SeminarOn

Temperature Compensation Techniques for MEMS based Pressure

Sensor

Balwan Singh College ID- 2007UEC151 Electronics &

Communication Engg. M.N.I.T.Jaipur, Rajasthan

Introduction

In India,VLSI & MEMS Fabrication, Assembly and Testing is done at:

1. Semi-Conductor Laboratory (SCL),S.A.S. Nagar, Punjab (also called as National Foundry for MEMS )

• Till recently were using 80µm technology• Now upgrading to 0.18µm with the help of Tower

Semiconductor Ltd., Israel (TSL) 2. Some work is also done at Bharat Electronics Limited, Bangalore

Pressure Sensor :

• Pressure Sensor: Transducer that converts pressure into an analog electrical signal.

• MEMS pressure sensor :A micro-machined silicon diaphragm.• Consist of four piezo-resistive strain gauges diffused into it• SCL manufactured Pressure Sensor:

Why Bridge ?

V2 V1 VB

IB

VOUT

R3

R4

R2

R1

Pressure

Sensor

VOUT=V1-V2

1.Increase in sensitivity 2.Noise Rejection

Typical Pressure Sensor Specifications

Applied Presssure Raw Sensor Output

Voltage MAP Sensor Output

Pmin 133mB -20mV -2V

Pmax 1200mB 40mV 4V

Full Span Output(FSO)=4VSpan=Vmax-Vmin=6VOffset=-2V

•Ideally offset should be zero at no pressure but due to mismatch in resistances during fabrication it is not zero

Pressure Sensor Parameters:

• Span: Change in bridge output voltage for minimum to full scale applied input pressure. 

• Offset: Output voltage at zero pressure applied.• Pressure Sensitivity: Span (S) of Pressure Sensor is given by S=G×VB×P Where, G=Normalized Pressure Sensitivity P=Rated Pressure VB=Bridge Voltage • Linearity: It refers to the straightness of the output signal at various

equally spaced pressure points applied in an increasing direction. A best fit line is then computed for the actual output values. Linearity error is calculated by taking the difference between the actual output and best fit value. It is expressed as the % of span. 

Pressure Sensor Parameters:

• Pressure Hysteresis:• It is due to hysteresis in the material of sensor• Measured by taking difference between two output signals

taken at the exactly the same pressure but during a sequence of increasing and decreasing pressure

• To Reduce hysteresis there should be some specific time difference between the readings.

Effect of Temperature on Pressure Sensor Parameters: 

• Temperature Coefficient of Resistance: With increase in temperature, Sensor bridge Resistance RBR increases (As TCR is positive for bridge resistors).

 • Temperature Coefficient of Span (Sensitivity):• Inherent property of a sensor.• SCL sensor has negative TCS in constant voltage mode.  • Temperature Coefficient of Zero (Offset): • This tells how the offset of sensor varies with temperature.• SCL sensor has negative TCZ.  

Temperature Compensation Techniques

V2Rx

V1 VB

IB

VOUT=V1-V2

R3

R4

R2

R1

Pressure

Sensor

1.Resistor Method for Zero(Offset) Compensation of Pressure Sensor:

Negative TCZVoltage Mode:• Insert a resistance Rx in series. Effective resistance R1 increases resulting an increase in overall resistance of that branch.

•Current in this branch decreases. V2 decreases while V1 remaining same.• Vout increases and TCZ effect can be compensated with proper selection of Rx.

TCZ Negative

Current Mode:• Insert resistance Rx in series, resistance of that branch increases•Resulting a decrease in current of that branch while current in other branch increases.• Therefore Vout= (V1 (increases)-V2 (decreases)) increases. •Hence TCZ effect can be compensated with proper selection of Rx

V2Rx

V1 VB

IB

VOUT=V1-V2

R3

R4

R2

R1

Pressure

Sensor

If TCZ is Positive:

Voltage Mode:• Insert a resistance Rx in parallel as shown in fig.•Effective R1 decreases resulting an increase in V2 while V1 remaining same.• Therefore Vout decreases .Hence TCZ effect can be compensated with proper selection of Rx

Rx

V2 V1 VB

IB

VOUT=V1-V2

R3

R2

R4

R1

Pressure

Sensor

If TCZ is Positive:

Rx

V2 V1 VB

IB

VOUT=V1-V2

R3

R2

R4

R1

Pressure

Sensor

Current Mode: •Insert resistanceRx, effective R1 decreases, overall resistance of that branch decreases.

• Current in this branch increases and in other branch decreases.V1 decreases and V2 increases

• Hence Vout Decreases. •TCZ effect can be compensated with proper selection of Rx.

Span Compensation(TCS Negative)

• Use bridge in current excitation mode• Bridge resistance RBR increases with temperature(TCR Positive) • Hence bridge voltage VBR also increases with temperature.• S=G×VB×P

• Increase in bridge voltage VBR will compensate decrement in span with temperature

2. Span Compensation using Diode

DIODE TEMPERATURE CHARACTERSTICS

Span Compensation Using a Diode: 

• Insert a diode in series .• The junction voltage decreases with increase in

temperature resulting an increase in bridge voltage as temperature increases (as shown in fig.) which directly effects span. ((S=G×Vb×P )

• Of limited utility as voltage across a diode can not change too much and hence span

V2 V1 VB

IB

VOUT=V1-V2

R3

R4

R2

R1

Pressure

Sensor

Span Compensation Using a Diode

3.Temperature Compensation using Signal Conditioner

1.Philip C.John , Abhay Joshi, Pramod Sindhanakeri , Ajayakumar.P.D , K. Natarajan ; “Signal Conditioner for MEMS based Piezoresistive sensor” IEEE International Conference on Industrial and Information Systems(ICIIS),August1,2010

Signal Conditioner : IC MAX 1452.Main Features:• Has768-byte internal EEPROM• 16-bit DACs • On chip temperature sensor• An uncommitted OPAMP

Features

• Internal 768 x 8 EEPROM stores the 16-bit DACs with calibration coefficients. This memory contains the following information, as 16-bit wide words:

• Offset Temperature Coefficient Register• FSO (Full-Span Output) Calibration Table

Temperature Index Pointer:

Example:Temperature Index Conversion Formula: Index = 0.6897 x Temp in C + 44.00

Temperature Compensation is accomplished by loading two tables with compensation coefficients and using an integrated temperature sensor to index into the tables.

Offset Compensation using Signal Conditioner

At Temperature T11.Apply Minimum Pressure and wait for Pressure Stabilization and

Measure Output Voltage2.Check Vout(Output Voltage )a. If Vout=Offset and Note the Current ODAC Valueb. Else Change ODAC Value and go to step 23. Write ODAC coefficients to EEPROM

Span Compensation using Signal Conditioner

1.Set Chamber Temperature = T1 and wait for stablization.2.Set Pressure = Pmax And Measure output Volatge(Vmax)3.Set Pressure = Pmin And Measure output Volatge(Vmin)4.Calculate Span = Vmax – Vmin (also note the current Bridge Voltage

VB1)5.Calculate Required Bridge Voltage VB for Desired Span: VB = VB1 * (Desired Span) / Calculated Span6.Adjust FSODAC until Bridge Voltage = VB.7.Note the Current FSODAC Value 8.Write FSODAC coefficients to EEPROM

Temperature Compensation

1.Repeat compensation steps for all other Temperature points T2, T3… Tn and Tabulate their respective FSODAC and ODAC Values

2.Perform Curve Fitting for calibration coefficients FSODAC and ODAC. Program EEPROM with the Curve Fitted Coefficients at other temperatures

• The number of calibration points decides the order of the equation used to calculate offset and full-scale values at any given temperature

• The order of the equation can be increased depending on the non-linear behaviour of sensitivity with respect to temperature

3.After EEPROM programming verify Calibration at different temperatures

Final Overview of Temperature Compensation of Pressure Sensor Using Signal Conditioner

Vout

Bridge Current

Span Temp. Compensation

∑PGA

∑

BLOCK DIAGRAM OF PRESSURE SENSOR COMPENSATION

Pressure

Sensor

Offset

Hardware Setup For Automatic Compensation Of Pressure Sensor

Temperature Chamber

Sensors

HARDWARE SETUP FOR AUTOMATIC COMPENSATION OF PRESSURE SENSOR

Pressure Chamber

RS232

GPIB

GPIB

Serial KEY

Power Supply

Host Computer with GPIB Card

DMM

Pressure Controller

Compressed Dry N2 Cylinder Vacuum

Pump

Advantages of Compensation using Signal Conditioner:

• Error is very less than 0.25% temperature ranges -10 °C to +70°C.

• Low power consumption .• The hardware components used for this approach are low cost,

reliable and readily available in the market.• Several Sensors can be Compensated simultaneously avoiding

repeated adjustment and stabilization of Pressure and Temperature for individual sensor saving stabilization time for each sensor.

Problems in Implementation:

Need of a serial interfacing hardware for communication between MAX1452 and Computer because:1. PC’s RS-232’s has two pins Tx and Rx are used for serial

transmission and serial receiver purpose while MAX1452 has only one pin for both transmission and receiving purpose.So multiplexing must be done.

2. RTS/CTS can’t be used as handshaking signals: MAX1452 has only one pin to communicate with RS-232 (For handshaking there must be at least two synchronization signals).

Problems in Implementation

RS232 Line Type & Logic Level RS232 VoltageData Transmission (Rx/Tx) Logic 0 +3V to +15V

Data Transmission (Rx/Tx) Logic 1 -3V to -15V

Control Signals (RTS/CTS/DTR/DSR) Logic 0 -3V to -15V

Control Signals (RTS/CTS/DTR/DSR) Logic 1 +3V to +15V

•While voltage levels of MAX1452 are CMOS compatible

3.The voltage levels of RS-232 signals:

References:

1.Philip C.John , Abhay Joshi, Pramod Sindhanakeri , Ajayakumar.P.D , K. Natarajan ; “Signal Conditioner for MEMS based Piezoresistive sensor” IEEE International Conference on Industrial and Information Systems(ICIIS),August1,2010.

2.GUAN Rong - feng, WANG Xiao-xue ; “ MEMS Pressure Sensor Temperature Compensation” Journal of Henan Normal University(Natural Science 2009).

3.Signal Conditioner MAX1452 Datasheet from www.maxim-ic.com4.Annual Report, SCL from www.scl.gov.in

5.Web Resources: www.isro.org, www.wikipedia.org.

THANK YOU Balwan Singh 2007 UEC 151 EC-3