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Analog Basics Workshop Offset

Analog Basics Workshop Offset. Rev 0.1. Input Offset. OPA827. OPA827. How is Vos Defined, Simulate it!. OPA835. OPA835. Drift Slope – Positive and Negative. For this example Vos drift is defined:. Drift Slope – Common Definition. OPA827.

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Analog Basics Workshop Offset

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  1. Analog Basics WorkshopOffset Rev 0.1

  2. Input Offset

  3. OPA827 OPA827

  4. How is Vos Defined, Simulate it!

  5. OPA835 OPA835 Drift Slope – Positive and Negative For this example Vos drift is defined:

  6. Drift Slope – Common Definition

  7. OPA827 Drift Slope – Positive and Negative(common definition) • Absolute value makes all results positive. • Actual drift may be positive or negative.

  8. OPA277 Warm Up Typical offset = 10uV Initial error is 10% of typical.

  9. Application Example What is the percentage error?

  10. Input Offset Drift Calculations

  11. Input Bias Current and Drift

  12. Simple Bipolar (No Ib Cancelation) Bias current in Bipolar amplifiers is from Base Current. It is typically larger then FET and it flows into the input terminals. Input offset current is the difference between the two currents. It is typically smaller then Ib for this simple configuration. Ibos = Ib1 – Ib2

  13. Bipolar with Ib Cancelation The input bias currents are mirrored and summed back in to cancel the bias current. This has the effect of significantly reducing input Ib. Note that when this is done, Ib can flow in both directions. Also Ibos is no longer smaller then Ib. Ibos = Ib1 – Ib2

  14. Bias current in MOSFET amplifiers is mainly from leakage into ESD diodes.

  15. OPA350 OPA277 CMOS amplifier: In this case you see a dramatic increase in bias current at 25C. Note the logarithmic graph. Doubles every 10C. Bipolar amplifier: In this case you see a dramatic increase in bias current at 75C

  16. Ib CalculationOPA211 At High Temp

  17. Simulating Bias Current and Offset

  18. OPA188 Macro Model No temperature effects are simulated. In our models. Ib, and Vosi at 25C. Typical Values are the target.

  19. Simulating

  20. Hand Calculation Simulated = 11.65mV Calculated = 24.9mV Vout1 = (Vos1 + Ib1p x R1) x Gain1 + Ib1n x R3 Vout1 = (6uV +160pA x 100k)x101 + 160pA x 100k = 2.238mV Vout2 = (Vout1 + Vos2) x Gain2 + Ib2n x R5 Vout2 = (2.238mV + 6uV) x 11 + 160pA x 10k = 24.9mV

  21. Why Sim <> Calculated Simulated = 11.65mV Calculated1 = 24.9mV Calculated2 = 11.22mV (both 1 and 2 are possible) Vout1 = (Vos1-Ib1p x R1) x Gain1 + Ib1n x R3 Vout1 = (6uV -160pA x 100k )x101 + 160pA x 100k = 1.026mV Vout2 = (Vout1 - Vos2) x Gain2 + Ib2n x R5 Vout2 = (1.026mV - 6uV) x 11 + 160pA x 10k = 11.22mV

  22. Dominant Terms – Vos1 and Ib1p Vout1 = (Vos1 + Ib1p x R1) x Gain1 + Ib1n x R3 Vout1 = (6uV +160pA x 100k)x101 + 160pA x 100k = 2.238mV Vout1 = (6uV +16uV)x101 + 160pA x 100k = 2.238mV Vout1 = 1.01mV+ 16uV = 2.238mV Vout2 = (Vout1 + Vos2) x Gain2 + Ib2n x R5 Vout2 = (2.238mV + 6uV) x 11 + 160pA x 10k = 24.9mV

  23. Offset Lab • Simulation • Calculation • Measurement

  24. Sim 1: OPA277, OPA188, OPA211 Output Offset No Filter Input Grounded Click the Links Below to Open Circuit.

  25. Sim 2: OPA277, OPA188, OPA211 Output Offset (with Rin = 5k, Ib effect) No Filter Input to R6

  26. MyDAQ Exp 1: OPA2188 & OPA2277 Offset Disconnect MyDAQ from PC. Set jumpers on fixture as shown. See table below for additional details. Connect MyDAQ to PC

  27. MyDAQ Exp 1: OPA2188 & OPA2277 Offset OPA211 OPA188 211: 127mV 188: 54mV

  28. Calculation: Output offset from vos(input grounded) This term is negligible Simplified equation has nearly the same result

  29. Calculated, Simulated, and Measured Output Offset

  30. Offset Homework • Calculation • Simulation • Measurement

  31. 1. For the circuit below: Calculate and simulate the typical and maximum output error voltage from Ib and Vosi. • For the circuit above: What is the dominant error source? Ib, or Vosi What is the percentage error?

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