Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd.

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Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd. Tucson, 85706 Tel: (602) 746-1111 Twx: 910-952-111 Telex: 066-6491 (602) 889-1510 Immediate Product Info: (800) 548-6132
NOISE SOURCES APPLICATIONS USING CAPACITIVE COUPLED ISOLATED AMPLIFIERS
Bonnie Baker (602) 746-7984
Noise typical problem confronting many isolation applications. Isolation products such analog isolation amplifiers, optocouplers, transformers digital couplers, used applications transmit signals across high voltage barrier while providing galvanic separation between grounds. Burr-Brown's isolated analog amplifiers digital couplers three coupling technologies their isolation products, each having advantages disadvantages noisy environments. These technologies inductive coupling, capacitive coupling optical coupling. Isolation amplifiers digital couplers used variety applications including breaking ground loops, motor control, power monitoring protecting equipment from possible damage. understanding design techniques used transmit signals across isolation barrier, well understanding sources noise, allows users quickly identify design layout problems make appropriate changes reduce noise tolerable levels. Noise defined this application note signal that present circuit other than desired signal. This definition excludes analog nonlinearities which pro-
duce distortion. shown Figure there three primary types noise endemic isolation applications, each with their possible solutions. first noise source device noise. Device noise intrinsic noise devices circuit. Examples device noise would thermal noise resistor shot noise transistor. second source noise that effects performance isolation devices conductive noise. This type noise already exists conductive paths circuit, such power lines, mixes with desired electrical signal through isolation device. third source noise radiated noise. Radiated noise emitted from sources such switches motors coupled into signal. This application bulletin will cover these three noise classifications they relate capacitive coupled isolation amplifiers. THEORY OPERATION CAPACITIVE COUPLED ISOLATION AMPLIFIERS capacitive coupled isolation amplifiers designed with input output section galvanically isolated pair matched capacitors. block diagram this type
Power Supply Noise Isolated Side Isolation Amplifier Spectral Noise +VS2 -VS2 GND2 +VS1 Isolation Barrier
Power Supply Noise System Side
Ripple Noise
Electric Field Coupling (EMI), Power Supply Noise
-VS1
GND1
Electric Field Coupling (EMI),
Power Supply Noise Device Noise Conducted Noise Radiated Noise
Transient Noise
FIGURE Three Basic Types Noise Isolation Applications Device Noise, Conducted Noise, Radiated Noise.
1993 Burr-Brown Corporation
AB-047
Printed U.S.A. April, 1993
isolation amplifier shown Figure capacitive coupled isolation amplifiers employ digital modulation schemes transmit differential signal across isolation barrier. modulation schemes used capacitive coupled isolation amplifiers duty-cycle modulation voltage-to-frequency, depending product. Both modulation schemes basically voltage time. internal oscillator used modulate analog input signal into digital signal which transmitted across isolation barrier. Most capacitive coupled amplifiers (ISO103, ISO107, ISO113, ISO120, ISO121, ISO122), shown block diagram Figure modulate analog signal dutycycle encoded signal; remainder isolation amplifiers (ISO102 ISO106), shown block diagram Figure modulate analog voltage frequency.
modulated signal transmitted other side isolation barrier through pair matched capacitors built into plastic ceramic package. value these capacitors varies from depending device. resulting capacitor simple reliable design. After modulated signal transmitted across isolation barrier, demodulated back analog voltage. output section isolation amplifier detects modulated signal converts back analog voltage using averaging techniques. Most undesired ripple voltages inherent demodulation process then removed. DEVICE NOISE CAPACITIVE COUPLED ISOLATION AMPLIFIERS Device noise generated devices circuit. Examples device noise generators would discrete resistor, which generates thermal noise, operational amplifier, which would generate noise, etc. Specifically, with Burr-Brown's capacitive coupled isolation amplifiers, there device noise specifications consequence.
+VS1
+VS2 Oscillator Modulator Demodulator VOUT
GND2 GND1 -VS2
-VS1
FIGURE Block Diagram Capacitive Coupled Isolation Amplifier.
Ripple Noise by-product demodulation scheme duty-cycle modulated isolation amplifiers ripple voltage output isolation amplifier. large part ripple voltage filtered output stage, however, small amount still present output. This ripple voltage varies from product product (5mVp-p 25mVp-p [typ]), dominated sample-hold droop capacitive feed through output stage isolation amplifier. example ripple voltage noise shown Figure
Isolation Barrier
200µA Sense 200k 150pF 100µA
200µA
Sense
150pF 100µA 200k Sense Signal
Signal
FIGURE Basic Block Diagram ISO103, ISO107, ISO113, ISO120, ISO121, ISO122 Isolation Amplifiers, which Duty-Cycle Techniques Transmit Signal Across Isolation Barrier.
This ripple voltage noise easily eliminated using pass active filter output isolation amplifier shown Figure This two-pole, unity-gain, Sallen-Key type filter designed with bandwidth 50kHz. OPA602 selected preserve accuracy ISO122. Figure dynamic range ISO122 changed from typical 9-bit resolution 11-bit resolution (see AB-023). ISO102 ISO106 isolation amplifiers have active filter built into their outputs. This pass filter provides significant reduction ripple voltage. remaining noise output isolation amplifier spectral noise. ripple noise isolation amplifier sufficiently reduced, spectral noise will begin dominate. Spectral Noise spectral noise, wideband noise, second type isolation amplifier device noise. This noise generated jitter modulation process. case ISO102 ISO106, jitter dominated time uncertainty one-shot. With ISO103, ISO113 ISO107 jitter noise dominated translation voltage noise comparator. Spectral noise reduced reducing signal bandwidth, again using pass filter output isolation amplifier. Another method reducing noise contribution from spectral noise well ripple voltage noise pre-gain stage isolation amplifier. This technique shown Figure gaining signal before transmitted across isolation barrier, signal-to-noise ratio will improved.
Offset Adjust +VCC1 -VCC1 Isolation Barrier
CONDUCTIVE NOISE EFFECT ISOLATION AMPLIFIER SIGNALS second source noise, conductive noise, coupled into signal path through three paths shown Figure Noise power supply lines coupled into signal through supply pins eventually signal path. Noise coming from input isolation amplifier transmitted directly across barrier. finally, fast change voltage difference between grounds isolated system corrupt signal some cases give erroneous output. Power Supply Noise Noise power supply lines coupled into isolation amplifier through supply pins. Isolation amplifiers require isolated supplies, typically DC/DC converters. DC/DC converters utilize high-frequency oscillators/drivers transmit voltage information across transformer barrier. output stage DC/DC converters rectify, filter some instances regulate output voltage. output voltage desired component well remnants switching frequency form complex ripple voltage. DC/DC converter regulation lack there switching frequency have effect performance isolation amplifier. cases where isolation amplifier self-powered (ISO103, ISO113, ISO107), DC/DC converter synchronized with isolation amplifier oscillator, however, unregulated. system power supply performance should evaluated possibly regulator chip added circuit system
+VCC2
-VCC2
Ref2
Ref1 0.5k Offset 2.5k ISO102 ISO106 Gain Adjust Common1 Common2 Digital Common 97.5k Osc. 24.5k
Sense
-Freq. Detector
VOUT Loop Filter Filter
FIGURE Basic Block Diagram ISO102 ISO106, Isolation Amplifiers, which Voltage-to-Frequency Modulation Techniques Transmit Signal Across Isolation Barrier.
side. isolation amplifiers that self-powered (ISO102, ISO106, ISO120, ISO121, ISO122) require power supplied external DC/DC converter battery. case where noise power supply line less than bandwidth isolation amplifier, noise manifests itself small signal offset voltage. magnitude this error specified data sheets
isolation amplifiers power supply rejection (PSR). Usually contribution power supply rejection error less than ripple voltage that generated demodulation process mentioned above. Power supply noise greater than bandwidth isolation amplifier come from several sources. Some these sources DC/DC converter switching frequency, switching noise from digital logic, switching noise from motors, from oscillator used isolation amplifier, name few. easy assume that isolation amplifier will filter noise that greater than bandwidth. That assumption erroneous, because aliasing between power supply noise isolation amplifier's oscillator. illustrate this point, refer performance curve from ISO122 data sheet shown Figure x-axis represents power supply noise frequency. left y-axis represents ratio between voltage supply voltage right y-axis represents frequency output signal generated aliasing effect. illustrated, supply line switching frequency 750kHz, there will noise ripple contribution output ISO122 about -33dBm frequency component that noise will 250kHz, which easily filtered using methods illustrated Figure supply line switching frequency noise 900kHz, there will noise ripple
FIGURE Unfiltered Output ISO122 Isolation Amplifier Showing Approximately 20mVp-p Output Ripple.
+VS1 +VS2
ISO122 GND2 -VS1 GND1
VOUT
4700pF
100pF OPA602
-VS2
GND2
Output OPA602 FIGURE ISO122 Isolation Amplifier with Two-Pole, Pass Filter Reduce Ripple Voltage Noise.
contribution output ISO122 about -20dBm with frequency component 50kHz. Since typical bandwidth ISO122 50kHz, this aliased noise will difficult filter without effecting signal bandwidth. danger zone power supply switching frequency noise this example frequency band ±50kHz around 500kHz multiples 500kHz. This because ISO122's bandwidth 50kHz modulation/demodulation oscillation frequency ISO122 500kHz. complicate matters further, DC/DC converter ripple voltage will never have frequency content simple sine wave, rather fairly complex summation several frequencies, usually multiples fundamental frequency. DC/DC converter switching frequency selected exactly same frequency multiple) modulation/demodulation oscillator frequency isolation amplifier, aliasing phenomena will problem. This, course, unrealistic because variances
variations temperature performance both DC/DC converter isolation amplifier. small difference between switching frequencies will generate frequency noise signal path that impossible filter. There design issues taken into consideration when selecting DC/DC converter switching frequency specific isolation amplifier. example, case ISO122, acceptable DC/DC switching frequency would 400kHz. this case, difference between DC/DC switching frequency isolation amplifier's oscillating frequency l00kHz. aliased noise will have fundamental frequency content l00kHz, which easily filtered isolation amplifier. Additionally, harmonic DC/DC converter harmonic ISO122 equal. Generally, amplitude DC/DC converter ripple having frequency content higher harmonic considerably smaller than that lower harmonics. Signals aliased back from higher harmonic elements DC/DC converter's ripple voltage will less. cases where isolation amplifier voltage-to-frequency modulation topology (ISO102 ISO106), selection DC/DC converter becomes more difficult. frequency modulation range ISO102 ISO106 0.5MHz (VOUT -10V) 1.5MHz (VOUT +l0V). these applications, proper by-pass designs help reduce noise caused switching frequency DC/DC converter. Figure illustrates resistor-capacitor inductor-capacitor decoupling networks that used isolate devices from power supply noise. These networks used eliminate coupling between circuits, keep power-supply noise from entering circuit suppress reflected ripple current DC/DC converter caused dynamic current component switching frequency. When
300pF
Isolation Barrier
OPA602 ISO122 Gain -10V/V
FIGURE Using Pre-Gain Stage Signal-to-Noise Ratio Improved. this Example Signalto-Noise Ratio Improved 20dB.
Isolation Barrier Power Supply Noise +VS1 +VS2 Power Supply Noise
Input Signal Path Noise
Isolation Amplifier GND2 GND2 -VS2
Output
Power Supply Noise
-VS1
Power Supply Noise
Transient Noise
FIGURE Three Sources Conductive Noise Isolation Application from Power Supply Lines, Signal Path Between Isolated Grounds.
SIGNAL RESPONSE INPUTS GREATER THAN 250kHz VOUT/VIN VOUT/VIN 100kHz Freq
Frequency
0.05 0.15
VOUT Ratio, (dB)
Resonant Frequency Damping Factor
0.25
500kHz
1MHz
1.5MHz
Input Frequency (NOTE: Shaded area shows aliasing frequencies that cannot removed low-pass filter output.)
Operating Frequency Resonant Frequency Ratio, f/fr
FIGURE Pi-Filter Response Design Formulas Input Signal Noise Noise signal path input isolation amplifier that within bandwidth isolation amplifier will transmitted across barrier with desired signal. This type noise impossible eliminate with filter before after isolation amplifier should eliminated source. Typically, noise coupled into signal path where there metal trace with high impedance node next metal trace where noise present. Signal path noise that above bandwidth isolation amplifier transmitted across barrier. Using performance curve ISO122 Figure easy deduce much noise will transmitted. this instance, x-axis represents input noise frequency. left y-axis represents ratio between voltage input voltage. right y-axis represents frequency output signal generated aliasing effect. there concern that there will high frequency noise input isolation amplifier, usually pass filter before isolation amplifier will reduce effects input noise aliasing into signal bandwidth. High dV/dt Changes Between Ground References Isolation Barrier third source conductive noise isolation applications caused transients between ground references across isolation barrier shown Figure 12). isolation mode voltage (IMV) voltage that appears across isolation barrier between input common output common. fault condition directly apply high voltage isolated common, forcing current through barrier capacitors. Finite isolation mode rejection results small output noise. Another specification that describes ability isolation product reject high transients between grounds called Transient Immunity (TI). These transients most commonly occur motor control applications. Transient Immunity specified volts seconds. high Transient Immunity indicates
FIGURE Noise Rejection Performance Curve ISO122.
VOUT
pi-filter VOUT
pi-filter
FIGURE Suggested Pi-Filter Designs Eliminate Power Supply Noise. filter used, voltage drop resistor causes decrease power-supply voltage (see AB-024 more details). circuit provides more filtering, especially high frequencies, however, resonant frequency network amplify lower frequencies. resistor placed series with inductor, this resonant frequency attenuated. Figure frequency response design equations network. This by-pass design approach known pi-filter. filter should positioned close noise source possible. Power supply noise reduced combination four methods. First, designer should carefully select DC/DC converter according power performance switching frequency. Second, filter output isolation amplifier eliminate high frequency noise. Third, pi-filter supply lines close switching source possible. fourth, some instances, external synchronization isolation amplifier makes possible synchronize multiple channels isolation amplifiers each other DC/DC power supplies.
MODEL ISO103 ISO107 ISO113 ISO212 3656 ISO100 ISO102 ISO106 ISO120 ISO121 ISO122
Isolation Function Buf-DC/DC Buf-DC/DC Buf-DC/DC Amp-DC/DC Amp-DC/DC Buffer Buffer Buffer Buffer Buffer
Isolation Barrier-Type (Signal/Power) Cap/Mag Cap/Mag Cap/Mag Opto
Signal Modulation Method Duty Cycle Duty Cycle Duty Cycle Balanced Flyback Linear Frequency Frequency Duty Cycle Duty Cycle Duty Cycle
Isolation Barrier Test Voltage(6) 4rms peak 4rms 1.2rms(2) 2.5DC 4rms peak 2.5rms(2) 5.6rms(2) 2.4rms(2)
Isolation Isolation Mode Barrier Rejection Impedance(1) Ratio 60Hz 10E12/9 10E12/13 10E12/9 10E10/12 10E12/6 10E12/2.5 10E14/6 10E14/6 10E14/2 10E14/2 10E14/2 130(1) 100(1) 130(1) 115(1) 108(1) 115(1) 115(1) 140(1)
Transient Immunity(1) kV/µs 0.006 0.6(3) 0.1(3) 1(3)
WideFull Scale band Signal Bandwidth/ Number Noise Output Small Signal Density(1) Ripple(1) Bandwidth DC/DC µV/Hz mVp-p kHz/µs Channels 0.02 0.117 20/75 20/75 20/75 2/400 1.3/500 5/100 5/100 5/100 20/50 20/50 20/50
DC/DC Output Ripple/External Filter Capacitor/ Frequency(1) mVp-p/µF/kHz 5/1/1600 10/0/1600 5/1/1600 10/10/25 100/.47/900
NOTES: Typical. Conforms with VDE884 partial discharge test methods. Value based limited evaluation; should used comparison purposes only.
greater ability reject isolation mode voltage transients. transient voltages between grounds exceed capabilities isolation amplifier, input sensor amplifier start false trigger output will display spurious errors. Transient immunity defined maximum rate change voltage that does interfere with normal transmission information across barrier. Errors high transients that less than full scale range isolation amplifier deemed within normal transmission range. high transient phenomena easy identify tracking difference between grounds correlating errors output isolation amplifier. transients predictable, this error filtered from signal timing data collection output isolation amplifier when data known valid. addition, selecting isolation amplifier with high Transient Immunity specification will reduce errors caused transients.
Isolation Barrier
problem experimenting with proximity circuit radiating device experimenting with shielding techniques. There numerous sources radiated noise such ground planes, power planes, metal traces close proximity, switching networks, inductors, toroids, etc. E-field B-field portion radiated field have effect isolation amplifiers. Specifically, high E-field vicinity capacitively coupled isolation amplifiers effect performance device. near-field emission areas, transmission radiated sources proportional inverse cube distance. Radiated noise transmit directly into signal, usually through capacitive barrier isolation amplifier. frequency content radiated noise multiple oscillating frequency isolation amplifier (plus minus bandwidth amplifier) radiated noise will appear signal bandwidth. example, refer Figure using left y-axis equal ratio output voltage isolation amplifier field strength radiate noise point entry. Although difficult quantify field strength radiated signal point entry, concepts Figure still apply. heavy fields, isolation amplifiers produce signals outside linear region. Radiated noise identified problem experimenting with shielding using scope probe identify spots. Various metallic materials used shielding long metal connected ground circuit. most effective shielding material found experimentation Mumetal, however, copper even conductive tape have been used identify eliminate problem areas. CONCLUSION Noise problems application difficult solve, particularly causes effects known. When investigating noise problem isolation application, combination three noise sources identified responsible noisy output isolated amplifier. understanding source noise, steps taken layout circuit design significantly reduce noise errors acceptable levels.
Isolation Amplifier
VOUT
FIGURE Transient Noise Caused High dV/dt Transients Between Grounds Isolation Application. RADIATED NOISE Radiated noise transmitted through into high impedance nodes. Some isolation technologies more sensitive radiated noise interference than others. Radiated noise, also called interference, easily identified
REFERENCES
Ott, Henry Noise Reduction Techniques Electronic Systems, John Wiley Sons, 1976. Morrison, Ralph, Noise Other Interfering Signals, John Wiley Sons, 1991. Zirngast, Mark Isolation Amplifiers-Design Implementation: Isolation, Transformer Optical Techniques, Part Electronic Engineering, April, 1989, 37-40. Zirngast, Mark Isolation Amplifiers-Design Implementation: Capacitive Isolation Part Electronic Engineering, May, 1989, 33-45. Burt, Rod, Noise Isolated Circuits with Variable Carrier Amplifier, Electronic Design, April 1988, 101-104. Baker, Bonnie, Improved Device Noise Performance 3650 Isolation Amplifier, Application Bulletin AB-044, Burr-Brown Corporation. Stitt, Mark, Very Cost Analog Isolation with Power, Application Bulletin AB-024, Burr-Brown Corporation. Stitt, Mark, Simple Filter Eliminates Ripple Keeps Full Bandwidth, Application Bulletin AB-023, Burr-Brown Corporation.
information provided herein believed reliable; however, BURR-BROWN assumes responsibility inaccuracies omissions. BURR-BROWN assumes responsibility this information, such information shall entirely user's risk. Prices specifications subject change without notice. patent rights licenses circuits described herein implied granted third party. BURR-BROWN does authorize warrant BURR-BROWN product life support devices and/or systems.

Mailing Address: 11400 Tucson, 85734 Street Address: 6730 Tucson Blvd.

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