -> ~V125
=> current in inverters defined by resistor value
Jan, 2001
CMS Tracker Electronics
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Experimental results with single chip pcb
APV25s1 test pcb altered to allow inverter V125
connection to V125 or to V250 via 100W resistor
response to internal calibrate pulse (16 channels)
V125
100
V250/100W
16 cal chans
50
100
50
remaining
112 channels
0
0
0
50
100
0
50
100
no common mode subtraction used for above plots
if algorithm used which calculates CM excluding channels
containing signal
V125
V250/100W
100
100
50
50
0
0
0
50
100
0
50
100
appears to be good solution – minor drawback of power
consumption in resistor
Jan, 2001
CMS Tracker Electronics
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Can go further ...
Why not derive input FET source voltage from V250 too?
Not as robust as previous solution to supply line resistance
Significant increase in power consumption (~25%)
But supply is now only two rails and currents balanced
Jan, 2001
CMS Tracker Electronics
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Experimental results 2 rail operation
cal pulse response
100
50
No CM subtraction
0
0
50
100
100
CM subtract using
average of channels
not containing signal
50
0
0
50
100
100
CM subtract using
median value of all
channels
50
0
0
50
100
performance not apparently affected by supplying
input FET current from V250 via 24W resistor
Jan, 2001
CMS Tracker Electronics
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Conclusions
Stability problems encountered with 1st hybrid version
understood and can be simulated
Proposed solution of deriving inverter V125 supply from
V250 via a resistor appears robust. Only significant drawback
a minor increase in power consumption.
Jan, 2001
CMS Tracker Electronics
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