SPR Surface Plasmon Resonance Chemical Sensing Microsystems

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Electronic Interfaces for Sensors and Sensor
Systems Denise Wilson, Associate
Professor Department of Electrical
Engineering University of Washington NSF/RISE
Workshop/Short Course Development and Study of
Advanced Sensors and Sensor Materials July 11,
2006
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Outline

• Basic Measurement Circuits
• Voltage
• Resistance
• Current
• Charge/Capacitance
• Phase Delay/Wave propagation
• Frequency
• Analog to Digital Conversion (ADC)
• Operational Amplifiers
• Power Supply Control

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Electronic Interfaces for Sensors and Sensor
Systems

• Voltage Measurement
• Sample Measurement Circuit Standard Inverting
  Amplifier
• Simplest configuration available
• Inverts effect of input (as input increases,
  output decreases vice versa)
• Ro limits current in over-voltage conditions

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Electronic Interfaces for Sensors and Sensor
Systems

• Voltage Measurement
• Sample Measurement Circuit Standard
  Non-Inverting Amplifier
• More inclined to saturate
• Retains effect of input (as input increases,
  output increases vice versa)
• Ro limits current in over-voltage conditions

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Electronic Interfaces for Sensors and Sensor
Systems

• Voltage Measurement
• Sample Measurement Circuit Standard
  Differential Amplifier
• Two inputs tend to shift in same direction for
  non-stimulus related fluctuations
• Useful for reducing the influence of drift and
  other systematic errors
• Common mode gain must lie below acceptable error
  levels

R
V1
Vout A (V2-V1) ACM ((V2 V1)/2)
V2
Ro
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Electronic Interfaces for Sensors and Sensor
Systems

• Voltage Measurement
• Issues to Consider
• High gain requires large resistors, which
  introduce higher noise
• Higher gain tends to induce greater instability
• Longer measurement windows (lower frequency)
  accumulate more noise
• Sensors that produce a voltage for output often
  require zero current to be drawn for accurate
  measurement.
• Filters can be used to
• Low-pass sensor signals Reduce aliasing (the
  appearance of high frequency information as lower
  frequency signals)
• High-pass sensor signals reduce impact of drift
  and systematic changes that occur over a time
  constant much longer than the sensor response
  time.
• Amplify at the same time as filtering
• Examples
• Piezoelectric Sensors, Electrochemical
  (potentiometric mode) sensors

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Electronic Interfaces for Sensors and Sensor
Systems

• Voltage Measurement Noise minimization
• A low-noise voltage amplifier minimizes addition
  of noise incurred by the process of amplification
  but cannot reduce the noise at the sensor itself
• Sensors are especially vulnerable to noise
• Environments inherently difficult to control
  and predict
• Sensor surface large percentage of overall volume
  high noise situation
• Solution Lock-in Amplification (retains portion
  of signal exclusively related to sensor activity)
  -- fo X Sensor output gives strong DC component
  at fo.

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Electronic Interfaces for Sensors and Sensor
Systems

• Resistance Measurement
• Sample Measurement Circuits

Voltage Divider Vout -Vcc (Vdd
Vcc)(Rload)/(Rsensor Rload)
Wheatstone Bridge Vout (R1/(R1Rsensor) -
R3/(R3R4))Vdd Balanced so that R1/Rsensor
R3/R4 when no stimulus is present (Vout 0)
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Electronic Interfaces for Sensors and Sensor
Systems

• Resistance Measurement
• Issues to Consider
• The Wheatstone bridge is larger and dissipates
  more power
• The Wheatstone bridge provides a differential
  measurement Zero voltage output in the presence
  of zero stimulus
• Enables best matching of dynamic range between
  circuits best resolution
• Reduces power dissipation in subsequent circuits
• Both circuits distort linearity a linear sensor
  now produces a non-linear output (and variable
  sensitivity)
• Pair
• 1 calculate sensitivity for the voltage
  divider
• 2 calculate sensitivity for the Wheatstone
  bridge
• Share
• Is there any difference in sensitivity? Why or
  why not?

Activity
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Electronic Interfaces for Sensors and Sensor
Systems

• Resistance Measurement (using the Howland Current
  Source)
• A more complex measurement circuit

When R2AR2B R1R4/R3 Io becomes independent of
Rsensor Vout Rsensor Io Sensitivity Io (is
constant!)
Io
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Electronic Interfaces for Sensors and Sensor
Systems

• Resistance Measurement
• Issues to Consider (cont)
• Resistors (in the measurement circuit and in the
  sensor) are typically the noisiest possible
  components in electronics!
• Both measurement circuits should be matched to
  the dynamic range of subsequent circuits. For
  example
• An A/D Converter with 12 bits of resolution at
  0-10V connected to a
• Resistive sensor whose voltage output varies from
  2-3.5V
• Provides only 9 Bits of resolution to the sensor
  
• 10/212 .00214V of resolution
• 1.5V/.00214V 614 distinguishable levels from
  2-3.5V
• 614 is closest to 29 ( 512)
• A 12 bit converter provides only 9 bits of sensor
  resolution

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Electronic Interfaces for Sensors and Sensor
Systems

• Resistance Measurement
• Sensor Examples
• Chemiresistors
• Composite polymer
• Metal-oxide
• Phthalocyanines
• Conductive Polymer
• Mechanical Sensors
• Piezoresistive elements in accelerometers/motion
  sensors
• Photoconductors

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Electronic Interfaces for Sensors and Sensor
Systems

• Current Measurement
• Sample Measurement Circuit

R
Vout -IsensorR
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Electronic Interfaces for Sensors and Sensor
Systems

• Current Measurement
• Issues to Consider For small currents (order of
  nA or smaller)
• The input current of the op amp must approach the
  ideal value of 0 (requires a MOSFET input)
• Parasitic capacitances/resistances in the
  breadboard/PCB/protoboard can overwhelm the
  current to be measured.
• Large resistors may be required in the feedback
  loop for desired gain
• Generate additional noise
• Can induce instability
• Many sensors require a constant non-zero (bias)
  voltage to increase current to be measured.
• Examples
• Photodiodes
• Electrochemical sensors (in amperometric mode of
  operation)

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Electronic Interfaces for Sensors and Sensor
Systems

• Current Measurement
• Constant reverse bias across a photodiode during
  current measurement
• Higher reverse bias increases sensitivity

Vout -IphotodiodeR
Photodiode
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Electronic Interfaces for Sensors and Sensor
Systems

• Analog to Digital Conversion
• Before Conversion
• Minimize Aliasing using Low-Pass filters
• Reduce Noise through
• Lock-in amplification
• Filtering, or
• Sensor averaging
• Amplify signal
• Differential amplification to reduce common mode
  influences
• Low noise pre-amplifier
• High gain post/primary amplifier
• Convert signal to voltage
• Frequency to voltage (commercially available
  ICs)
• Current to voltage (trans-resistance amplifier or
  configured op-amp)
• Resistance to voltage (Howland current source or
  Wheatstone bridge)

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Electronic Interfaces for Sensors and Sensor
Systems

• Analog to Digital Conversion
• During conversion
• Match input signal range to ADC range for maximum
  resolution
• Choose best scale (linear, logarithmic) to match
  sensor behavior
• Example
• An electrochemical sensor has an output voltage
  that varies logarithmically as a function of
  calcium ion concentration.
• Use a logarithmic rather than linear converter
• This same sensor generates output voltages
  between 0 and 60mV across the full range of
  calcium ion concentrations of interest. The ADC
  has a voltage range from 0-12V
• Use an inverting, low-noise/low-offset
  pre-amplifier with gain of -2
• And, in series with the pre-amplifier
• Use an inverting, amplifier of gain approximately
  -100

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Electronic Interfaces for Sensors and Sensor
Systems

• Operational Amplifiers Four basic types
• Voltage
• High input impedance
• Low output impedance
• Current
• Low input impedance
• High output impedance
• Transresistance (current input, voltage output)
• Low input impedance
• Low output impedance
• Transconductance (voltage input, current output)
• High input impedance
• High output impedance
• No op amp is truly general purpose!

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Electronic Interface Examples

• Chemiresistor Interface
• Chemiresistors are typically designed for large
  array implementations
• Electronic Interfaces require small,
  low-component count circuits that make the best
  compromise in performance for small size and low
  power consumption
• Surface Plasmon Resonance Portable Interface
• SPR-based sensors are typically designed for one
  component, distributed signal (wavelength or
  angle) implementation
• System on chip solutions must compensate for the
  influence of the background medium AND implement
  a (pre-optimized) calibration vector for
  determining analyte concentration/refractive
  index
• Fluorescence Analysis Systems
• Require relatively complex electronics at both
  the light source (input) and output
  (photodetection) stages of system
• Electronic interface, for best performance, must
  be well matched to the (biochemical) requirements
  and fluctuations of the system.

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Electronic Interfaces for Sensors and Sensor
Systems

• Example Electronic Interfaces for Chemiresistors
• Popular approach to chemical sensing
  (traditional)
• Small number (highly selective) sensors in an
• Application targeted to 1-2 analytes
• In an understood background
• Another approach to chemical sensing
  (olfactory)
• Large number (broad, overlapping selective)
  sensors in an
• Application targeted to many analytes
• And their (many) interferents
• In a cluttered and complicated background
• Candidates for high density arrays of chemical
  sensors are few
• Require small size, linear operation, broad
  selectivity, compatibility with integration, and
  room temperature operation

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• High resolution Sensor Arrays
• Require Integration
• Circuits produced in CMOS
• Gold post-deposited electrochemically
• Sensor coating sprayed on gold
• 1-2 layers of metal required for sensor
• Challenge Design processing circuits that
• Ignore large, variable baseline resistance
• Amplify very small changes in polymer resistance
  on top of large baselines
• Conform to VLSI footprint that addresses
• Electrode Geometry
• Required sensor density
• Circuit performance

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Differential Approach
• On-chip chemiresistor divided into
• One chemically sensitive resistor
• One or (three) reference resistors
• Passivated (responsive to zero analytes) or
• Exposed, not functionalized (responsive to all
  analytes)
• Resistive Bridge is part of sensor
• Remaining circuits are designed for maximum gain
  under constrained footprint ( sensor platform)

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Differential Approach
• Resistive Bridge output transferred to
• Differential Amplifier
• Comparator with ramping input for serial A/D
  conversion
• Design constraints
• Differential Amplifier maximum gain in small
  footprint
• Comparator fully serial (simple) A/D conversion
  acceptable because of slow sensor response time

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Differential Approach
• Circuit Gain
• 20 (Differential Amplifier)
• -20 (Comparator)
• Sensor Performance
• Bridge approach eliminates effect of broad range
  in baseline on circuit gain
• However, additional bias resistors add more noise
  (electrical and transduction)

• Translation
• 25mV detection limit
• Independent of baseline
• 0.01 (DR) detection limit and resolution

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Resistance to Frequency Conversion
• Sensor platform contains three terminals
• Outer ring terminals shorted together outside
  sensor platform to enable circuits to fit
  underneath
• Allows a single resistor per platform for
  chemical sensing
• More active area (fill factor) than previous
  approach.
• Electrode geometry more readily optimized for
  best noise performance.

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Resistance to Frequency Conversion
• Operation
• Sensor resistance charges Co
• As the capacitor charges, it trips the Schmitt
  trigger, causing the feedback to discharge the
  capacitor
• The frequency of the charge/discharge cycle
  becomes smaller with increasing resistance
  (smaller current)
• Hysteresis reduces impact of noisy sensor response

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Electronic Interfaces for Sensors and Sensor
Systems

• Electronic Interfaces for Chemiresistors
• Resistance to Frequency Conversion
• Sensitivity
• Baseline (730kW) .12/W
• Baseline (9.26kW) 4.1/W
• Resolution/Detection Limit
• Change in resistance from baseline
• Baseline (730kW) .07
• Baseline (9.26kW) .02

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Electronic Interfaces for Sensors and Sensor
Systems

• Example Electronic Interfaces for Chemiresistors
• Both circuits fit underneath sensor platform (.04
  mm2 area)
• Fill Factor
• Approach 1 25
• Approach 2 close to 100 (with exceptions for
  metal routing)
• Sensitivity
• Approach 1 400 (V/V)
• Approach 2 between .12/W and 4.1/W
• Resolution/Detection limit
• Approach 1 .01 change in resistance
• Approach 2 between .02 and .07
• Other
• Approach 2 more resilience to fluctuations in
  response due to built in hysteresis.

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Electronic Interfaces for Sensors and Sensor
Systems

• Example Surface Plasmon Resonance Systems
• Comparison of Four Approaches to SPR Interfaces
• Traditional (Software-based)
• Partial Integrated Circuit (Voltage-based)
• Full Integrated Circuit (Pulse-based)
• Full Integrated Circuit (Current-mode)
• All Interfaces assume
• Constant angle (variable wavelength)
  interrogation scheme
• Fiber-optic optical path
• Spectral dispersion (prism or similar) prior to
  photodetection

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Electronic Interfaces for Sensors and Sensor
Systems SPR
Approach 1 (Traditional)
Software
High Resolution Photodetection
Approach 2 (Voltage-Mode, Partially Integrated)
Software
Integration Time Programming
Low Resolution Photodetection
Flatlining Reference Ratio
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Electronic Interfaces for Sensors and Sensor
Systems SPR
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Electronic Interfaces for Sensors and Sensor
Systems SPR
Approach 2
All Designs are mixed signal, fabricated in
standard CMOS
Approach 4
Approach 3
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Electronic Interfaces for Sensors and Sensor
Systems SPR
15 pixel array fabricated on a 1cm2 die in the
1.5 micron AMI process through MOSIS
2mm
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Electronic Interfaces for Sensors and Sensor
Systems SPR