1 Ron LaTour Applications Engineer Photo Diode Signal Paths Using Decompensated Amplifiers.

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Ron LaTour

Applications Engineer

Photo Diode Signal Paths Using Decompensated Amplifiers

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© 2008 National Semiconductor Corporation

Customer Focus

• Learning's From Recent customer circuits• Photodiode signal path• Fill-in

– Clock Buffer differentiator– Comparator >1V Hysteresis– Comparator High and Low Side Sense

• 10 Question Quiz• Send Apps Group best customer or personal favorite

Op Amp circuit– Recent– Favorite all time

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Photodiode Signal Path Application

• Replace existing solution– Dual TIA, diff stage and bandpass filter

• NSC offered the LMV794/797

• Customer encounters– Oscillation - Bandpass– Gain peaking - TIA– Transient Recovery -TIA

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Bandpass - Schematic

• LM833 Vcc =5V– $.2

• LMV794– Oscillates– 797 won’t give

desired Q = 7

• Considering– Max4453 200 MHz

5


Bench Verify: Bandpass Oscillation Problem

• LMV794 vs LMV797

• LMV794 w/ Riso 1K, 2K

• Tried input R/C

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Intuitive Step: Good Practices when Analyzing a High Order or Multi-pole

Circuit?

• First simplify the circuit for an intuitive understanding • Use simple algebra to find the poles and zeros

• Use a simulator like Altium trying the simplified circuit then check the full circuit and the simulator results

• Finally go to the bench, build the circuit and take photos

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Photodiode – Signal Path Specs

• First stage Transimpedance Amp – Photoconductive topology

• Cap coupled photodiode, no precision

– High Gain RF ~1M @ 400 KHz – Single supply Vcc = 5V– Two identical TIA photodiode circuits

• 5-500 KHz • sq wave or pulse• Changing duty cycle from 10% to 100%

– Good input transient recovery– Low high freq noise

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Problem – LMV794 vs LMV797

• Is the LMV794 worth the $ difference over the LMV797?

• Will either work just as well?

• Packaging/layout - 2 duals or 1 dual and 2 singles?

• Poor High Freq Noise?

• Max Gain in one stage?

• Precision for free?

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Circuit Blocks

TIA

Diff

BP

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Intuitive Step: Which Values for Rf, Cf, Rg Determine a Single Pole Rolloff or Low

Pass Transfer Function?

• Rg=open(100M), Rf=1K, Cf=.1uF

• Rg=1K, Rf=open(100M), Cf =.1uF

• Rg=1K, Rf=1M, Cf= .1uF

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Intuitive Step: Which Values for Rf, Cf, Rg Cause Instability in the Transfer

Function?

Cg=1nF, Rf =1M, Cf=open

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Photodiode – Schematic?

• Guess– Introduced

Resistive “Tee”

– Capacitive “Tee”

– Customer cap 22pF

– Single supply bias

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Photodiode – Build a Tool?

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Formulas

• Some math and formulas provide insight

• Some Do Not (not shown)

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Photodiode – Bode Plot Compare the 797 &794

LMV797 any Cin/Cf ratio for min noise BWLMV794 ratio >10 or 20 dB

1/B = 1+ Rf/Rsh

Aol

Signal Bandwidth

1/B = 1 + Cin/Cf

LMV797 GBW =17 MHzPM =45 d

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)

LMV794 GBW = 88 MHzPM =45 d @ Acl =20dB

Fp = 1/2piCfRf

10M

Mag

(dB

)

100M1M100K10K1K100

Uncompensated PM = 0 d => OscillationStray C => peaking

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Intuitive step: What is the Application Total Phase Margin?

Phase Margin = The op amp open loop phase (-90 deg) plus 1/B phase at the point of intersection -180 degrees (negative feedback)

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Transimpedance Amp

critically dampedPM = 45 d

Transimpedance gainVo = -If * Rf

LMV794

Fp = 1/2piCfRf

damping or peaking

ROC = 20dB

ROC = 40dB

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)10M

Mag

(dB

)

100M1M100K10K1K100

LMV797

A

B

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Intuitive Step: Are these TIA Circuits Stable over the Freq Range Shown?

Transimpedance gainVo = -If * Rf

LMV794

ROC = 40dB

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)10M

Mag

(dB

)

100M1M100K10K1K100

LMV797

a) Yes LMV797

b) No LMV794

c) No both

d) Yes both

e) Yes a) and b)

FcFzf Fi

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What is the Advantage of the LMV794 over the LMV797?

Aol

Signal Bandwidth

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)10M

Mag

(dB

)

100M1M100K10K1K100

15 dB

5x

LMV797

LMV794

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TIA w/LMV794 Data

• Customer sees LMV794 as no better than LMV797

• Noise is a concern• Signal bandwidth/cost

advantage not understood

• No need for transient clamps with LMV794 or 797

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Follow Up

• Selling the customer– FAE on the bench with him taking data. – Poor results– Follow up

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Diff amp – Application Specs

• Customer needed – Differential Gain~20-50 @ 500 KHz– Single supply Vcc = 5V– Two TIA photodiode circuits are input signal

V1 and signal V2

– No precision

• Offered LMV797, LMV794– Which is best? Lowest cost?

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Diff Amp – Schematic Guess

• Gain of 20x

• Customer will use LMV797

• Customer is clear on how to use the part in a differential amp from the data sheet and apps notes

• Does he see that the LMV794 for Gain >10x will always be better?

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Intuitive Step: Which are Best Design Practices for Bandpass

Filters?

• Design Q<5 single stages

• Design without using the Op amp pole as part of the circuit

• Design using >20Q2 op amp GBW

• Use PSpice to investigate

• Build on the bench

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Tools

• Tools don’t work well with decompensated Op Amps

• Fixed Unity gain only

LMV792

LMH6628

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Bench Comparison w LM833

LMV794 @ 5V compensated

[email protected]

LM833 @ 12V

LM833 @ 4.2V

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Bandpass - Bode Plot

LMV797 any Cin/Cf ratio for min noise BWLMV794 ratio >10 or 20 dB

1/B = 1+ Rf/Rg

Aol

Signal Bandwidth

1/B = 1 + Cin/Cf

LMV797 GBW =17 MHzPM =45 d

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)

LMV794 GBW = 88 MHzPM =45 d @ Acl =20dB

Fp = 1/2piCfRf

10M

Mag

(dB

)

100M1M100K10K1K100

Fz = 1/2piCiRf

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Customer Data – PQA – Application Specs

• Small % bad LMC6035 parts• What’s wrong with them?

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Clock Buffer - Schematic

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Clock – Differentiator

Aol

ROC = 40dB after Closed loop plot intersects Aopen loop

critically dampedPM = 45 d

Fz = 1/2pi10pF*4M = 4 KHz@20dB/dec

Phase is -90 +90 = 0 d

~Fp=1/2pi2nF*200

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A Little Digging

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Bench Results

• This is one fix with very minor affect on slew rate

• All LMC6035 parts respond properly

• I recommended Cf =< 1pF to the designer instead

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Comparator Hysteresis - Specs

• Li-Ion Battery pack threshold sense

• ~1.5V Hysteresis

• Reset on drop below hysteresis

• Use range ~2.7 to 11.5VVref = 2.048V

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Comparator Hysteresis - Design

• Input 2 cell li-ion 6.7V drops to Vtrip @ 5.2V – ~1.5V Hysteresis

• For 100mV Hyst – Gain =1/15– R1/R3= 1/g –VBlw/Vref = 12.46– For R1=1M, R3 = 80.25K– R2 = 650.1K

Vref = 2.048V

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Intuitive Step: What Ratio Sets the Hysteresis Multiplier?

R1/R3

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Comparator Sense – Apps Spec

• Vin = 60mV from negative rail – design error

• Vcc=5v

• +/-5V hysteresis – error must be mV

• Rest is done to data sheet

• Send best shot!

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Comparator Sense - rail-or-elow-rail

Iin

• Design to data sheet - 5R’s

• Set Vtrip(-pin) = Iin*R1 ~1.5V– Chose Iset ~5uA – Chose R1~100K– Calc R2=(Vref-Iset*R1)/Iset for ~300k

• Set Vsense (+pin) to =>15mV +change for Vin=60mV

– Iin drops ~.15uA => R3~R1 and R4 slightly <R2

• +/-5V hysteresis – error must be mV change to 5mV

• • adj for Vos error R5 =1.76K

• Chose ratio R1/R2 = R3/R4 to reduce Ibias error

• Adjust for std values 1% or .1%

• Possible Combination Values from a solver R1=108.8K, R2=291.3K, R3=110K,R4=282.8K

R1

R1

R2Iset

R3

R4

R5

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Intuitive Step: How do I Start, Low Side Sense for the LMP7300?

Iin

R1

R2Iset

R3

R4

R5

• A) Chose ratio R1/R2 = R3/R4 to reduce Ibias error

• B) Set Vsense (+pin) to =>15mV +change for Vin=60mV

• C) Set Vtrip(-pin) = Iin*R1 ~1.5V

– Chose Iset ~5uA – Chose R1~100K– Calc R2=(Vref-Iset*R1)/Iset for ~300k

• D) Decide to use std values 1% or .1%

• E) answers B) and C)

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Intuitive Step: What Ratio Sets the High Side Sense Gain?

R1/R2

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Excel Tool

• Solver is a must!

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Which values for Rf, Cf, Rg determine a single pole rolloff or low pass transfer function?

A. B. C. D. E.

20% 20% 20%20%20%

1010


A. Rg=1K, Rf=1M, Cf= .1uFB. Rg=1K, Rf=open(100M), Cf =.1uFC. Rg=open(100M), Rf=1K, Cf=.1uFD. answers b) and c)E. all of the above

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Which values for Rf, Cf, Rg cause instability in the transfer function?

A. B. C. D. E.

20% 20% 20%20%20%

1010


A. Cg=1nF, Rf=1M, Cf= 10pFB. Cg=1nF, Rf =1M, Cf=openC. Cg=10pF, Rf=10K, Cf=1pFD. answers b) and c)E. all of the above

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What are good practices when analyzing a high order or multi-pole circuit?

1010


A. B. C. D. E.

20% 20% 20%20%20%A. From the start use matrix algebra to find the poles and zeros

B. First simplify the circuit for an intuitive understanding

C. Go to the bench immediately, build the circuit and take photos

D. Begin with a simulator like Altium using the full circuit and trust completely in the simulator results

E. None of the above

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What is the application total phase margin?

1010


A. B. C. D. E.

20% 20% 20%20%20%A. The difference in phase between the forward path and feedback path -180 degrees

B. The phase difference between F-3dB phase and FGBW phase @ minimum closed loop gain

C. The phase difference between the 1/B dominant pole and dominant zero @ unity gain -180 degrees

D. The op amp open loop phase plus 1/B phase at the point of intersection -180 degrees

E. None of the above

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How do I start, low side sense for the LMP7300?

A. B. C. D. E.

0% 0% 0%0%0%


A. Chose ratio R1/R2 = R3/R4 to reduce Ibias errorB. Set Vsense (+pin) to =>15mV +change for

Vin=60mVC. Set Vtrip(-pin) = Iin*R1 ~1.5V

-- Chose Iset ~5uA -- Chose R1~100K-- Calc R2=(Vref-Iset*R1)/Iset for ~300k

D. Decide to use std values 1% or .1%E. answers B) and C)

1010

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A. R1/R2B. R1/R3C. R3/R2D. R3/R4E. R1/R4

What ratio sets the Hysteresis multiplier?

A. B. C. D. E.

20% 20% 20%20%20%

1010


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A. R1/R2B. R1/R3C. R3/R2D. R3/R4E. R1/R4

What ratio sets the high side sense gain?

A. B. C. D. E.

20% 20% 20%20%20%

1010


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What is the advantage of the LMV794 over the LMV797?


Aol

Signal Bandwidth

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)10M

Mag

(dB

)

100M1M100K10K1K100

? dB

?x

LMV797

LMV794 1010

20%20%20%20%20% A. 10 dB Open Loop Gain

B. Lower cost $C. 3x Higher GBW D. Lower noise @ Gains <20dBE. None of the above

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Are these TIA circuits stable over the freq range shown?


1010Transimpedance gainVo = -If * Rf

LMV794

ROC = 40dB

0 dB

20 dB

40 dB

60 dB

80 dB

100 dB

Freq (Hz)10M

Mag

(dB

)

100M1M100K10K1K100

LMV797

A. B. C. D. E.

20% 20% 20%20%20%

A. Yes LMV797

B. No LMV794

C. No both

D. Yes both

E. Yes a) and b)

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Which are Best Design Practices for bandpass filters?

1010


A. B. C. D. E. F.

17% 17% 17%17%17%17%

A. Design high Q>10 single stagesB. Design using the Op amp pole as

part of the circuitC. Design using >20Q2 op amp GBWD. Use PSpice to investigateE. Build on the benchF. All except a) and b)

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1 Ron LaTour Applications Engineer Photo Diode Signal Paths Using Decompensated Amplifiers.

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