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Analog Basics Workshop RFI/EMI Rejection

Analog Basics Workshop RFI/EMI Rejection. Rev 0.1. Both are sources of radio frequency (RF) disturbance EMI – electromagnetic interference Often a broadband RF source RFI – radio frequency interference Often a narrowband RF source Terms are often used interchangeably. EMI or RFI?.

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Analog Basics Workshop RFI/EMI Rejection

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  1. Analog Basics WorkshopRFI/EMI Rejection Rev 0.1

  2. Both are sources of radio frequency (RF) disturbance EMI – electromagnetic interference Often a broadband RF source RFI – radio frequency interference Often a narrowband RF source Terms are often used interchangeably EMI or RFI?

  3. The necessary elements for EMI Coupling medium Receptor of Electromagnetic Energy 1 0 + _ Source of electromagnetic energy

  4. Sources of electromagnetic energy • RF generating sources • Intentional radiators • cell phones • transmitters & transceivers • wireless routers, peripherals • Unintentional radiators • System clocks & oscillators • Processors & logic circuits • Switching power supplies • Switching amplifiers • Electromechanical devices • Electrical power line services

  5. Reduce receptor circuit’s susceptibility to EMI (Filtering) Reduce the coupling medium’s effectiveness (Shielding) Minimize EMI radiation from the source (Keep sensitive analog away from digital, soften digital edges) Taming the EMI environment

  6. Analog receptors of electromagnetic energy Op-amps Low-speed: offset shift, RF noise High-speed: linear and non-linear amplification Converters EMI aliased into passband corrupted output levels or codes offset shifts Regulators Offset - output voltage error

  7. Operational amplifier voltage-offset shift resulting from conducted RF EMI in a 50Ω system -10dBm = 100mVpk 0dBm = 318mVpk +10dBm = 1.0Vpk

  8. Radiated EMI and its affect on an ECG EVM (Vin≈ 1mVp-p G = 2500V/V) Transmitter 470MHz Pout 0.5W d ≈1.5 ft (46cm) Significant DC Offset when RF present +4.0V offset RF present RF noise On ECG 1.5V Due to RFI Single Supply CMOS INA326 OPA335(s) Transmitter keyed 6 sec. +2.5V offsetnormal Fly wire Proto board ECG Full Scale 1Vp-p 0.5V/div EMI slide Information by John Brown

  9. Input RC filtering as applied to an instrumentation amplifier Differential Mode f-3dB= [2π(RA+ RB)(CA+ CB/2)]-1 let RB = RA and CC = CB f-3dB= 343Hz Common Mode f-3dB= [2π∙RA∙ CB)]-1 let RB = RA and CC = CB f-3dB= 7.2kHz

  10. Newer op-amps have built-in EMI filtering

  11. - VRF + + ΔVOS (DC) - EMIRR- a measure quantifying an operational amplifier’s ability to reject EMI • EMIRR - electromagnetic interference rejection ratio • Defined in National Semiconductor’s application note AN-1698 • Measured as a dB voltage ratio of output offset voltage change in response to an injected RF voltage having a defined level • Provides a definitive measure of EMI rejection across frequency allowing for a direct comparison of the EMI susceptibility of different operational amplifiers

  12. The EMIRR IN+ test set-upSee TI Application Report SBOA128 for details Simple schematic for EMIRR IN+ test Practical implementation Zin of Op-amp The complex RF input environment

  13. EMIRR IN+ equation solved for |∆VOS| • Use this equation to solve for |∆VOS| of a unity gain amplifier if VRF_PEAK and EMIRR IN+ are known such as when a plot is provided • EMIRR IN+ is frequency dependant • Doubling VRF_PEAK Quadruples |∆VOS|! • For example: Consider a 100mVP RF signal at 1.8GHz applied to a device with an EMIRR IN+ of 60 dB. The associated voltage offset shift would be 100uV

  14. EMIRR IN+ equation • VRF_PEAK = peak amplitude of the applied RF signal @ op-amp input • ΔVOS = resulting “input-referred” DC offset voltage shift @ op-amp output • 100mVP = standard EMIRR input level (-10 dBm) • Higher EMIRR IN+ means lower amplifier EMI sensitivity

  15. EMIRR IN+ measurement results forTI CMOS rail-to-rail operational amplifiers ModelGBWFilterModelGBWFilter OPA333/2333 350kHz Yes OPA376/377 5.5MHz Yes OPA378 500kHz Yes OPA348/2348 1MHz No

  16. Common-mode EMIRR Differential mode EMIRR IA under test IA under test EMIRR testing applied to instrumentation amplifiers Test Configuration Bipolar supplies (+/-V), reference pin grounded, RF level -10dBm • Common-mode Measurement • RF signal applied to both inputs • Differential measurement • RF signal applied to non-inverting input • Inverting input grounded

  17. INA118 • 3 op-amp current feedback design • Av range 1 to 10kV/V • 70kHz BW, G = 10V/V • Iq 350uA • circa 1994 • no internal EMI filter • INA333 • 3 op-amp CMOS auto-zero design • Av range 1 to 1kV/V • 35kHz BW, G = 10V/V • Iq 50uA • 2008 introduction • internal EMI filter EMIRR testing applied to instrumentation amplifiersINA118 – INA333 differential mode comparison

  18. INA118 • 3 op-amp current feedback design • Av range 1 to 10kV/V • 70kHz BW, G = 10V/V • Iq 350uA • 1994 introduction • no internal EMI filter • INA333 • 3 op-amp CMOS auto-zero design • Av range 1 to 1kV/V • 35kHz BW, G = 10V/V • Iq 50uA • 2008 introduction • internal EMI filter EMIRR testing applied to instrumentation amplifiersINA118 – INA333 common-mode comparison

  19. Shielding Effectiveness (S.E.) of enclosed material Emission Suppression S.EdB (Em. Supp.)≈ AdB Susceptibility S.EdB (Sus.) ≈ AdB + RdB (appropriate) where: A: absorption loss in dB R: reflection loss in dB From: COTS Journal, January 2004 – “Design Considerations In Building Shielded Enclosures.” Shielding and screening Minimizing the medium’s effectiveness Derived from: EDN – The Designer’s Guide to Electromagnetic Compatibility

  20. Ferrite shield RF absorber shield Shielding and screening Minimizing medium’s effectiveness • Metal Shielding • Magnetic field f < 20kHz • Ferrous metals • steel • Mu-metal – nickel, iron • RF fields 10kHz < f < 1GHz • Non-ferrous metals • Al foil ILoss > 90dB • Cu, Ni ILoss 40-60dB • Vacuum plating • ILoss > 80dB • Electroless deposition • ILoss > 80dB • From: EDN EMI/EMC guide

  21. Further Reading

  22. EMI/RFI Lab • Simulation • Calculation • Measurement

  23. Experiment: EMI Rejection Handheld transceiver, OPA188, OPA211 Key the transceiver to see the affect of EMIRR. Transmits at 467MHz ½ Watt Plug in small jumper “antenna” into JMP5 and JMP6

  24. Experiment: EMI Rejection Handheld transceiver, OPA188, OPA211 Transceiver Keyed OPA211 output offset 2V/div OPA188 output offset 20mV/div

  25. Experiment: EMI Rejection Handheld transceiver, OPA188, OPA211 OPA211 ∆Vout = 3.456V OPA188 ∆Vout = 18.25mV OPA211 EMIRR OPA188 EMIRR 65dB @ 467MHz 35dB @ 467MHz

  26. EMI/RFI Homework • Calculation • Simulation • Measurement

  27. The dc output of the circuit below is effected by RF interference from a cell phone. The RF signal is far beyond the bandwidth of the amplifier. Why does the RF signal effect the dc performance of the amplifier? What are three things that we can do to reduce the effect of EMI?

  28. In a design the RF peak interference signal is 10 larger then the customer expected it to be. How much change in EMIRR is required to cancel the 10x increase in RF signal?

  29. Revisions • 0.2 • Added homework • Changed formatting to be consistent

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