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1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( 赵 赵 ) June 1, 2011
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1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

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Page 1: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

1

Noise In Photodiode Applications

By Art Kay and Bryan Zhao ( 赵 伟 ) June 1, 2011

Page 2: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

2

Contents

• Noise Review• Photodiode Review• Photodiode Noise Theory• Bandwidth and Stability• OPA827 Hand Calculation• OPA827 Tina Spice Analysis• OPA827 Measurement Example

Page 3: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

3

Page 4: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

4

-

+

Vout

+

VinOp-Amp

Noise Model

Resistor Noise Model

Resistor Noise Model

Vout with Noise vs Time

-4

-3

-2

-1

0

1

2

3

4

5

0 2 4 6 8 10 12

Time (mS)V

ou

t (m

V)

Vout Ideal vs Time

-4

-3

-2

-1

0

1

2

3

4

0 2 4 6 8 10 12

Time (mS)

Vo

ut

(mV

)

Vin vs Time

-1.5

-1

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12

Time (mS)

Vin

(m

V)

Intrinsic Noise

• Error Source• Generated by circuit itself (not pickup)• Calculate, Simulate, and Measure

Page 5: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

5

0.04

-0.04

0.02

-0.02

0.00

Distribution of NoiseThermal Noise Measured In Time Domain

Vo

ltag

e (V

olt

s)

Counts Recorded During Measurement Period.

-0.04

-0.02

0

0.02

0.04

0.00E+00 5.00E-04 1.00E-03

Time (Sec)

Vo

ltag

e (V

olt

s)V

olt

age

(Vo

lts

)

Time (sec.)

Time Domain – White noise

normal distribution

Page 6: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

6

What is Spectral Density? (Noise per unit frequency)

T

1 1k 1M 1G

Out

put n

oise

(V

/Hz½

)

0

76n

150n

T

Time (s)

0.00 200.00m 400.00m

VG1

-999.39m

999.39m

VG2

-999.39m

999.39m

VG3

-1.00

1.00

VG4

-1.00

1.00

VG5

-999.39m

999.39m

VG6

-1.00

1.00

VG7

-999.85m

999.85m

VG8

-999.85m

999.85m

tot3

-5.49

7.46

Sum of all frequenciesRandom Noise

time

frequency

Page 7: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

7

Convert Spectral Density to RMS Convert RMS to Peak-to-Peak

fL

fH

fen2

d Erms

0.04

-0.04

0.02

-0.02

0.00

Distribution of NoiseThermal Noise Measured In Time Domain

Vo

ltag

e (V

olt

s)

Counts Recorded During Measurement Period.

-0.04

-0.02

0

0.02

0.04

0.00E+00 5.00E-04 1.00E-03V

olt

age

(Vo

lts)

Time (sec.)

0.04

-0.04

0.02

-0.02

0.00

Distribution of Noise Thermal Noise Measured In Time Domain

Vo

ltage (V

olts)

Counts Recorded During Measurement Period.

Vo

ltage (V

olts)

Time (sec.)

Page 8: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

8

Why Photodiode Noise?

• Noise is a key parameter in photodiode design– Wide bandwidth (integrate more noise)– Low signal levels (noise more critical)

• Photodiode amplifier noise is more complex– Parasitic capacitance and sensor capacitance– Poles and zeros– Gain peaking

Page 9: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

9

What can we do with the photodiode knowledge?

TI

Our new friends from National.

The competition is in trouble!

Page 10: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

10

Page 11: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

11

Photodiode Basics

Introduction • Photodiodes convert light into current or voltage.

Photodiode type• PN photodiode – more wavelength selective • PIN photodiode – wide spectral range (less selective) • APD (Avalanche photodiode) – sensitive to low light, fast

Page 12: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

12

+-

E-fieldMagnitude

E-field

NP

Photon

+-

Photon+

+

--

+

++

+-

--

-

Depletion Region

Basic Photodiode Physics

Page 13: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

13

Photodiode Basics

Figure1.3 Photodiode Equivalent Circuit

Figure1.4 Current VS. Voltage Characteristics

Page 14: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

14

Photodiode Basic

a

Io

IL

ID

I' IL

ISe

eVD

kT1

I'

o

IS : Photodiode reverse saturation current e: electron charge k: Boltzmann's constant T: Absolute temperature of the photodiode

Voc

kT

cln

IL

I'

IS

1

I

Use left equivalent circuit, the output current is given as :

The open circuit voltage Voc is the output voltage when Io equals 0. Thus Voc becomes:

Figure1.4 Photodiode Equivalent Circuit

Figure1.5 Current VS. Voltage Characteristics

Io

IL

ID

I' IL

ISe

eVD

kT1

I'

IS : Photodiode reverse saturation current

e: electron charge k: Boltzmann's constant T: Absolute temperature of the photodiode

Page 15: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

15

Photodiode and Control Source TINA model

Light exciting source: 1) Use VG1 and VG2 voltage sources to simulate light power wave. 2) Use R1 and C1 shape light signal 3) The Voltage Control Current Source (VCCS1) simulates photodiode sensitivity.Photodiode Equivalent Circuit: 1), Current Source Id simulates Dark current 2), Diode is a ideal diode 3), Cd and Rd simulate photodiode's junction capacitor and dark Resistance. 4), Rs is series resistor, which is far smaller than Rd.

Id

RdCd

Rs

Diode-

+

VCCS1 1u

+

VG1

+

VG2

C1

R1

Rf 1M

VG

Vi

Vo

-

+

Ideal OP

Simulate light source Photodiode equivalent circuit

VCCS transconductance simulate flux responsivity

Page 16: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

16

Page 17: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

17

-

+

VOUT

vn

Cf 4p

incopaRsincj

Photodiode Model

Op-Amp Model

Rf 100kvn

Resistor Model

Photo-Diode Amp Noise Model

Page 18: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

18

Photodiode noise

Thermal (Johnson Noise)

ij

4kb Tn

Rsh

Shot noise (dark)

isD 2q ID

Shot noise (w. Light)

isL 2q IL

Total Diode Current Noise

in ij2

isD2 isL

2 -

+

VOUT

vn

C1 4p

incinRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

kb Boltzmann constant 1.38*10-23J/K

q Electron Charge 1.6*10-19 C

Tn Temperature in Kelvin (25C)

fp Transconductance bandwidth

Rsh Shunt Resistance in photodiode

ID Dark Current in photodiode

IL Photo current in photodiode

Page 19: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

19

Noise Gain

Gain seen by the noise voltage source.

Noise_GainVout

Vn

Simplify the model to compute Noise Gain

-

+

VOUT

vn

Cf 4p

incopaRsincj

Photodiode Model

Op-Amp Model

Rf 100kvn

Resistor Model

-

+

VOUT

vn

Cf 4p

Cin = Cj + Copa

Rf 100k

Page 20: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

20

-

+

VOUT

vn

sCf

sCin

sRf

Node

Noise Gain

T

Frequency (Hz)

1.00 1.00k 1.00M 1.00G

Vol

tage

(V

)

-20.00

0.00

20.00

40.00

60.00

80.00

100.00

120.00

Aol

Noise Gain

fz

fp

Nodal Analysis on transimpedance amp

Vn

1

s Cin

Vn Vout Rf

Vn Vout

1

s Cf

0

Solve for noise gain Vout / Vn

Vout

Vn

Rf Cf Cin s 1

Cf Rf s 1

The numerator contains a Zero

fz1

2 Rf Cf Cin

The denominator contains a Pole

fp1

2 Rf Cf

Page 21: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

21

Noise Gain fi

Cf

Ci Cffc Intersection of the noise gain curve

with the Aol Curve

fcUnity Gain Bandwidth from Op-Amp Data Sheet

GPM 1Cin

Cf Gain Peak Magnitude

Page 22: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

22

Aol

Noise Gain

fz

fp fi

fc

T

Frequency (Hz)

1.00 1.00k 1.00M 1.00G

Outp

ut

nois

e (

V/H

)

10.00n

0.10u

200.00n 1/f region

Noise Peak

Aol

Roll Off

NoiseGainOp-Amp

Voltage Noise (Data Sheet)

Output Noise

Noise Gain

Page 23: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

23

Simulating Noise gain and noise bandwidth

AolVF3

VF1

Noise_Gain1

VF2

VF1

I_to_V_GainVF3

AM1

• Break the loop to measure Aol, 1/B, and I to V Gain

Page 24: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

24

Voltage Noise eni , eno and Eno

Region 1 Region 2 Region 3 Region 4 Region 5

ff zf pf if cfEno1 Eno2

Eno3

Eno4Eno5

eno An eni

1s

z

1fs

1s

p

1

s

i

Page 25: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

25

Voltage Noise eni , eno and Eno

22 21

2 2 22

22 2 3 323 2

22

24

2

25

ln

3

f

L

z

f

p

z

i

p

i

f

nif f fnoe f nif f

Lf

f

noe nif f nif z f

f

f

nif nif p znoe f

z zf

fnif i f

noe f nif i pff

nifnif cnoe f

f

e f fE d e f

f f

E e d e f f

e f e f fE d

f f

e C CE d e f f

C

e fe fE d

f

2c

if

2 2 2 2 2 21 2 3 4 5noe noe noe noe noe noeE E E E E E

Region 1 noise:

Region 2 noise:

Region 3 noise:

Region 4 noise:

Region 5 noise:

Total voltage noise:

Page 26: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

26

Voltage Noise eni , eno and Eno

eno

Log scale

eno

Linear scale

Eno^2

Linear scale

R.1

R.2 R.3R.4

R.5

Page 27: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

27

Resistor Noise and Current Noise

Poles Kn

1 1.57

2 1.22

3 1.16

Current noise and resistor noise are limited by the transimpedance (I-V gain) bandwidth

T

Aol

IVgain

Voltage Noise gain

Frequency (Hz)

1.00 5.48k 30.00M

Gai

n (d

B)

0.00

40.00

80.00

120.00

BWt=fp

Voltage Noise gain

IVgain

Aol

fP fBF

Small Signal BW

Noise BW

Skirt of1-Pole FilterResponse

Brickwall

Frequency (f)

0

0.1fP 10fP

-20

-40

-80

Filte

r A

tten

uati

on

(d

B)

Skirt of2-Pole FilterResponse

Skirt of3-Pole FilterResponse

BWn Kn fp

EnoR 4K T Rf BWn

Eno1 ini Rf BWn

Page 28: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

28

Page 29: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

29

Parasitic Capacitance Limits the Bandwidth

T

117dB, 317kHz

Gain

70.06

86.70

103.35

120.00

Frequency (Hz)

1 10 100 1k 10k 100k 1M 10M 100M

Phase

-90.00

-45.00

0.00

117dB, 317kHz

117dB, 317kHz

-

+OP AMP

Vo

Vp

Rf 1M

Cs 500f

Rd 50GCd 200pIG1

• Max bandwidth with Min Cf

• Low Cf may be unstable

• Wide BW increases noise

• As shown Cf=Cs (stray cap)

fp1

2 Rf Cf318kHz

Page 30: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

30

Feedback Capacitance Required for Stability

• Noise Gain is key to stability• Also called 1/Beta (in stability analysis)• ROC = Rate of Closure• ROC = (Aol slope) – (1/Beta slope)• Unstable when ROC > 20dB/decade

-

+

Rf 1M

Cin 70p

T

Frequency (Hz)1 1k 1M 1G

Gai

n (

dB

)

-20

0

20

40

60

80

100

120

Aol1/Beta

(Noise Gain)

40dB/decRate of Closure

+20dB/dec-20dB/dec

fc = 150MHzUnity Gain BW

Page 31: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

31

Feedback Capacitance Required for Stability

T

Time (s)

0.00 50.00u 100.00u 150.00u 200.00u

IG1

0.00

2.00u

Vo

-1.48

2.88

Applying a Step Input shows instability at output-

+

Rf 1M

Cin 70p

Page 32: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

32

Choosing a Minimum Cf for Stability

Frequency (Hz)1 1k 1M 1G

Gai

n (

dB

)

-20

0

20

40

60

80

100

120

fp

fc = 150MHzUnity Gain BW

fc 150MHz Op-amp Unity Gain Bandwidth

Cin 70pF Total input capacitance

Rf 1M Feedback resistance

Simplified equation forminimum feedback capAssumes Cin >> Cf

Cf

Cin

2 Rf fc272.5fF

Cc1

2 Rf fcIntermediate calculation usedin more exact formula

More exactformula forfeedbackcapacitance

Cfe

Cc

21 1

4Cin

Cc

273.1fF

-

+

Rf 1M

Cf 273f

Cin 70p

Page 33: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

33

Page 34: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

34

Noise Model for Simple Transimpedance Amp

-

+

VOUT

vn

C1 4p

incopaRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

Page 35: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

35

Example Photodiode: PDB-C158

Unfortunately Cj is not specified at Vr=0V.

We called the manufacturer for this info Cj=70pF for Vr=0V

Page 36: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

36

Calculate Diode Current Noise

-

+

VOUT

vn

C1 4p

incinRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

Thermal (Johnson Noise)

4kb Tn

Rsh10.472 10

15A

Hz

Shot noise (dark)

2q ID 25.314 1015

A

Hz

Shot noise (w. Light)

isL 2q IL 0

Total Diode Current Noise

ij2

isD2 isL

2 27.395 1015

A

Hz

kb 1.381023J

K Boltzmann constant

q 1.6021019C One electron Charge

Tn 298K Temperature in Kelvin (25C)

fp 397.887 103 Hz Transconductance bandwidth

Rsh 150106 Shunt Resistance in photodiode

ID 2 109 A Dark Current in photodiode

IL 0A Photo current in photodiode (our measurements are dark)

Page 37: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

37

OPA827 Noise Hand Calculation (key numbers)

-

+

VOUT

vn

C1 4p

incinRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

Page 38: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

38

OPA827 Noise Hand Calculation

-

+

vn

100k

4p

70pF

Photodiode Model

Op-Amp Model

18pF

0.1 1 10 100 1k 10k

100

10

1

enif

eat_f

fL

Page 39: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

39

Poles and Zeros in Noise Gain Curve

fz1

2 Rf Ci Cf 17.299 10

3 Hz

fp1

2 Rf Cf397.887 10

3 Hz

fi

Cf

Ci Cffc 956.522 10

3 Hz

fp

fz

fi

Page 40: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

40

efnorm eat_f fL 18.974 109 V

ff

efnorm2

enif2

24.931Hz

1/f (flicker) Noise Corner

fL

ff enif

eat_f

Page 41: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

41

Output Noise from OPA Noise Voltage

Enoe1 enif2ff ln

ff

fL

44.573 109 V

Enoe2 enif2fz ff 499.444 10

9 V

Enoe3

enif

fz

2fp3

fz3

3 31.828 10

6 V

Enoe4 enif

Ci Cf

Cf

2

fi fp 65.324 106 V

Enoe5

enif fc 2fi

85.479 106 V

Enoe Enoe12

Enoe22 Enoe3

2 Enoe42 Enoe5

2 112.193 106 V

Page 42: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

42

Thermal (Resistor) Noise at Output

-

+

VOUT

vn

C1 4p

incinRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

Page 43: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

43

Current Noise to Voltage Noise at Output

-

+

VOUT

vn

C1 4p

incinRsincj

Photodiode Model

Op-Amp Model

R1 100kvn

Resistor Model

Page 44: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

44

The Final Total Noise

Enoe 112.193 106 V Op-Amp Voltage Noise

EnoR 32.056 106 V Resistor Noise

EnoI 2.172106V Op-Amp Current Noise

Eno EnoR2

EnoI2 Enoe

2 116.703 106 V Total Output Noise for

OPA827 Transimpedance Amp

Page 45: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

45

Reducing Noise (Higher Cf = Lower BW & Noise)

T

Frequency (Hz)10k 100k 1M 10M 100M

I to

V G

ain

(dB

)

20

40

60

80

100

120

Cf=2pF

Cf=4pF

T

Frequency (Hz)1 4k 20M

Out

put n

oise

(V

/Hz½

)

0

76n

150n Cf=2pF

Cf=4pF

T

Frequency (Hz)1 271 74k 20M

Tot

al n

oise

(V

)

0

90u

180u

Cf=2pF

Cf=4pF

Page 46: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

46

Flux Capacitor

• Advantage– Reduces noise to zero– Available on E-Bay

• Disadvantage – Requires 1.21 gigawatts– Size

ADS1118

Quarter

Page 47: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

47

Page 48: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

48

OPA827 – test the model

V+V-

V-

V+J1

-

+ +3

2

7

4

6

U1 OPA827V1 18 V2 18

-

+CCVS1 1+ VG1

voltage noise

Current noise

T

Current noise density

voltage noise density

Frequency (Hz)

100.00m 1.00 10.00 100.00 1.00k 10.00k

Current noise

1.00E-15

2.00E-15

3.00E-15

voltage noise

1.00n

10.00n

100.00n

Current noise density

voltage noise density

The noise performance is the same as datasheet.

Low Noise : 4nV/√Hz at 1kHz Low Offset Voltage: 150µV max JFET Input: IB= 15pA typ

Wide Bandwidth: 22MHz

Page 49: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

49

Simulated Spectral Density and Total Noise

V-

V+

-

+ +3

2

7

4

6

U1 OPA827

Cd 70p

R2 100k

C1 4p

Vo

T

Frequency (Hz)

200.00m 2.00k 20.00M

Out

put

nois

e (V

/Hz?

)

6.64n

42.42n

78.19n

T

Frequency (Hz)

200.00m 2.00k 20.00M

Tota

l nois

e (

V)

0.00

54.54u

109.09u

Output Noise Density

Total Output NoiseCalculated

(rms)

Simulated

(rms)

116.7uV 109.1uV

Page 50: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

50

Page 51: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

51

Validating Test Equipment Capability

Spectrum AnalyzerOr Scope

OPA827 Transimpedance

Amp

Noise Spectral Density = 3.8nV/rtHzTotal Noise = 109.1uVrms = 654.6uVp-p

Photodiode (No light)

What is the noise floor?

Page 52: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

52

Tektronix DPO 4034 Oscilloscope

STDEV: 48uV (same as RMS)P-P: 6.6*STDEV=319uV40s P-P: 320uV

1) Set DC couple, 20MHz bandwidth limit2) Short input channel to measure noise floor

Page 53: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

53

Agilent 4395A Spectrum Analyzer1. Frequency Range: 10Hz~500MHz2. Noise floor: 10nV/rtHz3. Input Impedance: 50Ω

Reference voltage Noise Floor:

10nV/rtHz

Page 54: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

54

The Noise Floors are Not Good Enough

Spectrum AnalyzerOr Scope

OPA827 Transimpedance

Amp

Noise Spectral Density = 3.8nV/rtHzTotal noise = 109.1uVrms = 654.6uVp-p

Photodiode (No light)

Noise Floors:Scope (1mV range)= 48uV rms =320uVpp

4395A Spect. Analyzer = 10nV/rtHz

Page 55: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

55

Solution: Use A Post AmpWhat amp do we Choose?

Spectrum AnalyzerOr Scope

OPA827 Transimpedance

Amp

Noise Spectral Density = 3.8nV/rtHzTotal noise = 109.1uVrms = 654.6uVp-p

Photodiode (No light)

Noise Floors:Scope (1mV range)= 48uVrms = 320uVp-p

4395A Spect. Analyzer = 10nV/rtHz

Post Amp

Gain BW = 22MHz

Need Gain > 10

BW> 22MHz (in gain)Noise < 3.8nV/rtHz

Amplified Signal from OPA827With Negligible additional noise

Page 56: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

Use OPA847 as post amplifierWideband, Ultra-Low Noise, Voltage Feedback, Operational Amplifier

• At the gain of 150, the bandwidth is 26MHz

• 0.85nV/Hz Input Voltage Noise

• 2.5pA/Hz Input Current Noise

• ±100uV Input Offset Voltage (Typical))

575uVrms post amplifier noise in 20MHz bandwidth.

T

Frequency (Hz)

200.00m 2.00k 20.00M

Tot

al n

oise

(V

)

0.00

287.74u

575.48u

Post Amplifier Output Noise

V-

V+

R5 1.49k

R4 10

+

-

+DIS

U2 OPA847

R1 50 C1 33u Vo

Vin

Page 57: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

57

T

Frequency (Hz)

1 100 10k 1M 20M

Vn827

0.00

109.54u

Vn_post

0.00

16.03m

Post AmplifierRelatively small error!

Vn827 = 109.4uV rms

Vnpost = 16.03mV rms

Vnpost/Gain = 16.03mV/150 = 106.8uV rms

Gain=150

Page 58: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

58

Test the Noise Floor

Spectrum AnalyzerOr Scope

Post Amp

Disconnect The DUT

(Transimpedance amp)

GND input Measure Noise

Page 59: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

59

Test The Noise Floor – Post Amp Noise Scope

Simulated

(rms)

Measured

(rms)

575uV 518uV

STDEV: 518uVP-P: 6.6*STDEV=3.4mV40s P-P: 3.88mV

Page 60: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

60

Hardware Connections

V-V+

V-

V+

-

+ +3

2

7

4

6

U1 OPA827

Cd 70p

R2 100k

C1 4p

Vo

R5 1.5k

R4 10

+

-

+DIS

U2 OPA847

VF1

1. PDB-C158-ND photodiode2. 70pF junction capacitance at Vr=0 V3. 100dB I-V gain4. 4pF compensation capacitor5. ±5V power supply.

Page 61: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

61

Shield the Circuit if Possible

Power SupplyVcc,Vss,GND

Input & Output BNC connectors

Shield

Page 62: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

62

Divide by Gain for OPA827 Output Noise

Page 63: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

63

Measured Total Output Noise (DPO 4034 Scope)

21.7

1500.145 mV =145uV

OPA847 Measured at Scope:STDEV: 21.7mVP-P: 6.6*STDEV=143mV40s P-P: 170mV

At OPA827 (DUT) Output:Divide by gainVn827 = 21.7mV/150 = 144.6uV

Calculated

(rms)

Simulated

(rms)

Measured

(rms)

116.7uV 109.1uV 144.6uV

Page 64: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

64

Measured Spectral Density 4395A Spectrum Analyzer

Linear Scale

1. Agilent 4395A Spectrum Analyzer test 1Hz~20MHz span, 3uV/div, REF=24uV.

2. The tested noise density curve shape is the same as simulation.

Measurement 4395A

Linear Scale

T

Frequency (Hz)

0.00 2.00M 4.00M 6.00M 8.00M 10.00M12.00M14.00M16.00M18.00M20.00M

Outp

ut

nois

e (

V/H

z?)

-6.00u

-3.00u

0.00

3.00u

6.00u

9.00u

12.00u

15.00u

18.00u

21.00u

24.00u

Tina Spice

Linear Scale

T

Frequency (Hz)

200.00m 2.00 20.00 200.00 2.00k 20.00k 200.00k 2.00M 20.00M

Out

put

nois

e (V

/Hz?

)

0.00

3.00u

6.00u

9.00u

12.00u

Tina Spice

Log Scale

Page 65: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

65

OPA827-Noise Density 4395A Spectrum Analyzer

T

Frequency (Hz)

10k 309k 608k 907k 1.2M 1.5M 1.8M 2.1M 2.4M 2.7M 3.0M

Ou

tpu

t n

ois

e (V

/rtH

z)

(29KHz, 22.7nV/rtHz)

(552KHz, 46.7nV/rtHz)

(2MHz, 24.7nV/rtHz)

(3MHz, 16.7nV/rtHz)

(29KHz, 20.7nV/rtHz)

(2MHz, 22.3nV/rtHz)

(3MHz, 14.3nV/rtHz)

(552KHz, 40nV/rtHz)

Simulated

MeasuredO

utp

ut

no

ise

(V/r

tHz)

Page 66: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

66

Thanks• Bryan Zhao ( 赵 伟 )• Matt Hann• Collin Wells, Peter Semig, Curtis Mayberry

References• Jerald Graeme <Photodiode Amplifiers>• Art Kay < Op-Amp Noise Calculation and Measurement >• HAMAMATSU <Photodiode Technical Information>• Tim Green <Operational amp stability>

Before Questions, I must ask….

Page 67: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

67

You will move to a new topic my young apprentice

More noise presentations have we must

Do I move over to the dark side?

Page 68: 1 Noise In Photodiode Applications By Art Kay and Bryan Zhao ( ) June 1, 2011.

68

Contact Info:

[email protected]

520 750-2150 (phone)

520 208-1595 (cell)