MOTION SENSING AND DATA

ACQUISITION IN HIGH

TEMPERATURE ENVIRONMENTS

Jeff Watson

jeffrey.watson@analog.com

Maithil Pachchigar

Venkat Chandrasekaran

Analog Devices, Inc.

High Temperature Applications

Oil and Gas Exploration Avionics

Under-hood Automotive Heavy Industrial Processes Geothermal

High Temperature Applications

Oil and Gas Exploration Avionics

Under-hood Automotive Heavy Industrial Processes Geothermal

Common Signal Processing Needs:

• Multiple Sensors

• Precision Data Acquisition

• Low Power

• Small Form Factor

• Rugged Packaging

What is “High Temperature”, Anyway?

0-40-55 70

85

125 150 175 210

COMMERCIAL

INDUSTRIAL

MILLITARY/EP

AUTOMOTIVE

HT PLASTIC

HT CERAMIC

Temperature °C

Using Standard Temp ICs at Elevated

Temperatures is Problematic!

Process Changes

Expensive Qualification

Die Availability

Package Type

Package ReliabilityPerformance

Electromigration

Failure Analysis

HT Process Flow Highlights

Design

• Process Selection

• HT Design Practices

• Electromigration / Current Density

Characterization

• Design Verification

• HT Datasheet Specifications

• Multiple Lots

Qualification

• HT Operating Life

• HT Storage

• Temp Cycle

• MIL-STD-883 Group D Mechanical

• Shock

• Vibe

• Accel

• Seal

• Moisture

• ESD

• Latchup

Production Test

• HT Probe 175C

• HT Package

• 150C Plastic

• 190+C Hermetic

How Accelerometers Measure Acceleration

F = kxF = mA

Am

k

x

kx = mA x = A

known

At constant velocity, the spring does not stretch.

�

�

Typical displacement in

a low-g accelerometer

~10 nm/g

Measuring the Position of the Proof Mass

X

Y

�Differential capacitance used to pick off motion of mass

� C1 and C2 is the capacitance between the mass and a set of fixed fingers

� Keep monitoring (C1 – C2) to determine if the mass has moved in the X-axis

C1 C2

Gyro Building Blocks

x

x

x

xMass with

velocity

• The “resonator” (also referred to

as the moving mass) which

generates a force any time we

rotate it

• The “Coriolis” channel which is

the accelerometer that surrounds

the moving mass, and its sole

purpose is to detect the force

generated by the resonator

• Complicated couplingCouplingForce on

XL

Gyroscope Circuit Architecture

• Surface micro-machined gyros made using ADI specific “Integrated” BiMOS process

• “Integrated” means beams and circuits are made side by side on the same piece of Silicon

• Beam structures are made out of 4 um thick poly-silicon

• Signal conditioning circuitry also large geometry

• Single die solution (BiMOS die with both beams and circuits)

• BiMOS process allows laser trim

Sensor SpecificationsParameter Value Units

Supply Voltage 5.0±0.25 V

Temperature Range −40 to 175 °C

Quiescent Current 3.5 mA

Sensitivity 1.0 mV/°/s

Dynamic Range±2000

(extendable to ±5000)°/s

Bandwidth (±3dB) 2000 Hz

Noise Density 0.25 °/s/√Hz

Resolution Floor - 25°C

150°C

100

150°/hr

Linear-g sensitivity 0.1 °/s/g

Vibration rectification

25g rms, 50Hz to 5kHz 0.0006 °/s/g2

Bias Stability – Allen Variance

• Bias stability is often considered the most important parameter

• Measured by Allen Variance, which is function of averaging time

• Allan Variance plot shows the bias uncertainty between successive averaging periods

• Initial slope corresponds to broadband noise

• Longer time asymptote of the plot is equivalent to the resolution floor

Other Accuracy Considerations

�But Wait! It isn’t that simple. Other “real world” error sources

�Null Bias (Offset)

�Sensitivity Error (Gain)

�Variation over temperature

�For this reason there is a temp sensor on board for calibration

�Temperature hysteresis is very low, on order of 0.075% FS

�Plus, all of these other error sources could be dwarfed by effects of linear acceleration (vibration)

Error Due to Linear Acceleration and Vibe

� Ideally a gyroscope would be sensitive only to rotation rate

� In reality all gyros have sensitivity to acceleration due to asymmetry of mechanical design and other micromachining inaccuracies

�Most significant are sensitivity to linear acceleration (g sensitivity) and vibration reification (g2 sensitivity)

�Also shock events can saturate sensor response

�These errors can easily be largest in error budget when operating in a harsh environment

Dual Resonator Design

x

x

x

xMass

with

velocity

Resonator

Moving

RightForce on

XL if

Rotation

occurred

x

x

x

xMass

with

velocity

Second

Resonator

Moving Left Force on

XL if

Rotation

occurred

Quad Resonator Design

Linear-g sensitivity 0.1 °/s/g

Vibration rectification

25g rms, 50Hz to 5kHz 0.0006 °/s/g2

Temperature Compensation

• To operate over the wide temp range best performance is realized if offset and sensitivity are temperature compensated

• On chip temp sensor to facilitate this

• Need null voltage, sensitivity and temp sensor output for each cal point

• The variation over temp is non-linear, so at least six temp cal points may be needed

Device Calibration – Null Output (Offset)

Offset over temperature Residual error after 5th order

polynomial correction

Device Calibration – Sensitivity

Sensitivity over temperatureResidual error after 5th order

polynomial correction

Package – High Temperature Compliance

�Hermetic Ceramic Package � Established Assembly BOM

�Solder Seal Lid

�Mono-metallic wire bond (Al-Al)

�Au plated leads

�Through-hole leads

Package – Vertical Mount Ceramic

Application Example:

Downhole Drilling

• Accurate, robust measure of rotation needed

• Needed now more than ever with focus on drilling dynamics and abuse measurements

• Magnetometers can be used but subject to interference and problematic in high shock/vibe

• Need high temperature operation

Downhole Drilling – Stick-Slip

Drill Bit Stuck

Continues to

Rotate at Surface

…Leads to cyclical violent motion when drill string breaks free

Downhole Drilling – Stick-Slip

Downhole Tool Signal Chain

HT Data Acquisition Reference DesignOptimized for low power/low voltage with 2.5V reference

AD7981

ADR225 AD8634VSUPPLY Connections for

ADC, Amp, Reference

Space for

passive

filter

AD8634

SPI PMOD to

connect up

outside the oven

Solder links to allow user

to bypass the area

SMA connector

Space for

passive

filter

Specs of Reference Design

• Single Channel Data Acquisition Building Block

• 16 bit, 600kSPS SAR ADC

• 2.5V Micro-power Voltage Reference

• Dual Precision Op Amp– ADC Driver

– Voltage Reference Buffer

• PMOD SPI Digital Output

• High Temperature compliant PCB

• High Temperature passive components

• Tested and characterized over temperature

AD7981 175C/210C 16b 600KSPS SAR ADC

– 16-bit resolution with no missing

codes

– Throughput: 600 KSPS

– Low power dissipation scalable to

sample rate

– Pseudo differential analog input

range

– Serial SPI Interface, chainable

– Qualified for minimum 1000 hours

operation at max temp

– High temperature robust packaging

– 10 lead MSOP, BBFP or KGD

AD7981

REF

GND

VDD

IN+

IN–

VIO

SDI

SCK

SDO

CNV

1.8V TO 5.0V

3- OR 4-WIRE INTERFACE

(SPI, DAISY CHAIN, CS)

2.5V TO 5V 2.5V

0 TO VREF

ADC Analog Front End Design

Voltage Reference Considerations

High Temp Packaging Considerations

Wire Bond Integrity at Elevated Temp

• Au/Al wire bonding is industry standard

interconnect

• Au and Al fuse at the interface to form an

inter-metallic compound (IMC)

• IMC continues to grow by diffusion.

• Growth rate is temperature dependant,

accelerated at higher temp.

• Can be modelled with the Arrhenius equation

• Bond failures present as high resistance

(voiding) or open (separation)

• Halogens in mould compound promote

corrosion at Au-IMC boundary and accelerate

failure further

• Mono-metallic system is preferred!

HT Plastic Wire Bond Integrity

• Over-pad-metallization (OPM): NiPdAu

plated layers on Al Pad prevent AuAl

IMC growth

• Halogen-free BOM prevents corrosion

Au/Al Bond post 500hrs at 195°C Au/OPM Bond post 6000hrs at 195°C

Si Substrate

Al

Au

Ni

Pd

Nickel Palladium Gold (NiPdAu)

High Temp Packaging Options

• Hermetic Ceramic Package

• Bottom Brazed Flatpack

• SMT “dead bug” lead forming

• Al-Al wire bond (Mono-Metallic)

• 210C for 1000 hours

• Plastic MSOP

• Au-Au wire bond (Mono-Metallic)

• NiPdAu lead Frame

• Halide-Free BOM

• SMT and Small Footprint

• 175C for 1000 hours

PCB Design Considerations

• Laminate – Polyimide High Tg (240C+)

• Surface Finish (Intermetallics)

• High Melting Point Solder

– Lead vs Lead Free

• Hermetically Sealed Multi Chip Modules

– Require Die Availability

Lab Characterization Test Setup

HT Data Acquisition

Board (DUT)

DC Power Supplies

Audio Precision

SYS-2522

+5V, 2.5V, GND

Analog

InputSPI DIGITAL

OUT (PMOD)

PMOD InterposerSDP

Board

USB PC with

FFT Analysis

Software

SDP 120 PIN

CONNECTOR

AC performance with 1KHz input tone, 580 kSPS, 25°C

AC performance with 1KHz input tone, 580 kSPS, 175°C

83

83.5

84

84.5

85

85.5

86

86.5

0 25 50 75 100 125 150 175 200

SNR

(d

B)

Temperature (°C)

Signal to Noise Ratio

Total Harmonic Distortion

-105

-104

-103

-102

-101

-100

-99

-98

0 25 50 75 100 125 150 175 200T

HD

(d

B)

Temperature (°C)

For Further Information

• www.analog.com/hightemp

• “High Temperature Electronics Pose Design

and Reliability Challenges”

• “Harsh Environments Conquered – Precision

Components for Extreme High Temperatures”

• HiTEN Conference July 2015, Cambridge UK