Amplifier & ADC Interfacing: Tricks of the Trade John Oates CIFR Applications September 2011
Agenda • Pipeline ADC Frontends • Amplifier Types • Filter Topologies • Filter Design • Tricks of the Trade • Kick-back Control • Common mode filtering • Cap shifting • Cap splitting • Impedance Matching • Narrowband Resonance • Some Case Studies
The Basic Problem • The Amplifier desires to see a certain load impedance. (ZL) • The ADC desires to see a certain source impedance. (ZS) These impedances are NOT equal! ? ZL ZS
Two Types of ADC Input Architectures • Unbuffered • Input Impedance set by Switched-Capacitor Design • Lower Power • Input Impedance varies over time (sample clock – Track and Hold) • Charge Injection from sample caps kick back onto input network • Buffered • Highly Linear Buffer but requires more power • Easier to design input network to interface high impedance buffer since it provides a fixed input termination resistance • Buffer provides isolation between sample caps and input network resulting in reduced charge injection transients
Pipeline ADC Types Unbuffered Buffered
ADC Drivers High Zout Defined Zout Low Zout
The Open Collector (High-Zout) Amplifier • AD8375 or AD8376 • Bias Inductors provide inherent band-pass response along with AC coupling capacitors. • For DC Coupling this could be a problem. • Allows Rs=RL for easy filter design
The Defined Zout Amplifier • ADL5201 – Has a fixed differential Zout of 150ohms.
The Op-Amp Style (Low-Zout) Amplifier • ADL5562, ADL5565, AD8366
Filter Types & Topologies • Since we are primarily looking to provide anti-aliasing, we usually employ low pass or band-pass filters between the drive amplifier and ADC. • If Anti-Aliasing is not of concern, often LO rejection or specific interferer frequencies are. • Topologies: • Low Pass, High Pass, Band Pass, Band Stop (notch) • Butterworth, Chebyshev I & Chebyshev II. Eliptical (Cauer), Bessel • Typical Filter Specs & Trends: • Passband/Stopband Frequencies (MORE COWBELL BANDWIDTH) • Ap = Passband Attenuation (Insertion Loss) • As = Stopband Attenuation (Rejection) • Passband Ripple (Typ. 0.5-1dB) • Group Delay Variation (<10ns)
f Filter Specification Corner Frequency Pass-band Ripple dB Phase Linearity Group Delay
Steps to Design a Filter – Lookup Table Method • 1) Define Desired Response and Appropriate Filter Type • ω = rejection frequency ωc = 3dB cutoff frequency • 2) Calculate ω/ωc and determine order necessary for attenuation target using appropriate attenuation chart • 3) Calculate Rs/Rl or Rl/Rs and look into correct table to obtain coefficients • 4) Scale cofficients by Frequency & Impedance (using scaling equations for topology chosen) • 5) Transform (if necessary) to high-pass or bandpass and/or to differential.
Summary - Differential Filter Implementation • Steps • Select Filter Topology and Order • Look Up Normalized Prototype Values for source and load impedances • Scale Normalized Prototype Values by Frequency and Load • Convert Single-Ended Equivalent to Differential by Splitting Series Reactances
Filter Design/Network Design Tools • Old Fashion Paper and Pencil • Crude Excel Spreadsheet Approach • Low-Cost Filter Software, MathCad, Matlab…. • Agilent’s Advance Design System (ADS) • Genesys • Microwave Office • AADE • AppCAD • NuHertzFilterFree • Qucs • Pspice/Hspice
Tricks of the Trade: Filter topology matters • Series or Shunt element first? • End with which element? • Best to choose a topology that ends with a shunt C next to ADC • n=? • Should be the lowest order possible to meet selectivity requirements. • Ripple? • <=1dB typical. Can depend on DSP algorithms. • Amplitude & Phase Balance? • Component Tolerance & Matching? • Heavily effects the above mentioned AM & PM as well as overall frequency response. • Group Delay? • This matters for Modulation Quality (ISI & EVM).
Tricks of the Trade: Kick-back Control • Switch-Capacitor circuits kick-back charge currents onto the input network. These currents create transient voltage offsets on the input signal which causes distortion. • Given enough settling-time, the distortion of the currents on the input signal can be minimized. (SFDR)
Tricks of the Trade: Common-mode Capacitors • Often SFDR is set by HD2 and HD3. Single-ended HD2 and HD3 can come from the drive amplifier itself. Using some common mode capacitors to ground provides common mode filtering for these distortion products. • Both single-ended and differential filtering are important.
Tricks of the Trade: Cap Shifting • For Kick-back control, a differential capacitor as close as possible to the Ain+ Ain- pins can increase SFDR. • Capacitor location does not effect frequency response! Shift that Cap!
Tricks of the Trade: Cap Splitting • Another technique that can be used to increase kickback-control while providing common-mode filtering.
Tricks of the Trade: Impedance Matching • The Amplifier and ADC are voltage devices. Impedance matching (for max power transfer) is not always of primary importance. We care more about voltage amplitude.
Tricks of the Trade: Narrowband Resonance • For narrow bands, it is possible to choose an inductor to resonate out the ADC input capacitance. This allows the amplifier to see a purely resistive load. • Remember, a parallel resonant tank is an open circuit at Fr. • Let Fc=Fr 3 kohm || 3pF @ IF = 140MHz Set XC = XL @ Fc=140Mhz 1/(2*pi*f*C)=2*pi*f*L Solve for L L=431 nH **Try adding extra diff cap and calculating L value needed to resonate out C1+C2. Helps distortion.
Tricks of the Trade: Using the ADC as part of your Filter • For higher frequencies and load impedances, the capacitors in your filter design can become very small. In some cases the ADC input capacitance can be used to develop your last pole.
Tricks of the Trade Summary • Filter Topology • This matters! • De-Qing Circuitry (Kickback Control) • Reduces distortion (improves SFDR) • Limits bandwidth (harder to drive at higher frequencies) • Common-mode Capacitors • Provides common-mode path to ground. Provide common-mode filtering for possible amplifier HD2 distortions HD2. • Cap Shifting • A differential Cap close to the Ain pins can increase SFDR • Cap Splitting • Narrowband Resonance • Absorbing the ADC input Cap into your Filter
Interfacing – Cut & Paste! 30 ohms 200 ohms 30 ohms 200 ohms • Filter Design is all about Rs & Rl • Rs & Rl repeating allows for filter design reuse while doubling the stopband rejection. • Splitting the filter across the DGA is a good technique to achieve higher rejection or bandpass response without having to place a high Q circuit in front of the switched capacitor ADC input. (High Q can create resonance with charge kickback causing issues)