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Analog Basics Workshop RFI/EMI Rejection Rev 0.1.

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Presentation on theme: "Analog Basics Workshop RFI/EMI Rejection Rev 0.1."— Presentation transcript:

1 Analog Basics Workshop RFI/EMI Rejection Rev 0.1

2 EMI or RFI? 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

3 The necessary elements for EMI Coupling medium Source of electromagnetic energy _+_ Receptor 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 Taming the EMI environment 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)

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 = 100mV pk 0dBm = 318mV pk +10dBm = 1.0V pk

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

9 Input RC filtering as applied to an instrumentation amplifier Differential Mode f -3dB = [2π(R A + R B )(C A + C B /2)] -1 let R B = R A and C C = C B f -3dB = 343Hz Common Mode f -3dB = [2π∙R A ∙ C B )] -1 let R B = R A and C C = C B f -3dB = 7.2kHz

10 Newer op-amps have built-in EMI filtering

11 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 ΔV OS (DC) V RF

12 The EMIRR IN+ test set-up See TI Application Report SBOA128 for details Simple schematic for EMIRR IN+ test Practical implementation The complex RF input environment Z in of Op-amp

13 EMIRR IN+ equation solved for |∆V OS | Use this equation to solve for |∆V OS | of a unity gain amplifier if V RF_PEAK and EMIRR IN+ are known such as when a plot is provided EMIRR IN+ is frequency dependant Doubling V RF_PEAK Quadruples |∆V OS |! For example: Consider a 100mV P 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 V RF_PEAK = peak amplitude of the applied RF op-amp input ΔV OS = resulting “input-referred” DC offset voltage op-amp output 100mV P = standard EMIRR input level (-10 dBm) Higher EMIRR IN+ means lower amplifier EMI sensitivity

15 EMIRR IN+ measurement results for TI CMOS rail-to-rail operational amplifiers ModelGBWFilterModelGBWFilter OPA333/ kHzYesOPA376/3775.5MHzYes OPA378500kHzYesOPA348/23481MHz No Larger EMIRR is better

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

17 EMIRR testing applied to instrumentation amplifiers INA118 – INA333 differential mode comparison 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

18 EMIRR testing applied to instrumentation amplifiers INA118 – INA333 common-mode comparison 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

19 Simulation Calculation Measurement 19

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 20 ∆vos211 / ∆vos333 OPA211 EMIRR OPA188 EMIRR

21 Ex 6.1: EMIRR (Noise) Schematic 21 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 22 JumperPosition JMP7, JMP8DC — Right position for DC coupling JMP1, JMP3FLT — Top position for filtering. JMP2, JMP4GND — Right position for GND connection to input. JMP5, JMP6GND — Connect “antenna” to top position. This antenna is from the amplifiers noninverting input to GND. 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). 23

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


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