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

2. © 2008 National Semiconductor Corporation 2 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

3. © 2008 National Semiconductor Corporation 3 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

4. © 2008 National Semiconductor Corporation 4 Bandpass - Schematic LM833 Vcc =5V –$.2 LMV794 –Oscillates –797 won’t give desired Q = 7 Considering –Max4453 200 MHz

5. © 2008 National Semiconductor Corporation 5 Bench Verify: Bandpass Oscillation Problem LMV794 vs LMV797 LMV794 w/ Riso 1K, 2K Tried input R/C

6. © 2008 National Semiconductor Corporation 6 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

7. © 2008 National Semiconductor Corporation 7 Photodiode – Signal Path Specs First stage Transimpedance Amp –Photoconductive topology Cap coupled photodiode, no precision –High Gain R F ~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

8. © 2008 National Semiconductor Corporation 8 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?

9. © 2008 National Semiconductor Corporation 9 Circuit Blocks TIA Diff BP

10. © 2008 National Semiconductor Corporation 10 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

11. © 2008 National Semiconductor Corporation 11 Intuitive Step: Which Values for Rf, Cf, Rg Cause Instability in the Transfer Function? Cg=1nF, Rf =1M, Cf=open

12. © 2008 National Semiconductor Corporation 12 Photodiode – Schematic? Guess –Introduced Resistive “Tee” –Capacitive “Tee” –Customer cap 22pF –Single supply bias

13. © 2008 National Semiconductor Corporation 13 Photodiode – Build a Tool?

14. © 2008 National Semiconductor Corporation 14 Formulas Some math and formulas provide insight Some Do Not (not shown)

15. © 2008 National Semiconductor Corporation 15 Photodiode – Bode Plot Compare the 797 &794 LMV797 any Cin/Cf ratio for min noise BW LMV794 ratio >10 or 20 dB 1/B = 1+ Rf/Rsh Aol Signal Bandwidth 1/B = 1 + Cin/Cf LMV797 GBW =17 MHz PM =45 d 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB Freq (Hz) LMV794 GBW = 88 MHz PM =45 d @ Acl =20dB Fp = 1/2piCfRf 10M Mag(dB) 100M 1M 100K10K 1K 100 Uncompensated PM = 0 d => Oscillation Stray C => peaking

16. © 2008 National Semiconductor Corporation 16 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)

17. © 2008 National Semiconductor Corporation 17 Transimpedance Amp critically damped PM = 45 d Transimpedance gain Vo = -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) 100M 1M 100K10K 1K 100 LMV797 A B

18. © 2008 National Semiconductor Corporation 18 Intuitive Step: Are these TIA Circuits Stable over the Freq Range Shown? Transimpedance gain Vo = -If * Rf LMV794 ROC = 40dB 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB Freq (Hz) 10M Mag(dB) 100M 1M 100K10K 1K 100 LMV797 a)Yes LMV797 b)No LMV794 c)No both d)Yes both e)Yes a) and b) Fc Fzf Fi

19. © 2008 National Semiconductor Corporation 19 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) 100M 1M 100K10K 1K 100 15 dB 5x LMV797 LMV794

20. © 2008 National Semiconductor Corporation 20 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

21. © 2008 National Semiconductor Corporation 21 Follow Up Selling the customer –FAE on the bench with him taking data. –Poor results –Follow up

22. © 2008 National Semiconductor Corporation 22 Diff amp – Application Specs Customer needed –Differential Gain~20-50 @ 500 KHz –Single supply Vcc = 5V –Two TIA photodiode circuits are input signal V 1 and signal V 2 –No precision Offered LMV797, LMV794 –Which is best? Lowest cost?

23. © 2008 National Semiconductor Corporation 23 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?

24. © 2008 National Semiconductor Corporation 24 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 >20Q 2 op amp GBW Use PSpice to investigate Build on the bench

25. © 2008 National Semiconductor Corporation 25 Tools Tools don’t work well with decompensated Op Amps Fixed Unity gain only LMV792 LMH6628

26. © 2008 National Semiconductor Corporation 26 Bench Comparison w LM833 LMV794 @ 5V compensated LMV794@1.6V LM833 @ 12V LM833 @ 4.2V

27. © 2008 National Semiconductor Corporation 27 Bandpass - Bode Plot LMV797 any Cin/Cf ratio for min noise BW LMV794 ratio >10 or 20 dB 1/B = 1+ Rf/Rg Aol Signal Bandwidth 1/B = 1 + Cin/Cf LMV797 GBW =17 MHz PM =45 d 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB Freq (Hz) LMV794 GBW = 88 MHz PM =45 d @ Acl =20dB Fp = 1/2piCfRf 10M Mag(dB) 100M 1M 100K10K 1K 100 Fz = 1/2piCiRf

28. © 2008 National Semiconductor Corporation 28 Customer Data – PQA – Application Specs Small % bad LMC6035 parts What’s wrong with them?

29. © 2008 National Semiconductor Corporation 29 Clock Buffer - Schematic

30. © 2008 National Semiconductor Corporation 30 Clock – Differentiator Aol ROC = 40dB after Closed loop plot intersects Aopen loop critically damped PM = 45 d Fz = 1/2pi10pF*4M = 4 KHz@20dB/dec Phase is -90 +90 = 0 d ~Fp=1/2pi2nF*200

31. © 2008 National Semiconductor Corporation 31 A Little Digging

32. © 2008 National Semiconductor Corporation 32 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

33. © 2008 National Semiconductor Corporation 33 Comparator Hysteresis - Specs Li-Ion Battery pack threshold sense ~1.5V Hysteresis Reset on drop below hysteresis Use range ~2.7 to 11.5V Vref = 2.048V

34. © 2008 National Semiconductor Corporation 34 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

35. © 2008 National Semiconductor Corporation 35 Intuitive Step: What Ratio Sets the Hysteresis Multiplier? R1/R3

36. © 2008 National Semiconductor Corporation 36 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!

37. © 2008 National Semiconductor Corporation 37 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 R1R1 R1 R2 Iset R3 R4 R5

38. © 2008 National Semiconductor Corporation 38 Intuitive Step: How do I Start, Low Side Sense for the LMP7300? Iin R1 R2 Iset 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)

39. © 2008 National Semiconductor Corporation 39 Intuitive Step: What Ratio Sets the High Side Sense Gain? R1/R2

40. © 2008 National Semiconductor Corporation 40 Excel Tool Solver is a must!

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42. © 2008 National Semiconductor Corporation 42 Which values for Rf, Cf, Rg determine a single pole rolloff or low pass transfer function? 10 Photo Diode Signal Paths Using Decompensated Amplifiers A.Rg=1K, Rf=1M, Cf=.1uF B.Rg=1K, Rf=open(100M), Cf =.1uF C.Rg=open(100M), Rf=1K, Cf=.1uF D.answers b) and c) E.all of the above

43. © 2008 National Semiconductor Corporation 43 Which values for Rf, Cf, Rg cause instability in the transfer function? 10 Photo Diode Signal Paths Using Decompensated Amplifiers A.Cg=1nF, Rf=1M, Cf= 10pF B.Cg=1nF, Rf =1M, Cf=open C.Cg=10pF, Rf=10K, Cf=1pF D.answers b) and c) E.all of the above

44. © 2008 National Semiconductor Corporation 44 What are good practices when analyzing a high order or multi-pole circuit? 10 Photo Diode Signal Paths Using Decompensated Amplifiers 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

45. © 2008 National Semiconductor Corporation 45 What is the application total phase margin? 10 Photo Diode Signal Paths Using Decompensated Amplifiers A.The difference in phase between the forward path and feedback path -180 degrees B.The phase difference between F -3dB phase and F GBW 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

46. © 2008 National Semiconductor Corporation 46 How do I start, low side sense for the LMP7300? Photo Diode Signal Paths Using Decompensated Amplifiers 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) 10

47. © 2008 National Semiconductor Corporation 47 A.R1/R2 B.R1/R3 C.R3/R2 D.R3/R4 E.R1/R4 What ratio sets the Hysteresis multiplier? 10 Photo Diode Signal Paths Using Decompensated Amplifiers

48. © 2008 National Semiconductor Corporation 48 A.R1/R2 B.R1/R3 C.R3/R2 D.R3/R4 E.R1/R4 What ratio sets the high side sense gain? 10 Photo Diode Signal Paths Using Decompensated Amplifiers

49. © 2008 National Semiconductor Corporation 49 What is the advantage of the LMV794 over the LMV797? Photo Diode Signal Paths Using Decompensated Amplifiers Aol Signal Bandwidth 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB Freq (Hz) 10M Mag(dB) 100M 1M 100K10K 1K 100 ? dB ?x LMV797 LMV794 10 A.10 dB Open Loop Gain B. Lower cost $ C.3x Higher GBW D.Lower noise @ Gains <20dB E.None of the above

50. © 2008 National Semiconductor Corporation 50 Are these TIA circuits stable over the freq range shown? Photo Diode Signal Paths Using Decompensated Amplifiers 10 Transimpedance gain Vo = -If * Rf LMV794 ROC = 40dB 0 dB 20 dB 40 dB 60 dB 80 dB 100 dB Freq (Hz) 10M Mag(dB) 100M 1M 100K10K 1K 100 LMV797 A.Yes LMV797 B.No LMV794 C.No both D.Yes both E.Yes a) and b)

51. © 2008 National Semiconductor Corporation 51 Which are Best Design Practices for bandpass filters? 10 Photo Diode Signal Paths Using Decompensated Amplifiers A.Design high Q>10 single stages B.Design using the Op amp pole as part of the circuit C.Design using >20Q 2 op amp GBW D.Use PSpice to investigate E.Build on the bench F.All except a) and b)

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53. © 2008 National Semiconductor Corporation 53 Team Scores 0Team 1 0Team 2 0Team 3 0Team 4 0Team 5