Analog Basics WorkshopRFI/EMI Rejection

Rev 0.1

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

The necessary elements for EMI

Coupling medium

Source of electromagnetic energy

1

0

+

_

Receptor of

ElectromagneticEnergy

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

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)

Analog receptors of electromagnetic energy

Op-ampsLow-speed: offset shift, RF noiseHigh-speed: linear and non-linear

amplification

ConvertersEMI aliased into passbandcorrupted output levels or

codesoffset shifts

RegulatorsOffset - output voltage error

Operational amplifier voltage-offset shift resulting from conducted RF EMI

in a 50Ω system -10dBm = 100mVpk 0dBm = 318mVpk +10dBm = 1.0Vpk

OPA376 dc offset voltage change (RTI) vs. RF level and frequency

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

1.E+07 1.E+08 1.E+09 1.E+10

RF Frequency (Hz)

dc o

ffse

t cha

nge

(V)

-10dBm

+10dBm

0dBm

Radiated EMI and its affect on an ECG EVM

ECG Full Scale 1Vp-p 0.5V/div

Transmitterkeyed 6 sec.

+2.5V offset normal

+4.0V offset RF present

1.5VDue to RFI

Single SupplyCMOSINA326

OPA335(s)

Fly wire Proto board

(Vin ≈ 1mVp-p G = 2500V/V)

Transmitter470MHzPout 0.5W

d ≈1.5 ft (46cm)Significant DC Offset

when RF present

RF noiseOn ECG

EMI slideInformation

by John Brown

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

++

-R1

R1

R2

U1 INA326

R3 400k

V1 5

C4

100n R5 400k

Vo

R4 400k

CB 4.7n

CC 4.7n

R2 1M

R1 1M

CA 47n

RB 4.7k

RA 4.7k

+VDM/2

+

VDM/2

+

VCM

First-orderLow pass filter

Newer op-amps have built-in EMI filtering

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

+

-

-

+ΔVOS (DC)

VRF

The EMIRR IN+ test set-upSee TI Application Report SBOA128 for details

Simple schematicfor EMIRR IN+ test

Practical implementation

The complex RF inputenvironment

Zin ofOp-amp

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

VOS1

10

EMIRR

20

VRF_PEAK2

100mVp

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

EMIRR 20LogVRF_PEAK

VOS

20LogVRF_PEAK

100mVp

EMIRR vs. Frequency (-10 dBm RF Input Signal)

0

20

40

60

80

100

120

140

1.E+07 1.E+08 1.E+09 1.E+10Frequency (Hz)

EM

IRR

(dB

)

EMIRR IN+ measurement results forTI CMOS rail-to-rail operational amplifiers

Model GBW Filter Model GBW Filter

OPA333/2333 350kHz Yes OPA376/377 5.5MHz Yes

OPA378 500kHz Yes OPA348/2348 1MHz No

Larger EMIRR is better

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 IA under test

Differential mode EMIRR Common-mode EMIRR

EMIRR testing applied to instrumentation amplifiersINA118 – 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

INA118 INA333 Differential Mode EMIRR Comparison

0

20

40

60

80

100

120

140

160

180

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency Hz

EM

IRR

dB INA118 G=1

INA118 G=10

INA333 G=1

INA333 G=10

EMIRR testing applied to instrumentation amplifiersINA118 – INA333 common-mode comparison

INA118 INA333 Common-mode EMIRR Comparison

0

20

40

60

80

100

120

140

160

180

1.00E+07 1.00E+08 1.00E+09 1.00E+10

Frequency Hz

EM

IRR

dB INA118 G=1

INA118 G=10

INA333 G=1

INA333 G=10

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

EMI/RFI Lab • Simulation • Calculation • Measurement

19

Ex 6.1: Hand Calculations1. 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

V+

V-

V- V+

V+

V-

R13

5k

SW

2

SW1

R12

10

R11 10k

+Vin

SW3 C1 1n

Vo277

R10 20kR9 1kR8 100kR7 1k

V4 18V3 18

-

++

U1 OPA277_TG

-

++

U2 OPA277_TG

OPA211 EMIRR

OPA188 EMIRR

From graph

v os211v os188

10

65

20

10

35

20

31.6

From myDAQ

v os211v os188

3.46

.0183189

Ex 6.1: EMIRR (Noise) Schematic

21

0.1µF

C3+15V

-15V

GND

GND

0.1µF

C5

1 2 3GNDAO(0)

JMP2

GND

1.0k

R1

100k

R2

GND+15V

-15V

1.0k

R3

GND

100k

R5

100R6

GND

1000pF

C1

1

2

3

FLT

JMP1

AI(0+)

AO(0)

0.1µF

C10+15V

-15V

GND

GND

0.1µF

C12

1 2 3GNDAO(0)

JMP4

1.0k

R7

100k

R8

GND+15V

-15V

1.0k

R9

GND

1000pF

C9

1

2

3

JMP3

AI(1+)

2

31

AV+

V-

84U0A

OPA2188AIDR5

67

BV+

V-

84U0B

OPA2188AIDR

5

67

BV+

V-

84U1B

OPA2188AIDR

2

31

AV+

V-

84U1A

OPA2188AIDR

Vin0

Vin1

1 2 3GNDRIN

JMP5

100kRIN0

GND

GND

1 2 3GNDRIN

JMP6

100kRIN1

GND

20.0k

R4

20.0k

R10

Vout0

Vout1

1µF

C2

1µF

C4

1

2

3ACDC

JMP7

1

2

3ACDC

JMP8

1.00MegR11

1.00MegR12

GND

GND

Two copies of the same two stage amplifier is on the board. Each two stage amplifier has four jumpers to configure the circuit.

Ex 6.1: Amplifier I/O PCB Setup

22

Jumper Position

JMP7, JMP8 DC — Right position for DC coupling

JMP1, JMP3 FLT — Top position for filtering.

JMP2, JMP4 GND — Right position for GND connection to input.

JMP5, JMP6 GND — Connect “antenna” to top position. This antenna is from the amplifiers noninverting input to GND.

U0 = OPA2211U1 = OPA2188

Connect antenna to JMP5 & JMP6.

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

Ex 6.1: Expected Results

1. Does the relative change in offset match the theoretical EMIRR plots from the hand calculations?

24

OPA211 output offset2V/div

OPA188 output offset20mV/div

Transceiver Keyed

AnswerFrom graph

v os211v os188

10

65

20

10

35

20

31.6

From myDAQ

v os211v os188

3.46

.0183189