Amplifier Noise Principles for the Practical Engineer

Amplifier Noise Principles for the Practical Engineer

The World Leader in High Performance Signal Processing Solutions Amplifier Noise Principles for the Practical Engineer Matt Duff Applications Engineer – Instrumentation Amplifiers Analog Devices, Inc. copyright 2009 Outline Noise z Extrinsic z Intrinsic Three main sources of intrinsic noise z Resistance z Amplifier Voltage Noise Current Noise z ADC Why so many units? z nV/rt(Hz), µVrms, µVp-p z Unit conversion Noise Math & Shortcuts Tips z Applying gain z Source resistance z Feedback resistance 2 Where does noise come from? -Extrinsic - Intrinsic 3 Extrinsic Noise RFI Switching Noise Noise coupling in from external Power sources Supply + Examples z RFI Coupling – z Power Supply Noise z Ground loops Digital circuitry 50/60 Hz Ground Not focus of this talk Loop Noise 4 Intrinsic Noise Intrinsic z Internal noise from components in signal chain Sensor Resistors Amplifier A/D z Specified on the datasheet z Focus of this talk 5 Main Sources of Intrinsic Noise -Resistor - Amplifier -A/D 6 Resistor noise vn 7 Types of Resistor noise Excess Noise z Depends on type: Carbon composition = bad performance vn Thick film = OK performance Thin film, wirewound = good performance z Increases with applied voltage z 1/f characteristic z Can ignore if using good resistors Intrinsic thermal noise z Independent of type z Independent of voltage applied z White noise characteristic z Need to calculate in design 8 Thermal noise of an ideal resistor bandwidth vn = √4k T R Δf resistance rms noise constant Temperature (in Kelvin) Too Complicated! 9 Resistor noise shortcut 1 kΩ→ 4 nV/√(Hz) Noise scales as square root of resistance 4 kΩ→ (2)(4) nV/√(Hz) = 8 nV/√(Hz) 9 kΩ→(3)(4) nV/√(Hz) = 12 nV/√(Hz) 16 kΩ→(4)(4) nV/√(Hz) = 16 nV/√(Hz) 100 kΩ→(10)(4) nV/√(Hz) = 40 nV/√(Hz) 10 Common Resistor Circuits: Resistor Divider VIN R1 R1 R1 || R2 Theory: VOUT VOUT VOUT R2 R2 VIN 200 kΩ 200 kΩ 100 kΩ √(100) * 4 nV /√(Hz) = Example: 40 nV /√(Hz) 200 kΩ 200 kΩ 11 Common Resistor Circuits – Bridge Theory: Example: 12 Amplifier Noise 13 Amplifier Voltage Noise Units: z nV/√(Hz) z µVrms z µV p-p example from AD8295 datasheet: 14 Amplifier Voltage Noise: Referred to What? Options z Referred to Input (RTI) z Referred to Output (RTO) If not stated, referred to Input Multiply by “noise gain” RTI RTO vx + 10 vx – 9 1 15 Instrumentation Amplifiers are Different Op Amps In Amp z All noise dependent on gain eNI In Amps + eNO z Some noise dependent on gain (eNI) z Some noise independent of gain (eNO) – Total In Amp Noise, referred to input 2 2 z √( eNI + (eNO/G) ) example from AD8221 datasheet: 16 Amplifier Current Noise Units: z fA/√(Hz) z pArms z pA p-p example from AD8295 datasheet: 17 Amplifier Current Noise: Effect depends on resistance Source resistance Feedback network + + ix1 ix1 R R ix1 – – ix2 ix2 18 ADC Noise: Sometimes voltage units provided: But most of the time, signal to noise ratio (SNR) z With distortion: SINAD In emergency use ideal equation z SNR = 6.02 * Bits + 1.76 z Gives better performance than reality 19 ADC Noise – Converting SNR to µVrms Full Scale Voltage in µVrms Conversion Equation: = ADC noise in µVrms SNR 20 10 Key Point: Use RMS value for full scale range! Example: z 2 V ADC input range z 69 dB SNR 20 Why so many units? - µVrms, µVp-p, nV/√Hz - RMS, peak to peak, spectral density - converting between units 21 Peak to Peak and RMS noise Peak to peak noise z Distance from min and max points on waveform z Depends on only two points Less accurate z Variable z Measure longer -> bigger result Easy to compute rms z Max - Min RMS noise z One standard deviation (mean is zero) peak to z Depends on all points peak More accurate z Repeatable z Measure longer -> more accurate result Lengthy to compute 2 2 2 x1 + x2 +...+ xn n 22 RMS and Peak to Peak: Dependent on Bandwidth AD8222 Noise (G=100) When measuring: z Circuit bandwidth Bandwidth = 10 Hz z Measurement instrument bandwidth Bandwidth = 100 Hz When specifying z Bandwidth must be noted Bandwidth = 1000 Hz Voltage (0.5 µV/DIV) (0.5 Voltage Time (1 s/DIV) 23 Spectral Noise Density 1k Frequency Domain z Noise density at specific AD8295 frequency Units z nV/√Hz 100 z fA/√Hz NOISE (nV/ Hz) Includes many, many points z Datasheet figure has lots of averaging 10 1 10 100 1k 10k 100k FREQUENCY (Hz) 07343-067 24 Spectral Noise Density – 1/f corner 1k 1/f noise z Noise density increases at AD8295 low frequencies 1/f noise White noise z Noise density flat at high 100 frequencies NOISE (nV/ Hz) white noise 1/f corner z Where 1/f noise and white noise trendlines intersect 10 1 10 100 1k 10k 100k FREQUENCY (Hz) 07343-067 noise corner 25 How to convert Conversion process z nV/√(Hz) → µVrms z µVrms → µVp-p 26 µ nV/√(Hz) -> Vrms: theory 1k Theoretical Equation f HIGH 2 Noise = (SD ) df 100 RMS ∫ f fLOW NOISE (nV/ Hz) Where z SDf: spectral density at frequency f 10 1 10 100 1k 10k 100k z f : low frequency cutoff FREQUENCY (Hz) LOW 07343-067 z fHIGH: high frequency cutoff Too Complicated! 27 µ nV/√(Hz) -> Vrms shortcut1k Assumption z Mostly white noise 1/f corner << bandwidth Equation 100 z RMS Noise = NOISE (nV/ Hz) spectral density * √(bandwidth) spectral density 10 1 10 100 1k 10k 100k FREQUENCY (Hz) 07343-067 bandwidth 28 Equivalent Noise Bandwidth 1k Equivalent Noise bandwidth for Butterworth filters z 1 pole: 1.57 z 2 pole: 1.11 z 3 pole: 1.05 100 NOISE (nV/ Hz) Example: z Given: 40 nV/rt(Hz) 10 10 kHz 1 pole filter 1 10 100 1k 10k 100k FREQUENCY (Hz) z Answer: 07343-067 40 nV/rt(Hz) * sqrt(10 kHz * 1.57) ≈ 5000 nVrms = 5 µVrms 29 nV/√(Hz) -> µVrms Not mostly white noise? z 1/f corner near bandwidth Options 50 Hz z Use theoretical formula? z Use amplifier with low 1/f corner z Compare uVp-p numbers 0.1 to 10 Hz z Autozero/Chopper 200 Hz 30 µVrms -> µVp-p To get peak to peak noise z In theory: peak to peak noise infinite z In practice: multiply rms by 6 Multiplier of ‘6’ is rule of thumb: 99.73% of points Examples z 1 µVrms * 6 ≈ 6 µVp-p z 1 µVrms * 6.6 ≈ 6.6 µVp-p 6 standard deviations covers 99.7% 6.6 standard deviations covers 99.9% graph created by Jeremy Kemp 31 Math and Shortcuts 32 Noise Math Addition Multiplication z Noise adds as sum of z Gain and attenuation work squares just like normal signals Example Example 100k Ω + 40 nV/√Hz ideal √(40^2+40^2) = in amp 57 nV/rt(Hz) 100k Ω G=1 – 40 nV/√Hz 33 Noise shortcuts 1 k Ω resistor -> 4 nV/√Hz When adding noise sources, larger sources dominate z Sum of squares addition z Ignore signals < 1/5th largest signal If first stage gain is large z Later stages typically have little effect 34 Tips 35 Noise Tip #1: Apply Gain Early Noise adds as sum of squares 36 Noise Tip #2: Watch out for source impedance Source Resistance adds noise Current noise calculation Sensor + R SOURCE – 37 Source impedance example Sensor Given the following sensor: 9 kΩ Which op amp is better? Sensor 9 kΩ Noise contributors (RTI): Sensor noise = 4 x √9 = 12 Amp Voltage noise = 1.1 Amp Current noise = 2.3 x 9 = 20.7 Total Noise = √(122+1.12 + 20.72) = 24 nV/rt(Hz) 38 Noise Tip #3: Watch out for feedback resistors Rf acts like voltage source at output Rg acts likes inverting configuration voltage source What noise dominates? z Rf dominates Noninverting config: G<2 Inverting configuration: G > -1 z Rf and Rg noise equal Noninverting config: G=2 Inverting config: G = -1 z Rg dominates Noninverting config: G>2 Inverting config: G<-1 Watch out for current noise 39 Summary Noise z Extrinsic z Intrinsic Three main sources of intrinsic noise z Resistance z Amplifier Voltage Noise Current Noise z ADC Why so many units? z RMS noise, peak to peak noise, spectral density z How to convert between units Noise Math & Shortcuts Tips z Gain Early z Source Impedance z Feedback resistors 40 Where to learn more Application Notes z AN-940: Low Noise Amplifier Selection Guide for Optimal Noise Performance z AN-358: Noise and Operational Amplifier Circuits Analogue Dialogue z Ask The Applications Engineer -7: What should I know about op-amp noise? z Ask The Applications Engineer -8: What is “noise gain”? Webinar z Noise Optimization in Signal Conditioning Circuits (Three part series) Tutorials z MT-047: Op Amp Noise z MT-048:Op Amp Noise Relationships: 1/f Noise, RMS Noise, z and Equivalent Noise Bandwidth z MT-049: Op Amp Total Output Noise Calculations for Single-Pole System z MT-050: Op Amp Total Output Noise Calculations for Second-Order System z MT-065: In-Amp Noise 41 The World Leader in High Performance Signal Processing Solutions Questions? Contact Info: Matt Duff (781) 937-2123.

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