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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 Larger EMIRR is better 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. EMI/RFI Lab • Simulation • Calculation • Measurement

  20. Ex 6.1: Hand Calculations 1. The figures below illustrate the EMIRR for two different op-amps. Assume the same magnitude and frequency (476MHz) of RF signal is applied to the circuit below. H OPA211 EMIRR OPA188 EMIRR

  21. Ex 6.1: EMIRR (Noise) Schematic Two copies of the same two stage amplifier is on the board. Each two stage amplifier has four jumpers to configure the circuit.

  22. Ex 6.1: Amplifier I/O PCB Setup U0 = OPA2211 U1 = OPA2188 Connect antenna to JMP5 & JMP6.

  23. Ex 6.1: Instrument Setup The instrument setup above will configure the signal source and scope for the circuit below so that we can see the bandwidth limitations. Use the curser to determine the bandwidth (-3dB).

  24. Ex 6.1: Expected Results Transceiver Keyed OPA211 output offset 2V/div OPA188 output offset 20mV/div 1. Does the relative change in offset match the theoretical EMIRR plots from the hand calculations?

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