The Design of a HighPrecision ReadOut Circuit for Thermistors

1
The Design of a High-Precision Read-Out Circuit
for Thermistors

• Rong Wu1, K.A.A. Makinwa1, Johan H.Huijsing1,
• Stoyan N. Nihtianov1,2
• 1Electronic Instrumentation Laboratory, Delft
  University of Technology,
• 2ASML Netherlands B.V. Veldhoven
• The Netherlands
• 16-Nov-2007

2
Overview

• Introduction
• Design specifications
• Design challenges and consideration
• Simulation result
• Conclusion

3
Project Motivation

• To design read-out circuit for high
  precision(1µK) thermistor sensors in
  wafer-steppers.
• ultra low-power
• Vacuum compatiable

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Thermistors
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Read-out System for a Thermistor
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Overview

• Introduction
• Design specifications
• Design challenges and consideration
• Simulation result
• Conclusion

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Specification of the thermistor bridge

• Noise (3s) 1µK
• Sensitivity 27mV/K
• Thermal noise level 14nV/
• Temperature range 21.1-22.9 ºC
• Bandwidth 3mHz-50mHz
• Gain error 0.5

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Noise budget in the thermistor read-out system
0.74µK lt 1µK (spec)
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Overview

• Introduction
• Design specifications
• Design challenges and consideration
• Simulation result
• Conclusion

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Design Challenges

• Low drift, low 1/f noise (17nV/rt(Hz)) at low
  frequency (3mHz-50mHz)
• Low power (lt250µA)
• Ground sensing input, 0-5V output.

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Amplifier Behavior Near DC
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Chopping in Frequency domain
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Design Challenges

• Low 1/f noise (17nV/rt(Hz)) at low frequency
• (3mHz-50mHz)
• Low power (lt250µA)
• Ground sensing input, 0-5V output.

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Design Challenges

• Low 1/f noise (17nV/rt(Hz)) at low frequency
• (3mHz-50mHz)
• Low power (lt300µA)
• Ground sensing input, 0-5V output.

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Read-out System for a Thermistor
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Current Mode Instrumentation Amplifier (CMIA)

• Input and output voltage level are isolated
• Gain accuracy is good enough

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CMIA topology
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Overview

• Introduction
• Design specifications
• Design challenges and consideration
• Simulation result
• Conclusion

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Simulated noise spectrum
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Chip Photo
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Overview

• Introduction
• Design specifications
• Design challenges and consideration
• Simulation result
• Conclusion

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Conclusion

• A low noise current-mode instrumentation
  amplifier
• has been realized.
• As intended, the measured noise level is 19nV/
  
• and the 1/f noise corner frequency is very
  low (30mHz).
• The supply current is 226µA, which satisfies the
  specifications.

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• Thank you!

24

• Some answers

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Gain accuracy

• The nonlinearity of the two gm stages are
  compensated if these two gm stages are biased at
  the same condition
• Loop-gain is large enough.

26
LPFs cut-off frequency
Ripple can be reduced to 10uV
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Noise from the voltage reference
AD1580B
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3s noise calculation

• Thermistor 14nV/
• Preamp 17nV/
• ADC 2nV/
• Reference 6nV/
• 1/f noise 2.5nVrms
• BW 0.05Hz
• 3s noise 20nVrms ? 0.74µK lt 1µK (ASML spec)

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Noise budget in the thermistor read-out system