Chapter3: Signal conditioningSignal conditioning circuits are used to process the output signal from sensors of a measurement system to be suitable for the next stage of operation

The function of the signal conditioning circuits in clude the following items: Signal amplification (opamp) , Filtering (opamp), Interfacing with µµµµP (ADC), Protection (Zener & photo isolation), Linearization , Current – voltage change circuits, resistance change circuits (Wheatstone bridge) , error compensation

Operational Amplifiers

• Generally the opamp has the following properties:Gain : being of the order greater than 100000, ideally = infiniteInput impedance : ideally infiniteoutput impedance: ideally zero; practical values 20-100Ω

Operational amplifiers are the basic element of many signal conditioning modules

Opamp Circuit Configurations (1)

Voltage Comparator– digitize input

Inverting AmpNon-Inverting Amp

Voltage Follower

Opamp as comparator (1)

If the voltage applied to v1 is greater than V2 then the output is constant voltage equal to (-10V) if (V2>V1) then the output is constant voltage =(+10V). This can be used in the following example:

The output indicates which of the two voltages is hi gh (V1 or V2). When used with no feedback connection

Opamp as comparator (2)The circuit is designed to control temperature with a c ertain range. When the temp. is below certain value, the thermisto r R1 is more than R2 and the bridge is out of balance, it gives an out put at its lower saturation limit which keeps the transistor OFF. When t emperature rises and R1 falls the opamp switch to +ive saturation value and switch the transistor ON

• Summing Amp

• Differential Amp

• Integrating Amp

• Differentiating Amp

Opamp Circuit Configurations (2)

Differential Opamp Circuit Example (3)The difference in voltage between the emfs of the tw o junctions of the thermocouple is being amplified.

If a temperature difference between the thermocoupl e junctions of 10 0C produces an emf difference of 530 µµµµV, then the values of R1 and R2 can be chosen to give a circuit with an output of 10mV.

For the circuit we have V out= (V2-V1)R2/R1

9.18

105301010

1

2

6

1

23

=

××=× −−

R

R

R

R

So if we select R1 as 10 k Ohm then

R2=189 k Ohm

Opamp Circuit Configurations (4)

Current-to-Voltage

Voltage-to-Current

Logarithmic amplifier

Vout= k ln (Vin )

The voltage- current relationship can be approximated b y:

V=Cln (I) = Cln (Vin/R) = kln (Vin); so if Vin=Aexp(at) then Vout= K ln(Aexp(at)) =klnA+at which is linear relationship

Instrumentation Amplifier

• It is available as single IC is designed to have:-high input impedance (300M ohm)– High common mode rejection gain

(more than 100 dB)– High voltage gain

input stage

gain stage

+=3

4

1

12d

2

R

R

R

RRG

total differential gain

Instrumentation Amplifier

By selection suitable resistance ratios show that this circuit is capable of rejecting common mode noise.???

See Mechatronics, Bolton

Signal conditioning: Wheatstone Bridge• One of the most used signal conditioning circuit. It can be used to

convert a resistance change to a voltage change as in the following example:

4

3

2

1

R

R

R

R =At balanced condition V 0 =0 and in result

When not balanced V BD =

s

s

s

VRR

RVwritecanwethen

RthansmallermuchisRif

RR

R

RRR

RRVV

RR

R

RRR

RRVVV

21

10

11

21

1

211

110

43

3

211

1100

)(

)(

+=

+−

+++=

+−

+++=+

δδ

δδ

δδ

δδδ

)(43

3

21

10 RR

R

RR

RVVV sBD +

−+

==

KreeVVRR

RV s deg/012.0

100100

0.639.0

21

10 =

+×=

+= δδ

Signal conditioning: Wheatstone BridgeEg. A platinum resistance temperature sensor has a resi stance of 100 ohm at 0 0C is placed in one arm of a Wheatstone bridge with ea ch of the other arms also being 100 ohm. If the resistance temperaturecoefficient of the platinum is 0.0039/K, find the o utput voltage from the bridge per degree change in temp. if the load acro ss the output can be assumed to be infinite. V s=6.0 volt.

The variation of the resistance of the platinum with temperature can be represented as R t =R0 (1+ααααT)

Sol.

Change in resistance= R t -R0=R0ααααT

=100x0.0039x1=0.39 ohm/k

Since the resistance change is small compared to the 100 ohm, the approximate equation can be used. Hence:

Compensation for leads

Signal conditioning: Wheatstone Bridge

In many measurement system involving a resistive sensor, the actual sensing element may be affected by the connecting leads. so to compensate for such effects Wheatstone bridge may be used as arranged in the figure

Connecting Lead 1 to R3 and Lead 3 to R1 (the sensor) and so if R1 is equal to R3 and of the same size and type then the leads effect can be canceled out.

Temperature compensation with strain gauges: (a) us e of dummy gauge, (b) four active arm bridge

Signal conditioning: Wheatstone Bridge

Strain gauge in tension increase resistance , in comp ression decrease resistance which increase the output voltage

ss V

R

R

R

R

R

R

R

R

RRRR

RRVV )(

))(( 4

4

3

3

2

2

1

1

4321

410

δδδδ +−−=++

=

Signal conditioning: PROTECTION

• Normally protection is provided against high current and high voltage which may damage the important components.

• Examples of protection in mechatronics :• Series resistor to limit line current• Fuse to break if the current does exceed a safe

level• Zener diode circuit to protect against high

voltage and wrong polarity.• Optoisolator to isolate circuits completely

Protection: Zener Diode

• Zener diodes operate in the breakdown region.• Zener diodes have a specified voltage drop when

they are used in reverse bias. So normally used for voltage regulation in reverse bias

• Zener has the ability to maintain a nearly constant voltage under conditions of widely varying current.

Zener Diode characteristics

When the reverse voltage VR is increased, the leakage current remains essentially constant until the breakdown voltage VZ (Zener voltage) is reached.

Optoisolator Background• Operation similar to relays• Used to control high voltage devices• Excellent noise isolation because

switching circuits are electrically isolated• Coupling of two systems with transmission

of photons eliminates the need for a common ground

Ideal for applications requiring High isolation surge voltageNoise isolationSmall size

Signal cannot travel in opposite directionUsed to control motors, solenoids, etc.

Optoisolator Schematic

• Input Stage = infrared emitting diode (IRED)• Output Stage = silicon NPN phototransistor

Optoisolators: (a) transistor, (b) Darlington, (c) triac

(see Bolton Mechatronics)

Protection circuit

Signal conditioning: Filtering (1)• Filtering is the process of removing a certain band

of frequencies from a signal and permitting others to be transmitted.

• The Pass Band: the range of frequencies passed by the filter

• The Stop Band: the range not passed by the filter.• CUT OFF frequency: the boundary between

stopping and passing

Technically CUTOFF frequency is defined as the frequ encyat which the output voltage is 70.7% of that in the pass band.

Attenuation dB (decibels )=in

out

in

out

V

V

P

Plog20log10 =

The output voltage of 70.7% of that in the pass band is thus an attenuation of 3dB=20 log (0.707)

Characteristics of ideal filters: (a) low-pass filter, (b) high-pass filter, (c) band-pass filter, (d) band-stop filter

Signal conditioning: Filtering (2)

Low-pass filter: (a) passive, (b) active using an o perational amplifier

Signal conditioning: Filtering (3)

Passive Filters made up using only resistors, capacitors and inductors

Active filters involve an operational amplifier

Intrinsic Noise SourcesIntrinsic Noise Sources

Thermal Noise or Johnson NoiseThermal Noise or Johnson NoiseShot NoiseShot NoiseContact NoiseContact NoisePopcorn NoisePopcorn Noise

Thermal Noise Thermal Noise J.B. Johnson discovered in 1928.J.B. Johnson discovered in 1928.From thermal agitation of electrons within a resistance.From thermal agitation of electrons within a resistance.NyquistNyquist formed formed rmsrms voltage for thermal noise asvoltage for thermal noise as

kk: : BoltsmannBoltsmann constant, 1.38 x 10constant, 1.38 x 10--2323 J/KJ/KTT: temperature, K: temperature, KBB: equivalent noise bandwidth, Hz : equivalent noise bandwidth, Hz RR: resistance: resistance

4tV kTBR=

Equivalent Noise BandwidthEquivalent Noise Bandwidth

For any network transfer function For any network transfer function A(fA(f), there is ), there is an equivalent noise bandwidth of constant an equivalent noise bandwidth of constant magnitude of transmission Amagnitude of transmission A00 and bandwidth ofand bandwidth of

Normally greater than filtering bandwidthNormally greater than filtering bandwidth

22

00

1 ( )B A f dfA

∞

= ∫

Shot NoiseShot Noise

Vacuum tubes and semiconductorsVacuum tubes and semiconductorsAssociated with current loss across a barrier.Associated with current loss across a barrier.W. W. SchottkySchottky (1918) shows (1918) shows rmsrms current ascurrent as

qq: electron charge, 1.6 x 10: electron charge, 1.6 x 10--1919 CCIIdcdc: average current, A: average current, ABB: equivalent noise bandwidth, Hz : equivalent noise bandwidth, Hz

2sh dcI qI B=

Contact NoiseContact NoiseImperfection of contact (usually in switch and relay) Imperfection of contact (usually in switch and relay) changes in conductancechanges in conductance1/f noise1/f noise

KK: constant dependent on contact material: constant dependent on contact materialIIdcdc: average current, A: average current, Aff: frequency, Hz: frequency, HzBB: equivalent noise bandwidth, Hz : equivalent noise bandwidth, Hz

f dcBI KIf

=

Popcorn NoisePopcorn Noise

Burst NoiseBurst NoiseDue to manufacturer imperfection of Due to manufacturer imperfection of semiconductor devices especially at the junction semiconductor devices especially at the junction due to metallic impuritydue to metallic impurity

Active Device NoiseActive Device Noise

Noise FactorNoise Factor

Noise Power at Output of Actual DeviceNoise Power at Output of Ideal Device

F =

11; if actual device is an ideal device

FF≥=

/

/

( / )( / )

i p

o p

S NF

S N= Power of Signal/

Power of NoiseS N =

Noise Figure, 10logNF F=

Measurement of Noise FactorMeasurement of Noise Factor

Two methodsTwo methodsSingle frequency methodSingle frequency methodNoise diode or white noise methodNoise diode or white noise method

For both methods,For both methods,the noise power at output of an ideal device isthe noise power at output of an ideal device is

,ideal no tv Av=

Single Frequency MethodSingle Frequency Method

Device or network withvoltage gain Asv

sR

LR

With generator turned off, measure With generator turned off, measure

2 2( ) (device noise)no tv Av= +

Single Frequency MethodSingle Frequency Method

Next, generator is turned on and its magnitude is Next, generator is turned on and its magnitude is increased until output power doubles (increases increased until output power doubles (increases by 3dB over that previously measured), thenby 3dB over that previously measured), then

2 2 22 ( )no s no

s no

v Av v

Av v

= +

=

2

2

2

( )no

t

s

t

vFAv

vFv

=

⎛ ⎞= ⎜ ⎟⎝ ⎠

Single Frequency Method (cont.)Single Frequency Method (cont.)

Adv.Adv.Any value of Any value of RRLL may be used.may be used.

DisadvDisadv..Noise bandwidth of the device must be known.Noise bandwidth of the device must be known.

20

2

20

At 290K, 1.6 10

1.6 10

t s

s

s

v BR

vFBR

−

−

= ×

=×

Noise Diode or White Noise MethodNoise Diode or White Noise Method

Device or network withvoltage gain AsR LR

NoisediodedcI

Blockingcapacitor

With no diode current, the With no diode current, the rmsrms noise voltage is noise voltage is measured.measured.

2 2( ) (device noise)no tv Av= +

Noise Diode or White Noise Method Noise Diode or White Noise Method (cont.)(cont.)

Next, the diode current is increased until output Next, the diode current is increased until output noise power doubles (increases by 3dB).noise power doubles (increases by 3dB).The shot noise generator isThe shot noise generator is

193.2 10sh dc

sh sh s

i I B

v i R

−= ×

=2 2 22 ( )no sh no

no sh sh s

v Av v

v Av Ai R

= +

= =

Noise Diode or White Noise Method Noise Diode or White Noise Method (cont.)(cont.)

The noise factor is a function of The noise factor is a function of RRss but in this but in this method the other influencing factor is the direct method the other influencing factor is the direct current through the diode.current through the diode.AdvAdv

The method is frequency independent.The method is frequency independent.Both Both RRss andand IIdcdc are easily measured.are easily measured.

2

2

( )

20

sh s

t

dc s

i RFv

F I R

=

=

Noise Factor in Cascade CircuitNoise Factor in Cascade Circuit

sR LR

Gain, G1Noise

Factor, F1

Gain, G2Noise

Factor, F2

Gain, G3Noise

Factor, F3

Gain, GNNoise

Factor, FN

321

1 1 2 1 2 1

1 11 N

N

F FFF FG GG GG G −

− −−= + + + +L

L

Basic Noise Reduction TechniquesBasic Noise Reduction Techniques

Definition of InterferenceDefinition of InterferenceGroundingGroundingShieldingShielding

InterferenceInterference

SheingoldSheingold (1980) classifies interference problem into three areas:(1980) classifies interference problem into three areas:1.1. Problems generated locally by the materials used in the signal Problems generated locally by the materials used in the signal

path (e.g., unwanted thermocouples, path (e.g., unwanted thermocouples, ohmicohmic contact of switches contact of switches and terminals)and terminals)

2.2. Problems within a subsystem (e.g., grounds)Problems within a subsystem (e.g., grounds)3.3. Problems originating in the outside world [e.g., electric, Problems originating in the outside world [e.g., electric,

magnetic, and RF (radiomagnetic, and RF (radio--frequency) interference]frequency) interference]

Webster (1977) classifies interference into three types of Webster (1977) classifies interference into three types of couplingcoupling

1.1. capacitive (electric fields)capacitive (electric fields)2.2. inductive (magnetic fields)inductive (magnetic fields)3.3. resistive (resistive (ohmicohmic voltages in ground conductors). voltages in ground conductors).

GroundingGrounding

Ground = a reference connection (reference Ground = a reference connection (reference potential)potential)Earth = a connection to earthEarth = a connection to earth

Point of interest: interference created by Point of interest: interference created by resistive coupling in ground conductorsresistive coupling in ground conductors

Grounding: Analog CircuitsGrounding: Analog Circuits

Interference voltages may develop on ground lines.Interference voltages may develop on ground lines.RRxx = nonzero = nonzero resistivityresistivity of the wire (in this case, 3 mof the wire (in this case, 3 mΩΩ per 15 cm of per 15 cm of No. 18 copper wire)No. 18 copper wire)

Parallel Distribution of Power

Grounding: Analog Circuits (cont.)Grounding: Analog Circuits (cont.)

Radial or star distribution minimizes voltage drops in both hot Radial or star distribution minimizes voltage drops in both hot and ground wires.and ground wires.Circuit 3 has no difference between the parallel and radial Circuit 3 has no difference between the parallel and radial distributions.distributions.

Radial or Star Distribution of Power

Grounding: Analog CircuitsGrounding: Analog Circuits

Better solution: connect circuit 3 (higher current) closer to thBetter solution: connect circuit 3 (higher current) closer to the power e power supply, if possible.supply, if possible.Circuit 3 should be connected to an extra power supply, if availCircuit 3 should be connected to an extra power supply, if available, to able, to avoid the resistance of a long wire while sharing the same poweravoid the resistance of a long wire while sharing the same powersupply.supply.

If the voltage drop on the power supply path does not affect theIf the voltage drop on the power supply path does not affect theoperation of the circuits, a combination of parallel and radial operation of the circuits, a combination of parallel and radial distribution could be used (star connection for the ground wire)distribution could be used (star connection for the ground wire)..

Grounding: AnalogGrounding: Analog--Digital CircuitsDigital Circuits

Digital signals Digital signals →→ large current spikes along ground paths large current spikes along ground paths →→interference in analog circuitsinterference in analog circuitsWhen analog and digital circuits sharing one power supply, the When analog and digital circuits sharing one power supply, the ground wire each must be different, with only one common ground wire each must be different, with only one common point. point. →→ to minimize common impedances between digital and to minimize common impedances between digital and analog circuits.analog circuits.

One power supply, one common point

Grounding: AnalogGrounding: Analog--Digital Circuits Digital Circuits (cont.)(cont.)

When using separate analog and digital power supplies, When using separate analog and digital power supplies, each circuit is connected to its ground and both each circuit is connected to its ground and both grounds are tied to a single point.grounds are tied to a single point.

Magnetic FieldMagnetic FieldLarge current Large current magnetic field magnetic field Magnetic field cuts a conductor Magnetic field cuts a conductor current is induced in current is induced in that conductor that conductor

AC currents AC currents DC currents via switches, relays, electronics, and brushesDC currents via switches, relays, electronics, and brushes

SourcesSourcesRfRf signals e.g. digital signal of processor, CATV, broadband, signals e.g. digital signal of processor, CATV, broadband, or baseband data communications cables (high frequency or baseband data communications cables (high frequency noise) and noise) and the high current signal of the power output stages the high current signal of the power output stages produces significant magnetic fields within the electronic produces significant magnetic fields within the electronic chassis chassis

Protection against Magnetic FieldProtection against Magnetic Field

Separate sensitive input signal conditioning from Separate sensitive input signal conditioning from the other portions of the electronics. the other portions of the electronics. Put small signal analog circuitry on a separate Put small signal analog circuitry on a separate card, covered by a magnetic shield, from the card, covered by a magnetic shield, from the computer and power electronics.computer and power electronics.If not possible, group sensitive analog If not possible, group sensitive analog processing components together and as far away processing components together and as far away from sources of magnetic fields as possible. from sources of magnetic fields as possible. (may be covered by a small magnetic shield box)(may be covered by a small magnetic shield box)

Protection against Magnetic Field Protection against Magnetic Field (cont.)(cont.)

Never run ac power line in the same raceway or Never run ac power line in the same raceway or conduitconduitShieldingShielding

Using ferromagnetic conduit for power lines or for Using ferromagnetic conduit for power lines or for lowlow--level analog signallevel analog signalSpray some coatings to shield against high frequency Spray some coatings to shield against high frequency magnetic field.magnetic field.

ShieldingShielding

Magnetic absorptive Magnetic absorptive lossloss depends ondepends on

MaterialMaterialThicknessThicknessFrequency of the Frequency of the magnetic fieldmagnetic field

Electric FieldElectric Field

Difference in Difference in potentials potentials electric electric field (free charges, field (free charges, primarily electrons in primarily electrons in conductors respond to conductors respond to this field)this field)

ShieldingShielding

The shield must be tied to an infinite The shield must be tied to an infinite source/sink of charge.source/sink of charge.Not only must the signal high be shielded, Not only must the signal high be shielded, but the signal common must be as well. but the signal common must be as well. Do not use the shield as the signal common. Do not use the shield as the signal common. Otherwise, external electric fields can float Otherwise, external electric fields can float the ground up and down.the ground up and down.