Instrumentation Amplifier Noise Analysis

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Instrumentation Amplifier Noise Analysis
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Short Review of 3 Amp INA
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Three Stage IA
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Real World Input to Mathematical Model
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Analyze the Input and Output Separately
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Split Input Stage in Half
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Use Superposition on Output Amp
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Gain For Three Amp IA
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Noise Model of 3 Amp INA
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Complex Noise Model
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The Complex Model is Simplified
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The Input amplifier dominates at High Gain
G Total Input-Referred Noise (nV/rtHz) Total Output Noise (nV/rtHz)
1 206.2 206.2
2 111.8 223.6
5 64 320
10 53.9 539
100 50 5000
1000 50 50,000
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Two Ways to represent INA Spectral Density
G Input-Referred Noise (nV/rtHz)
1 206.2
10 53.9
100 50
1000 50
G Input-Referred Noise (nV/rtHz)
1 110
10 12
100 8
1000 8
Calculated using graphs and formula
Taken directly from the graph
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Hand Analysis of 3 Amp INA
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Find the total RMS Noise Voltage at the Output
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Look at Noise Sources Bridge, INA333,
Reference Buffer
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Noise Equivalent Model for Reference Pin Buffer
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Reference buffer
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The reference voltage directly adds to the output
noise
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The bridge generates thermal noise, in x
R_bridge
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Noise From Bridge / Current Sources
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Combine all the noise sources
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Rule of 3x
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(No Transcript)
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(No Transcript)
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Calculate RMS Output Noise for INA333 From
Voltage Noise
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Simulation of 3 Amp INA
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Simulate the Circuit
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Using Tina Spice
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Noise Spectral Density at the Output
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Total RMS Noise at the Output
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Why doesnt calculation match simulation exactly?
Bandwidth from Data Sheet and simulated bandwidth
is different.
The roll-off was approximated as first order in
the calculations. Simulation shows that it is
not first order.
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Reduce Noise Using Averaging Circuit
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Averaging Circuit
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Noise in Averaging Circuit
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Averaging Circuit with INA333
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Experiment with 20 Parallel INA333
20 INA333 amps in parallel (jumper selectable)
Socketed Gain Set Resistors
OPA333 Averaging Circuit
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Standard Noise Measurement Precautions
Linear Power Source
Steel Paint Can for Shielding
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Total Output Noise vs Number of Amplifiers Being
Averaged
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Measured vs simulated spectral density
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References

• 1 Hann, Gina. "Selecting the right op amp -
  Electronic Products." Electronic Products
  Magazine Component and Technology News. 21 Nov.
  2008. Web. 09 Dec. 2009. lthttp//www2.electronicpr
  oducts.com/Selecting_the_right_op_amp-article-facn
  texas_nov2008-html.aspxgt.
• Henry W. Ott, Noise Reduction Techniques in
  Electronics Systems, John Wiley and Sons

Acknowledgments

• R. Burt, Technique for Computing Noise based on
  Data Sheet Curves, General Noise Information
• T. Green, General Information
• B. Trump, General Information
• Matt Hann, General INA information and review

Noise Article Series (www.en-genius.net)
http//www.en-genius.net/site/zones/audiovideoZONE
/technical_notes/avt_022508
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Thank You for Your Interest in INA Noise
Calculation and Measurement