2/June/2009LHCb Upgrade1 Single ended ADC Differential ADC –Convert single ended signal to differential (use AD8138 amp) –ASIC differential output ADC interface options

2/June/2009LHCb Upgrade2 A differential ADC can be used to convert single-ended signals: –Signal applied in V in+ –V ref = V in- = ½ V DD –All codes present at output Single ended ADC

2/June/2009LHCb Upgrade3 –Analog Devices amp AD8138: Used as single-ended-to-differential amp Common-mode level shifting Buffering of the analog signal It can operate with a single 3 or 5V supply (=ADC) –Outputs are Matched and balanced differential outputs Identical amplitude 180º out of phase Single-ended to differential ADC interface Need to design single-ended-to-differential conversion stage before ADC: –Using a differential amplifier -> ease of design –Configuring a dual op amp, or –Using an RF transformer Differential amplifier circuit: conversion circuit

2/June/2009LHCb Upgrade4 Adapt the signal to the differential ADC input requirements: (1)Apply a differential offset of -0.5V to the signals -> dynamic range of 1V (2)Adjust the common mode to the same value of the ADC ASIC to differential ADC interface PS ADC conversion circuit (1)(2) PS example. It can be included inside the ASIC

2/June/2009LHCb Upgrade5 The offset dynamic range has to be the 10% of the ADC dynamic range –ASIC option: –Change of levels before ADC: –Take into account the offset values (almost all V OFF are within +/-3σ): –Which leads to an offset dynamic range: Apart from the ADC quantization noise, we should study any other source of noise (like interface circuits) Offset and noise

2/June/2009LHCb Upgrade6 Extra slides

2/June/2009LHCb Upgrade7 Analog inputs V in+, V in- –Equal but they are 180º out of phase –Equal amplitude to V ref to use the full dynamic range Common mode voltage –Determines the value at which V in+ and V in- are centered –Externally set Reference voltage V ref –Determines the range –Externally set Conversion –ADC converts only the difference between V in+ and V in- –Common mode is rejected; the ADC subtracts all signal in phase (noise,…) –Input range 2xV ref Advantages over single-ended –Common mode rejection –Better distortion performance The operation of a differential ADC