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ET8.017El. Instr.Delft University of Technology
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Electronic Instrumentation
Lecturer: Kofi Makinwa
015-27 86466Room. HB13.270 EWI building
Teaching Assistant: Saleh Heidary
ET8.017El. Instr.Delft University of Technology
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Introduction
� Goal of a measurement system:– Get the most out of a measurement or sensor– in terms of information quality
� How?– Filtering (averaging) and shielding– Compensation (e.g. feed-forward)– Correction (e.g. auto-zeroing)– Feedback– Modulation and correlation
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ET8.017El. Instr.Delft University of Technology
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Precise butnot accurate
Not accurate andnot precise
Accurate butnot precise
Accurate andprecise
Time TimeTimeTime
Stable butnot accurate
Not stable andnot accurate
Accurate (on the average)
but not stable
Stable andaccurate
0
f fff
Accuracy, precision (resolution) and stability
ET8.017El. Instr.Delft University of Technology
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Signal processing techniques
Add-on techniques:� filtering
– prefiltering
– postfiltering
– correlation
� correction– post-processing
(e.g. auto-zeroing, linearization)
Built in techniques:� compensation
– balancing– bridge
� feedback– requires an inverse
transducer (actuator)
� modulation– chopping– spinning– swapping
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ET8.017El. Instr.Delft University of Technology
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Filtering: general principle
� reduces additive errors� in a specified frequency band� reduces the effects of thermal noise
interference
prefilter postfilter
sensor processormeasurand out
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Pre-filtering
� Pre-filtering (prior to transduction)� Mitigates the effects of:
– electrically induced signals (capacitive) ⇒ shielding– magnetically induced signals (inductive)
⇒ reducing loop area; shielding– changes in environmental temperature ⇒ isolation– unwanted optical input ⇒ optical filters– mechanical disturbances (shocks, vibrations) ⇒ dampers– .....
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Electrically-induced interference
, ,s
n o n ig
CV V
C=
sensor system interface system
Vn,i
Cs
Cg
Vn,o
s gC C<<
Vn,i: voltage noise source
Cs: capacitance between noise source and signal leads
Cg: capacitance between signal leads and ground
ET8.017El. Instr.Delft University of Technology
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Shielding
, , 0restn o n i
sh
CV V
C= →
Vn,i
Vn,o
Cg
Csh
Crest
interface systemsensor system
Crest: rest capacitance between noise source and signal leads
Cg: capacitance between noise source and ground
Csh: capacitance between shield and signal leads
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Post-filtering
� post-filtering (after transduction)� in the electrical domain
– low pass, high pass, band pass– first order, .... higher order– characteristic (Butterworth, Bessel,...)– analog, digital– Matched filtering
� If the time-domain characteristics of the measurand are known, correlation and windowing techniques can be used
ET8.017El. Instr.Delft University of Technology
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Post-filtering
� High-pass filter removes 50Hz interferencefrom ambient light sources
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Correction
� Add-on to an existing system– model-based correction (after calibration)– correction of cross sensitivities (extra sensors)
� Correcting for additive errors– auto-zeroing– CDS
� Correcting for multiplicative errors– linearization– 3-signal method (calibration of linear systems)
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Correction - Overview
interference
sensorsystem
processormeasurand out
modeldata
additional sensor
measureddata
reference
� Can cancel both additive and multiplicative errors� But additional sensors and multiple errors limit the
achievable accuracy
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Compensation
� where?– pre-compensation (inherent design)– post-compensation (model-based signal correction)
� which strategy?– balancing– feedforward– feedback (with an actuator)
ET8.017El. Instr.Delft University of Technology
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Balancing: General principle
� reduces (common) additive errors� improves dynamic range
Examples� dummy sensor: used to compensate for
environmental influences� Extra temperature sensor: used to compensate
for unwanted temperature dependencies
compensatingelement
sensor processormeasurand out
interference
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ET8.017El. Instr.Delft University of Technology
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Bridge compensator
VRR
RRR
RV io
+−
+=
43
4
21
2
Vi
R1
R2
R3
R4
Vo 4132 RRRR =
( )3412 RRRR =?
• for accurate absolute measurements of resistances
• Insensitive to CM disturbances e.g. temperature
• but requires an accurate, adjustable resistor
ET8.017El. Instr.Delft University of Technology
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Switched Cap “Resistor”
� Switched cap network emulates an adjustable resistance- Effective resistance value: Rref = Tclk/C2
M. Perrot et al., ISSCC 2012
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ET8.017El. Instr.Delft University of Technology
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Bridge indicator
VRR
RRR
RV io
+−
+=
43
4
21
2
Vi
R1
R2
R3
R4
Vo
• but output is sensitive to drift and error in Vi
initially: resistances have values such that
04010302 RRRR =
• measures small changes with respect to "an initial” value
ET8.017El. Instr.Delft University of Technology
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Ratiometric readout
ADC and bridge use the same reference voltage Vref
⇒ errors in Vref do not affect digital output
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Assignment 8
� Consider the ratiometric system shown on the previous slide. If the noise densities shown for the bridge, preamp and ADC, correspond to the situation when the bridge is balanced:
1. Calculate the resistance of each arm of the bridge2. If the measurement BW is 10Hz, how many bits of
resolution must the ADC have in order to ensure that the resolution of the system is determined by thermal noise rather than by quantization errors.
3. If the ADC has 20-bit resolution (not necessarily the answer to question 2 above) what percentage change in the resistance of a bridge element corresponds to a 1LSB change in its digital output.
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Differential measurements
1 1 1
2 2 2
m m n n
m m n n
y S x S x
y S x S x
= += − +
( ) ( )1 2 1 2comp m m m n n ny S S x S S x= + + −
2 22
ncomp m m n n m m n
m
Sy S x S x S x x
S
∆= + ∆ = +
2 m
n
SCMRR
S=
∆
opposite sensitivity
by design!
nx
compensatingsensor
sensor
processor
measurand
out
interference
1y
mx
2y
compy
Sm1: sensitivity sensor 1 to measurand
Sm2: sensitivity sensor 2 to measurand
Sn1: sensitivity sensor 1 to interference
Sn2: sensitivity sensor 2 to interference
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Differential capacitive displacement sensor
differential ∆x
+Vi -Vi
IoV1
I
V2
C2
C1
Cf
2
1 2o io f
CI j V
C Cω ∆= ⋅ ⋅
+
C2C1 Cf
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Differential sensor interface
R
Vo
Cf
_
+
Vi
C2
-Vi
C1
2 io
f
VV C
C= ∆
1 2i i
of f
V VV C C
C C= − +
1
2
o
o
C C C
C C C
= + ∆= − ∆
� The resistance R reduces drift� But it introduces extra noise and gives rise to a
frequency-dependent transfer function
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The utility of feedback
� If A(f)•β >>1 ⇒ ACL ≅ 1/β.
� For moderate 1/β, op-amp� DC gain is large enough!
� β ⇒ Resistor/Capacitor ratios.� Resistors ⇒ 0.01%, 5 ppm/°C� Capacitors ⇒ 1%, 500 ppm/°C
� ⇒ ACL can be very accurately defined.
ET8.017El. Instr.Delft University of Technology
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Feedback: General principle
� reduces multiplicative errors� improves dynamic behaviour
interference
feedback
sensor processormeasurand out
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General application of feedback
� Prerequisites for effective error reduction are:– high forward path transfer– stable feedback path transfer
0
01f
SS
S k=
+ ⋅
0
0 0
1
1f
f
dS dS
S k S S= ⋅
+ ⋅
S0
k
yoxm
+
−−−−
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Example 1: Accelerometer
sensor interface
seismic mass
differential capacitor
V(∆C)
Vo
actuator
∆x
a
inertia Vospring sensor interface gain
actuatorFa
Fi
a
∆x ∆C VC
+ −
Fs
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Analysis of a feedback accelerometer (1)
1s e
o ia s e
H HV H a
H H H=
+
io
a
HV a
H=
inertia Vospring sensor interface gain
actuatorFa
Fi
a
∆x ∆C VC
+ −
Fs
Hi: transfer from acceleration a to inertial force Fi
Hs: transfer from force Fs on sensor to displacement ∆x
He: transfer from displacement to output voltage Vo
Ha: transfer of actuator: from output voltage Vo to Fa
: 1:a sif H H >>
Independent of system parameters of the spring, sensor, interface, gain factor
Transfer function depends only on mass and actuator
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Example 2: Wind sensor
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ET8.017El. Instr.Delft University of Technology
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δT
� Airflow induces an on-chip temperature gradient δT� This is sensed by an on-chip thermopile and
cancelled by a pair of heaters� Transfer function only depends on the heaters
(resistors) and their supply voltage
Chip
Airflow
Heater
Thermal balancing
Controller
Heater
SensorCross-Section Thermopile
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Wind sensor chip
� On-chip heaters
� Thermopiles ⇒⇒⇒⇒ wind induced temperature differences δTNS & δTEW
� On-chip controllers nulls δTNS & δTEW
� |δP| ⇒⇒⇒⇒ wind speed� arg(δP) ⇒⇒⇒⇒ wind direction
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Smart wind sensor chip
� Digital outputs δPNS & δPEW
� After calibration:Speed error: ± 4% Angle error: ± 2°
� Comparable to conventional wind sensors
� But no moving parts!
ET8.017El. Instr.Delft University of Technology
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Modulation: General principle
� Basic ideas: shift information to a quiet frequency bands OR make information more robust
� Reduces additive errors� In particular, offset, drift, LF noise and gain error
interference
sensor processormeasurand out
modulation demodulation
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Modulation types
� AM modulation: Chopping, synchronous detection, dynamic element matching
� FM modulation: Used to make information more robust to amplitude variations
� Pulse-width (or duty-cycle) modulation: also used to make information more robust to amplitude variations, also used for simple digital-to-analog conversion
modulator
carrier
in out
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The Thermal Management Challenge!
� SoCs and multi-core processors ⇒⇒⇒⇒ multiple hot spots � Hot spot location depends on (dynamic) workload ⇒⇒⇒⇒ multiple (20+) temperature sensors
Intel 45nm Xeon Courtesy of S. Rusu Intel
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Thermal Diffusivity Sensing
� Heater generates small heat pulses (mWs)� Temperature sensor picks up delayed pulses (mVs)� Resulting thermal delay is temperature-dependent � Exploits the strengths of CMOS technology:
– Lithography � accurate spacing s– Pure silicon substrate � DSi is well-defined– Fast circuits � accurate delay
measurements
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A CMOS ETF
• Heater = n+-diffusion resistor
• Temp. sensor= p+/Al thermopile
• Can be made in any IC process!
• Circular, differential layout for maximum output
S
Heater Thermopile
100 µm
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Smart TD Sensor
� At room temp, s =20µm, f = 100kHz ⇒⇒⇒⇒ φETF ~ 90°� Quartz-crystal clock generates fdrive (typ. 100s of kHz)
� φETF is digitized by a phase-domain Σ∆ modulator
� Resolution is mainly limited by ETF’s thermal noise
[van Vroonhoven, Makinwa, ISSCC 2008]
φ ∝ /drive SiETF s f D
ET8.017El. Instr.Delft University of Technology
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Phase Digitizer
� How to accurately measure the phase of a small (1mVpp) and noisy signal?
� Use feedback, an ADC and a digital phase generator
� Then digitally filter the result (few Hz BW)
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Phase-Domain ∆Σ∆Σ∆Σ∆ΣM
� Since the sensor’s BW is so low, we can over-sample its output and then average ⇒ a 1-bit ADC is enough!
� This technique is known as sigma-delta modulation
� 12-bit resolution in 0.5Hz BW � fs > 2kHz � easy!
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Subtracting phase?
� Use a synchronous detector!
� DAC references are just phase-shifted copies of fdrive� multiplier inputs are at same frequency
� DC ∝ cos(φETF – φfb ) = 0 when φfb = φETF – 90°
� Approx. linear close to (φETF – φfb ) = 90°
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Performance
– ±0.2ºC (3s) in 0.18µm CMOS (same ETF layout)
� Accurate, and scales!
� Untrimmed spread is mainly limited by lithography
– ±0.6ºC (3σ) in 0.7µm CMOS
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Overview: Signal processing techniques
interference
sensor processormeasurand out
basic system
compensatingsensor
feedback
modulation demodulation
measureddata
inherent design
prefilter postfilter
correction
modeldata
additional systems
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THE END!
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Assignment 9
� The thermal diffusivity of silicon is proportional to 1/T1.8 where T is absolute temperature
� If an electrothermal filter has a phase shift of 90° at 300K, calculate its phase-shift at the extremes of the military temperature range, i.e.-55°C and 125°C
� A smart TD sensor employs a phase digitizer. How many bits of resolution should it have in order to achieve a temperature-sensing resolution of 0.1°C?
� If the phase digitizer employs the feedback architecture shown on slide 38 and uses a chopper as a phase detector, what range of phases must its phase DAC provide in order for the sensor to cover the military range?
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