MEMS Signal Conditioning Circuits Dr. Lynn Fuller · MEMS Signal Conditioning Circuits Rochester Institute of Technology. MEMS Signal Conditioning Circuits Rochester Institute of

© February 24, 2017 Dr. Lynn Fuller

MEMS Signal Conditioning Circuits

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Rochester Institute of Technology

Microelectronic Engineering

ROCHESTER INSTITUTE OF TECHNOLOGYMICROELECTRONIC ENGINEERING


Dr. Lynn FullerElectrical and Microelectronic Engineering

Rochester Institute of Technology82 Lomb Memorial DriveRochester, NY 14623-5604

Email: [email protected]. Fuller’s Webpage: http://people.rit.edu/lffeee

MicroE Webpage: http://www.rit.edu/microelectronic/

2-24-2017 Signal_Conditioning.ppt

mailto:[email protected]

http://people.rit.edu/lffeee

http://www.rit.edu/



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OUTLINE

Op Amp IntroductionInverting AmplifierNon-inverting AmplifierUnity Gain BufferIntegratorInverting SummerDifference AmplifierComparator, Comparator with hysteresisVoltage AmplifierCurrent to Voltage ConverterRC OscillatorChanging AC Capacitance to VoltageChange in Resistance to VoltageDiode Temperature to VoltageResistor Bridge and Differential AmplifiersPower Op Amp CircuitReferences



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INTRODUCTION

This document discusses single-supply, low-voltage, rail-to-rail, Operational Amplifier (Op Amp) circuits. Although all op amps can operate with single supply or dual supply, most engineers are familiar with dual-supply Op Amp circuits such as those shown on the following page. The dual supply allows the input and output to be easily referenced to zero volts. (analog ground = earth ground)

Single supply Op Amps usually refers to low voltage Op Amps using voltages of 5, 3.3 or smaller and ground. Some types of Op Amps will not work at these voltages. (some Op Amps use BJT current source biasing that takes a couple of diode drops of voltage to work thus the output voltage of these Op Amps can only get within 1.4 volts of the supply rails. For example at 5 volts, output is limited between 1.4 volts and 3.6 volts and with 3.3 volts supply some Op Amps may not work at all. With single supply Op Amp circuits we also can not have negative output voltages. There are several techniques for working with these limitations.



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VOLTAGE SUPPLIES

Dual DC Power Supply

Single Supply

Multiple OutputSupplies



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CREATING A SPLIT SUPPLY FROM A SINGLE SUPPLY

Signal

Ground

Chassis

GroundEarth

Ground

Virtual

Ground

Analog

Ground

Digital

GroundCommon

Ground

Floating

Ground

Single

Supply

R

R

C

C

Simple Voltage Splitter+V/2

-V/2

The simple voltage splitter draws a lot of power if R’s are low. C’s ensure AC short (for AC signals).

Example: 20V single supply can be split giving +/- 10V. If R’s are 10 ohms then I in each is 10/10=1 Amp and if the Op Amp draws only a few mA the voltages will be ~+/- 10V. Resistor Power = IV=10 watts. (be sure to get resistors for 10 watts) If C=500uf the corner will be ~1/RC = 200r/s = 31.8hz good for AC signals above ~300hz



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VIRTUAL GROUND / VOLTAGE SPLITTER

Virtual Ground Using Op Amp

Virtual ground is simply a voltage reference typically half of the supply voltage.

This virtual ground can supply/sink only as much current as the maximum Op Amp output current. The output current can be increased as shown with the BJT’s

Vout = V/2-

+

100K+V

+V+V

100K



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TLE2426 RAIL SPLITTER (COMMERCIAL VIRTUAL GND)



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OPERATIONAL AMPLIFERS

The 741 Op Amp is a general purpose bipolar (BJT) integrated circuit that has input bias current of 80nA, and input voltage of +/-15 volts @ supply maximum of +/- 18 volts. The output voltage can not go all the way to the + and - supply voltage. At a minimum supply of +/- 5 volts the output voltage can go ~6 volts p-p.

The newer Op Amps have rail-rail output swing and supply voltages as low as +/- 1.5 volts. The MOSFET input bias currents are ~ 1pA. The NJU7031 is an example of this type of Op Amp.



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BASIC TWO STAGE CMOS OPERATIONAL AMPLIFIER

1. Low Voltage operation2. Rail to Rail input and output voltages3. Low Input bias ~ 1pA or smaller4. Low Output Current (depends on M6 and M7)5. Unity Gain Bandwidth depends on Cc



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LOW VOLTAGE, RAIL-TO-RAIL OP AMP

1. 3 to 16 Volt operation2. Rail to Rail input and output voltages3. Low Input bias ~ 1pA4. Output Current ~1mA5. Unity Gain Bandwidth 1.5 MHz6. Power Dissipation 1mA at 3 V = 3000uW



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NJU703X OP AMP DATA SHEET



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LTC6078 OP AMP

1. 2.7 to 5.5 Volt operation2. Rail to Rail input and output voltages3. Low Input bias ~ 1pA4. Output Current ~5mA5. Unity Gain Bandwidth ~350Khz6. Power dissipation 54 uA at 3 V = 162uW



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SOME BASIC DUAL SUPPLY OP AMP CIRCUITS

These dual supply circuits should be familiar:

-

+Vo

Vin

R2R1

Non-Inverting Amplifier

-

+VoVin

C

R

Integrator

Vo= Vin (1 + R2/R1)

Vo= -1/RC Vin dt

-

+

Unity Gain Buffer

VinVo

Vo= Vin

+V

- V

+V

- V

+V

- V

-

+VoVin

R2

R1

Inverting Amplifier

Vo= - Vin R2/R1

+V

- V



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SOME BASIC ANALOG ELECTRONIC CIRCUITS

Inverting Summer

Difference Amplifier

Vo= Rf/Rin (V1-V2)

Vo-+

Rin

Rf

V1

V2

RfRin

-

+VoV2

R3

R1

V1

R1

Vo= ( -R3/R1) (V1 + V2)+V

- V

+V

- V

These dual supply circuits should be familiar:



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INVERTING AMPLIFIER SINGLE SUPPLY EXAMPLE

-

+ VoVin

R2

R1

Inverting Amplifier

Vo= - Vin R2/R1+V

+-

1. This is a DC and AC amplifier.2. The input is referenced to the analog ground typically ½ of +V3. The output voltage is referenced to the virtual ground or to earth ground. 4. If using a scope to measure Vo the scope ground is earth ground. If the Vin is

ac you can AC couple the scope.5. If the input Vin is DC you can measure the output relative to the analog

ground using a multimeter (not the oscilloscope)

These Grounds are not the same



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INVERTING AMPLIFIER SINGLE SUPPLY EXAMPLE

-

+

R2R1

Single Supply DC Inverting Amplifier

Vo= - Vin R2/R1

+V-

+

Vin

+

-Vo

Gnd1 is analog ground ~1/2 of supply voltage. Vout can

be take relative to Gnd1 or Gnd2 however there is a +V/2

DC added to Vo if relative to Gnd2.

Gnd1

Gnd2Thermopile



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INVERTING AMPLIFIER EXAMPLES

The two 100K resistors create an analog ground ~1/2 V+the gain = -R2/R1, offset of V+/2 or virtual ground

offset



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SINGLE RESISTOR SENSOR AMPLIFIER DESIGN

-V

-+

+V

-V

R1

R4

Vout

+V

R2

R3

Sensor

Non-Inverting Amp

Gain = 1 + R3/R4



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SINGLE SUPPLY NON-INVERTING AMPLIFIER

-

+

20K

+V = 3.3

20

K

+V=3.3

100K

+3.3

10K

To

10.07K

10K

Sensor

Vo

1. The two 20K resistors can be replaced by its Thevenin equivalent of V/2 and 10K

2. This sets up the analog ground at V/2 and the voltage gain to 11

3. Vin is V/2 (or zero if referenced to analog ground) if the sensor is 10K

4. If the sensor is not exactly10K then Vo will have a value of 11 x (Vin – (+V)/2)

Vin



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RESISTOR R3 PARAMETER CHANGE (LIST)

Resistor changes by +/- 0.1 ohm and supply voltage sweeps from 3.5V to 5V

If V+ is 5 volts what is

the gain in volts per ohm



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NON-INVERTING AMPLIFIER EXAMPLES

What is the gain in V/V



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SINGLE SUPPLY COMPARATOR

-

+VoVin

Vo

VinVref

+V

Vref

+V

+V

Theoretical

0



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SINGLE SUPPLYBISTABLE CIRCUIT WITH HYSTERESIS

-

+Vo

Vin

+V

RR

Vo

Vin

VTH

+V

VTL

Theoretical+V

R

0VT

= 2/3 V

=1/3 V

1. The R’s set up the threshold voltage at V/3 and 2V/32. Vout is either +V or Ground



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SINGLE SUPPLY OSCILLATOR (MULTIVIBRATOR)

-

+Vo

C

+V

R2R1

R

VT

+VR3

Vo

t

t1

+V

0

Let R1 = 100K, R2=R3=100K

and +V = 3.3

Then VTH = 2.2 when Vo = 3.3

VTL = 1.1 when Vo = 0

VTH

VTL



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OP AMP COMPARATOR WITH HYSTERESIS RC OSCILLATOR

C2 Represents Scope Probe If C1 was a sensor and changed by

1pF what is the change in frequency?



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CAPACITANCE CHANGE TO VOLTAGE

+

9V

-9 V

TL081

i

V

R

CVo

i

i = V Cm 2 p f cos (2pft)Co = Average value of CCm = amplitude of C changeC = Co +Cm sin (2pft)V is constant across C

Vo = - i R

i = d (CV)/dt

Vo = - 2pf V R Cm cos (2pft)

amplitude of Vo



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DUAL SUPPLY OP AMP DC TO VOLTAGE

What is the frequency response?

Output vs frequency



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SINGLE SUPPLY VERSION OF SIGNAL PROCESSING

+

9V

TL081

i

Vdc=9V

5.6 MEG

CVo

i

100K

100K

earth ground



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SINGLE SUPPLY MICROPHONE DC TO VOLTAGE



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SINGLE SUPPLY PHOTO DETECTOR I TO V AMP

+Light

I

NJU7024

470K

+5

Vout

Vishay BPW46Digikey No. 751-1017-ND

The voltage across the diode is zero volts in the dark and the current is zero

In the light I is 5uA (in direction shown, i.e. out of p-side)

What is Vout?



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SIGNAL CONDITIONING FOR TEMPERATURE SENSOR

p

nI

3.3VR1

20K

0.2 < Vout < 0.7V

+

-

~ - 2mV/°C

Vout



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RESISTIVE PRESSURE SENSOR

R1R3

R2R4

Gnd

+5 Volts Vo2

Vo1

Resistors on a DiaphragmGnd

5 Volts

R1=427 R3=427

R2=427R4=427

Vo2=2.5vVo1=2.5v

No Pressure

Vo2-Vo1 = 0



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WHEATSTONE BRIDGE AND DIFFERENTIAL AMP

Vs

R1 R3

R2 R4Vo+

Rsb

Rst

Vo- Vo1-

+

Rin

Rf

-+

-+

RfRin

1. The R1=R2=R3=R4 make a Wheatstone bridge and are sensor resistors that will change in response to pressure.

2. Vo+ and Vo- should be equal to each other and ~Vs/2 with no pressure.

+V+V

+V



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SENSOR AND INTERNAL ELECTRONICS



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SUMMARY

Low voltage Op Amps are often used with a single supply.

Some circuits work just fine with single supply such as the comparator.

Other circuits use a virtual ground typically ½ of the supply voltage.

Since signal generators and oscilloscopes are referenced to earth ground. Op Amp circuits need to consider this if powered by a single supply referenced to earth ground. In that case earth ground and virtual ground are at different voltages.



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REFERENCES

1. MOSFET Modeling with SPICE, Daniel Foty, 1997, Prentice Hall, ISBN-0-13-227935-5

2. Operation and Modeling of the MOS Transistor, 2nd Edition, Yannis Tsividis, 1999, McGraw-Hill, ISBN-0-07-065523-5

3. UTMOST III Modeling Manual-Vol.1. Ch. 5. From Silvaco International.4. ATHENA USERS Manual, From Silvaco International.5. ATLAS USERS Manual, From Silvaco International.6. Device Electronics for Integrated Circuits, Richard Muller and Theodore

Kamins, with Mansun Chan, 3rd Edition, John Wiley, 2003, ISBN 0-471-59398-27. ICCAP Manual, Hewlet Packard8. PSpice Users Guide. 9. Using Single Supply Operational Amplifiers – from Microchip10. Designing Single Supply, Low-Power Systems – from Analog Devices11. Designing Circuits for Single Supply Operation – from Linear Technology12. Single Supply Design – from TI13. Design Trade-Offs for Single-Supply Op Amps – from Maxium



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HOMEWORK – MEMS SIGNAL CONDITIONING

1. Do SPICE analysis for a single op amp with dual supply to amplify the output for a single resistor sensor similar to that shown on page 18.

2. If you want to measure a small value (0.1pF) slowly changing capacitance, what circuit could be used? Show it works using SPICE.

MEMS Signal Conditioning Circuits Dr. Lynn Fuller · MEMS Signal Conditioning Circuits Rochester Institute of Technology. MEMS Signal Conditioning Circuits Rochester Institute of

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