Abstract
This paper describes a front-end for hybrid pixel readoutchips, which was developed for the Timepix3 and Smallpix ASICs. Thefront-end contains a single-ended preamplifier with a structure for leakagecurrent compensation which can handle both signal polarities, and asingle-threshold discriminator with compensation for pixel-to-pixelmismatch. Preamplifier and discriminator are required to be fast, to allow aTime-Of-Arrival (TOA) measurement with a resolution of 1.56 ns.Time-Over-Threshold (TOT) is also measured; the monotonicity of TOT withrespect to the input charge is greatly improved as compared to the previousTimepix chip. The analog area is only 55 μm × 13.5 μm. Timepix3has already been fabricated and the first test results are also presented inthis paper.
Highlights
TOA is measured using a reference clock at a frequency of 40 MHz for the most significant 14 bits; the additional 4 bits of fine TOA measurement are provided by some Voltage-Controlled Oscillators (VCO), which are distributed across the pixel matrix
In the latter mode of operation, the number of hits is stored in a 10 bit register, while another 14 bit register computes the integrated TOT of all hits occurring during a period of the external shutter signal
A requirement that marks a difference between Timepix3 and the previous Timepix is that the measurement of TOT vs input charge must be monotonic for large positive charges up to 300 kh+
Summary
Because the measured quantity is time, signal amplitude linearity is not required. TOT must be monotonic for large positive charges and the TOA measurement must be adequate to the fine VCO timing resolution. The time-to-peak of the front-end should be in the order of 25 ns, in order to yield a sufficiently low timewalk. This matches Large Hadron Collider (LHC) applications, where the main clock has a frequency of 40 MHz, and is important in view of the VELOpix chip. Each pixel is equipped with a threshold-equalization DAC (Digital-to-Analog Converter), which corrects for pixel-to-pixel mismatches. The quadratic sum of noise and residual mismatch after equalization should be in the order of 90 e− r.m.s. or less
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