1. AIDA design review 12 May 2008 Davide Braga Steve Thomas
ASIC Design Group
12 May 2008

2. Overview
Low Energy Channel: pulsed reset
High Energy Channel: drive strength
pulsed reset
Diode Connection

3. Pulsed Reset (Low Energy Channel)
shaper reset:
switch to a shorter shaping time (~1/20)
preAmplifier reset:
activates a low impedance feedback path to discharge the feedback capacitor Cf
DC fdbk

4. Pulsed Reset (Low Energy Channel)
time [μs]

5. Pulsed Reset (Low Energy Channel)
When shaper and preamp are reset at the same time:
reset time <2us from shaper peak (1.7us to 0.1% accuracy)
(<3us from signal's arrival, with tshaping_time=0.6us)

6. Pulsed Reset (Low Energy Channel)
charge injection
When the reset is released there is an unavoidable charge injection. The low frequency feedback responds too slowly for the shaper.

7. Pulsed Reset (Low Energy Channel)
The existing shaper architecture saturates for one polarity because of the undershoot caused by the preamp reset: it cannot be reset until it has recovered from this condition.
A new rail to rail shaper is a more solid architecture, moreover it allows the use of the same reference voltage for both polarities and has lower power consumption.
saturation
recovery

8. High Energy Channel: drive strength
The output stage of the preamplifier (nMOS source follower) is asymmetrical:
Pull-down strength (hole collection): the maximum current is limited
Pull-up strength (electron collection): output current virtually unlimited, can match the input one
Input pulse: 20GeV, 50ns, Ipk=17.5mA

9. High Energy Channel: drive strength
Switching between nMOS & pMOS source follower is not possible because the latter cannot drive the output below ~1V
The drawback of unlimited current in one direction is the possible appearance of huge current spikes from/towards vdd/gnd
→ crossover & self-triggering
The h+ collection time is likely to be longer than the e-, so the fast pull-up preamp should be able to cope with the fast e- signal, while the slower rise time of the other configuration should be mitigated by the longer h+ collection time.

10. High Energy Channel Input current parameterized
With a steady output current of ~5mA→no slew rate limitation for t input> 350ns (figure strongly dependant on rise time)

11. High Energy Channel: Pulsed Reset
The big feedback capacitor stabilizes the amplifier during the reset
Reset time<0.5us

12. Diode connection
Bypass switch activated by comparator with selectable threshold
(see AIDA design study 09/06)
Trade-off between charge injection (small transistor) and low offset (big t.)

13. Diode connection
reset LEC
reset HEC
LEC preAmp saturates