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Amplifier & ADC Interfacing: Tricks of the Trade

John Oates CIFR ApplicationsSeptember 2011

2

Agenda

Pipeline ADC FrontendsAmplifier TypesFilter TopologiesFilter DesignTricks of the Trade

Kick-back ControlCommon mode filteringCap shiftingCap splittingImpedance MatchingNarrowband Resonance

Some Case Studies

3

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

UnbufferedInput Impedance set by Switched-Capacitor DesignLower PowerInput Impedance varies over time (sample clock – Track and Hold)Charge Injection from sample caps kick back onto input network

BufferedHighly Linear Buffer but requires more powerEasier to design input network to interface high impedance buffer

since it provides a fixed input termination resistanceBuffer provides isolation between sample caps and input network

resulting in reduced charge injection transients

4

5

Pipeline ADC Types

SHA

Vcmin

Vcmin

AVcc

AVcc

Gnd

InternalSampleClock

VIN+

VIN-

n

ESD

ESD

Input Switch

Input Switch Sampling

Switches

Flip-Around Switch

Flip-Around Switch

Sampling Cap

Sampling Cap

InternalInputClock

InternalInputClock

SHA

Vcmin

Vcmin

Gnd

InternalSampleClock

n

Input Switch

Input Switch Sampling

Switches

Flip-Around Switch

Flip-Around Switch

Sampling Cap

Sampling Cap

InternalInputClock

InternalInputClock

VIN+

VIN-

AVcc

AVcc

ESD

ESD

AVccAVcc

InternalBufferStage

R

R

Unbuffered

Buffered

6

Buffered ADC Input Impedance - Real

7

Buffered ADC Input Impedance - Imaginary

Switched-Capacitor ADC

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150 200 250 300 350 400 450 500

Frequency (MHz)

Par

alle

l Res

ista

nce

(K

oh

m)

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Par

alle

l Cap

acit

ance

(p

F)

Rpar (kohm) Track Mode

Cpar (pF) Track Mode

R

ADC Internal Input Z

R || jX

Parallel Configuration

VIN-

VIN+

jX

9

ADC Drivers

Low ZoutHigh Zout Defined Zout

10

The Open Collector (High-Zout) Amplifier

AD8375 or AD8376Bias 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

11

The Defined Zout Amplifier

ADL5201 – Has a fixed differential Zout of 150ohms.

12

The Op-Amp Style (Low-Zout) Amplifier

ADL5562, ADL5565, AD8366

13

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)

14

Filter Specification

1E7 1E81E6 1E9

-80

-70

-60

-50

-40

-30

-20

-10

-90

0

freq, Hz

dB(S

(2,1

))

10 20 30 40 50 60 70 80 900 100

-9

-8

-7

-6

-5

-4

-3

-2

-1

-10

0

freq, MHz

dB(S

(2,1

))

Forward Transmission, dB Zoomed Forward Transmission, dB

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0 1.0

-150

-100

-50

0

50

100

150

-200

200

freq, GHz

phas

e(S(2

,1))

20 40 60 80 100 120 140 160 1800 200

2.0E-9

4.0E-9

6.0E-9

8.0E-9

1.0E-8

0.0

1.2E-8

freq, MHz

dela

y(2,

1)

Forward Transmission, degrees Group Delay, sec.

Corner Frequency

Pass-band Ripple

Phase Linearity Group

Delay

dB

f

15

Various Filter Types

16

Butterworth (aka Maximally Flat)

LL2

R=1e-12 OhmL=225.299997 nH

LL1

R=1e-12 OhmL=167.043428 nH

TermTerm1

Z=50 OhmNum=1

TermTerm2

Z=50 OhmNum=2

CC2C=25.120626 pF

CC1C=33.88147 pF

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(4

,3))

20 40 60 80 100 120 140 160 1800 200

1E-9

2E-9

3E-9

4E-9

5E-9

0

6E-9

freq, MHz

dela

y(4,

3)

17

Chebyshev

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(2

,1))

20 40 60 80 100 120 140 160 1800 200

2.0E-9

4.0E-9

6.0E-9

8.0E-9

1.0E-8

0.0

1.2E-8

freq, MHz

dela

y(2,

1)

LL3

R=1e-12 OhmL=124.188765 nH

LL4

R=1e-12 OhmL=51.440671 nH

CC4C=49.675506 pF

CC3C=20.576269 pF

TermTerm4

Z=50 OhmNum=4

TermTerm3

Z=50 OhmNum=3

18

Elliptical

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(6

,5))

20 40 60 80 100 120 140 160 1800 200

2.0E-9

4.0E-9

6.0E-9

8.0E-9

1.0E-8

0.0

1.2E-8

freq, MHz

dela

y(6,

5)

CC6C=1.926709 pF

CC5C=53.668271 pF

LL5

R=1e-12 OhmL=100.545205 nH

TermTerm5

Z=50 OhmNum=5

TermTerm6

Z=50 OhmNum=6

CC7C=58.880737 pF

LL6

R=1e-12 OhmL=101.220508 nH

19

Inverse Chebyshev (aka Type II)

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(8

,7))

20 40 60 80 100

120

140

160

180

0 200

1E-9

2E-9

3E-9

4E-9

5E-9

0

6E-9

freq, MHz

dela

y(8,

7)

TermTerm7

Z=50 OhmNum=7

TermTerm8

Z=50 OhmNum=8

LL8

R=1e-12 OhmL=51.25765 nH

CC10C=49.016707 pF

LL7

R=1e-12 OhmL=120.822775 nH

CC9C=697.379292 fF

CC8C=19.815464 pF

20

Bessel

TermTerm9

Z=50 OhmNum=9

CC11C=12.697234 pF

LL9

R=1e-12 OhmL=11.017196 nH

TermTerm10

Z=50 OhmNum=10

CC12C=42.298236 pF

LL10

R=1e-12 OhmL=51.047423 nH

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(1

0,9

))

20 40 60 80 100

120

140

160

180

0 200

2.0E-104.0E-106.0E-108.0E-101.0E-91.2E-91.4E-91.6E-91.8E-92.0E-92.2E-92.4E-92.6E-92.8E-93.0E-93.2E-93.4E-93.6E-93.8E-9

0.0

4.0E-9

freq, MHz

dela

y(10

,9)

21

Gaussian

20 40 60 80 100

120

140

160

180

0 200

2.0E-104.0E-106.0E-108.0E-101.0E-91.2E-91.4E-91.6E-91.8E-92.0E-92.2E-92.4E-92.6E-92.8E-93.0E-93.2E-93.4E-93.6E-93.8E-9

0.0

4.0E-9

freq, MHz

dela

y(10

,9)

1E7 1E81E6 1E9

-40

-30

-20

-10

-50

0

freq, Hz

dB

(S(1

0,9

))

CC13C=9.722397 pF

CC14C=41.165855 pF

LL11

R=1e-12 OhmL=8.121478 nH

LL12

R=1e-12 OhmL=42.727503 nH

TermTerm11

Z=50 OhmNum=11

TermTerm12

Z=50 OhmNum=12

22

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.

23

Step 1 & 2 - Butterworth Response

24

Step 3 - Normalized Prototype Filter Tables

25

Step 3 - Normalized Prototype Filter Tables

26

Step 3 - Normalized Prototype Filter Tables

27

Step 3 - Normalized Prototype Filter Tables

28

Step 4 - Frequency and Impedance Scaling Equations

Lc

nscaled Rf

CC

2

c

Lnscaled f

RLL

2

Summary - Differential Filter Implementation

Steps1) Select Filter Topology and

Order2) Look Up Normalized

Prototype Values for source and load impedances

3) Scale Normalized Prototype Values by Frequency and Load

4) Convert Single-Ended Equivalent to Differential by Splitting Series Reactances

Lc

nscaled Rf

CC

2

c

Lnscaled f

RLL

2

30

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)GenesysMicrowave OfficeAADEAppCADNuHertz FilterFreeQucsPspice/Hspice

31

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 ADCn=?

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).

32

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)

33

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.

34

Differential vs Common-mode Capacitor ComparisonSFDR Improvement

35

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!

36

Tricks of the Trade: Cap Splitting

Another technique that can be used to increase kickback-control while providing common-mode filtering.

37

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.

Passband Zoom

72 82 9262 102

-6

-5

-4

-3

-2

-1

-7

0

freq, MHz

dB(S

(2,1

))V

_atte

nV

_atte

n2

38

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 = 140MHzSet 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.

39

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. m3

freq=dB(S(2,1))=-5.135

2.000MHzm4freq=dB(S(2,1))=-5.961

200.0MHz

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0 1.0

-50

-40

-30

-20

-10

-60

0

freq, GHz

dB(S

(2,1

))

Readout

m3

Readout

m4

m3freq=dB(S(2,1))=-5.135

2.000MHzm4freq=dB(S(2,1))=-5.961

200.0MHz

40

Tricks of the Trade Summary

Filter TopologyThis matters!

De-Qing Circuitry (Kickback Control)Reduces distortion (improves SFDR)Limits bandwidth (harder to drive at higher frequencies)

Common-mode CapacitorsProvides 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 SFDRCap SplittingNarrowband ResonanceAbsorbing the ADC input Cap into your Filter

41

Example – High Zout Amplifier

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.0 1.0

-80

-60

-40

-20

0

-100

20

freq, GHz

dB(S

(2,1

))

Readout

m1

Readout

m2

Readout

m3

m1freq=dB(S(2,1))=18.177

154.0MHzm2freq=dB(S(2,1))=17.800

140.0MHzm3freq=dB(S(2,1))=17.884

167.0MHz

110 120 130 140 150 160 170 180 190100 200

123456789

10111213141516171819

0

20

freq, MHz

dB

(S(2

,1))

Readout

m4

Readout

m5

Readout

m6

m4freq=dB(S(2,1))=18.177

154.0MHzm5freq=dB(S(2,1))=17.800

140.0MHzm6freq=dB(S(2,1))=17.884

167.0MHz

Interfacing – Cut & Paste!

42

30 ohms 30 ohms 200 ohms200 ohmsFilter 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)