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AIDA design review Davide Braga Steve Thomas ASIC Design Group 09 October 2008 PowerPoint PPT Presentation


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AIDA design review Davide Braga Steve Thomas ASIC Design Group 09 October 2008. X10 stage - comparator. Comparator threshold adjustment to reduce offset Clamping of the x10 gain stage performed by latched comparator to avoid saturation and hence big current spikes.

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AIDA design review Davide Braga Steve Thomas ASIC Design Group 09 October 2008

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Aida design review davide braga steve thomas asic design group 09 october 2008

AIDA design review

Davide Braga

Steve Thomas

ASIC Design Group

09 October 2008


X10 stage comparator

X10 stage - comparator

  • Comparator threshold adjustment to reduce offset

  • Clamping of the x10 gain stage performed by latched comparator to avoid saturation and hence big current spikes


Offset monte carlo results

Offset (Monte Carlo results)

  • Offset preamp:σ~0.5mV(AC coupled to shaper, does not propagate to comparator)

  • Offset shaper:σ~2mV

  • Offset x10 stage:σ~3mV

  • Offset comparator:σ~30mV!(~ 10 times offset previous stage!)


Comparator threshold

Comparator Threshold

  • The threshold is added to the mean value of the x10 stage output voltage.

  • It is generated by a selectable current (8bit) flowing in a resistor

  • The RC filter has a distributed reset for recovery


Comparator threshold1

Comparator Threshold

Selectable threshold:

Minimum value: Vth_ min~=7mV ~=9800eV

(Noise after x10gain stage expected to be σn~=3.5mV → 5σn=17.5mV)


Comparator hysteresis

Comparator Hysteresis

Built-in feature (not selectable) to avoid noise on comparator output: comparator fires at:

Vin>Vth+Vhyst.

Vin<Vth-Vhyst.

Vhyst now set to ~10mV


X10 gain stage clamp

X10 gain stage clamp

Most signal would saturate the x10 stage output, causing big (and long!) variations in current

This can be avoided clamping the amplifier to a unity-gain configuration after the comparator has fired


X10 gain stage clamp latch

X10 gain stage clamp - latch

The comparator responsible for the clamping must be latched not to let the amplifier come back to the x10 gain configuration

A timed signal will reset the latch once the signal has disappeared


Current stabilization

Current stabilization

Another measure has been taken to reduce current oscillation in the comparator by inserting a dummy branch which counterbalances the drop in current consumption in the device


Transient false firing

Transient: false firing

When the reset comes off it causes “small” activity in the shaper, which is enough to fire the comparator and also to be store on the peak hold.

The comparator must be masked and the peak hold could be set in hold mode to decouple it from the shaper output

Shaper

Out comparator

Peak Hold

Resert shaper


Layout

Layout

“left to right” channel layout, with LEC/MEC and HEC in parallel

Grid distribution of power supply (“Horizontal” and “vertical”) to minimize impedence and voltage fluctuation across the chip


Aida design review davide braga steve thomas asic design group 09 october 2008

Bond-wire parasitics:

C=peL/cosh-1(a/d)

a=~7mm,d~25mm, L~1mm

which gives C~4fF

Injected charge Q=CV~12fC=75ke for single wire

“Dummy” wires have opposite polarity signals. Electric field drops as r2 for a dipole, giving much lower coupling capacitance

~7mm

~2mm

~2mm


Pads detail

PADs (detail)


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