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Requirement: Sample Depth

The SCAs shall have a depth of 128 time samples.


The depth is dictated by the electron cloud spread.

Table: SCA specifications and requirements

Requirement: Stored Charge Retention

Charge leakage or injection on each SCA capacitor shall be less than .05% during the readout interval ( 10 ms).


The stored charge leakage requirement arises because the charge must be retained on a capacitor to within 1/2 least significant quantization bit independently of when in the readout cycle it is actually read. This could be as long as 10 ms.

Requirement: Readout Procedure

The SCA shall be in read mode when the R/W signal is active in the R (read) state. Readout shall be initiated when a token is received the two TI (token input) line. Readout shall occur at the clock rate (approximately 3 MHz or 330 ns per sample). Channel 1 through channel 16 are multiplexed onto the analog signal output line. After each channel has been read out, the TO (token output) line shall go active enabling passing of the token to the next SCA.

All channels shall read out the same sample number while the token is active. A channel read consists of switching a channel capacitance array into the readout operational amplifier (op-amp) which drives the analog signal bus line for one clock interval. When the token is not active, the SCA amplifier shall be disconnected from the analog signal output line. When disconnected impedance shall be at least 10 M.


The readout clock rate is close to the minimum needed to operate the SVT at the required 100 Hz service rate ( <10 ms readout time is available). The Hybrid area is limited. Multiplexing channel analog signal output lines is needed to reduce the number of traces and minimize the number of pads on the Hybrid. Token passing permits SCA controls to be daisy chained. This makes external multiplexing switches unnecessary and reduces the number of control signals, Hybrid traces and power consumption.

Requirement: Multiplexing

Each SCA shall be connected to (multiplexed with) 14 other SCAs (fifteen in all) and shall feed into a common low gain amplifier to an analog signal output line. Each SCA has 16 channels. This provides a total multiplex ratio of 240:1. Only one SCA, the one with the active token, shall drive the amplifier at any time.


Each anode side of the SDD is served by one Hybrid and has 240 channels. The number of channels per SCA was set to 16. This is a compromise among issues of cost, production yield and chip size for the best available chip feature size (1.2 m) technology. The 240 channels will require 15 SCAs of 16 channels each. The 15 SCAs are multiplexed onto a common analog signal line to reduce the number of Hybrid signal lines from 15 to 1. A Hybrid multiplex ratio of 240 to 1 reduces power and size by avoiding the need for 16 op-amp drivers.


Under discussion. ASIC includes correction by default.

Requirement: Token Passing

The first SCA shall receive an external token on a TI line as a START signal. All TOs of each SCA but the last are connected to the TI input of the following SCA. The START signal shall be initiated after every 240 readouts.


Token passing simplifies the control structure and minimizes the number of interface signals in a interconnection-limited area.

Requirement: Readout Termination

The readout of each SCA shall be controlled by externally metering the clock during readout. (It is possible to repeat the readout sequence indefinitely, though the storage capacitor charges will degrade with time.)


The readout may be interrupted at any time. If there are fewer than 4096 clocks per SCA, then all read values will be unique. This is desirable because the SCA can be returned to sampling (placed in the write mode) earlier to improve the event service rate. External readout control simplifies the SCA design and permits more flexible control.

next up previous
Next: Settling Time Up: SCA Previous: Sampling

Claude Andre Pruneau
Thu Oct 12 17:29:54 EDT 1995