ELECTRONIC

INSTRUMENTATION & PLC DKT314

3. Signal Conditioning Circuits

• Amplification– Increase the level of input signal to better suit the

DAQ.– Improve the sensitivity and resolution of the

measurement.

• Filtering– Reject useless noise within certain frequency range.– Prevent signal aliasing and distortion.

• Attenuation– Contrary to amplification.

Signal Conditioning Functions

Signal Conditioning Functions …continued

• Isolation– Solve improper grounding problem of the

system.• Multiplexing

– Sequentially transmit a number of signals into single digitiser.

• Simultaneous Sampling– Issue of measuring more than one signals at

the same time.• Digital Signal Conditioning

DC Signal Conditioning System

DC Signal Conditioning System… continued

• DC bridge can be Wheatstone’s Bridge which can be balanced by a potentiometer or can be calibrated for unbalanced conditions.

• Amplifier should be thermally good and stable for a long term.

• Low pass filter for eliminating high frequencies components or noise.

• Main disadvantages – problem of drift.

AC Signal Conditioning System

AC Signal Conditioning System… continued

• AC system is to overcome the problem of DC system.

• Transducer can be variable resistance or variable inductance types.

• Bridge circuit for modulating the amplitude of the output from the transducer stage.

• The signal is then amplified and demodulated before pass through the low pass filter.

Amplifiers

• Required in the system to improve the signal strength which is typically in the low level range of less than a few mV.

• In some cases, amplifiers is necessary in providing impedance matching and isolation.

• Basic characteristics involved in designing amplifiers are:– Input impedance– Output impedance– Gain and frequency response– Noise

Input Impedance

• Input impedance of an amplifier regularly depends on the output impedance from the transducer stage.

• Source impedance may vary from few Ω up to hundred MΩ.

• Considering the loading effect of the input impedance of the amplifier to the transducer, the effective input, ei can formulated as follows.

ssin

ini e

RR

Re

Input Impedance… continued

• The error between effective input and source voltage reflect the overall sensitivity of the system.

• A very high input impedance (approaching infinity) amplifier can be used to reduced an error.

• Practically it is simpler to design an amplifier with input impedance of 10 to 50 times source impedance and calibrate the system sensitivity combining the effect of the amplifier itself and the transducer.

• In some cases, the amplifier with a very high input impedance is needed to overcome the changes in the sensitivity of the system.

Output Impedance

• Output impedance required for an amplifier depends on the input impedance of the next sub-system.

• The value of output impedance can be explicit so that the loading effect for next sub-system can be calibrated.

• Generally the output impedance on an amplifier need to be sufficiently low enough (less than an Ω).

• Beside output impedance, the factor of output drive capability of an amplifier also need to considered.

Gain

• Gain of an amplifier is the result of an amplification of the input signal.

• Gain factor of an amplifier can be generally expressed as:

• aout and ain can be power or voltage.• Amplifier gain also can be expressed in Decibel

(dB).

in

out

a

aA

in

out

in

out)dB( V

Vlog

P

PlogA 2010

• Considering the relativity of the input impedance of an amplifier and the output impedance of the transducer, there are attenuation occur before amplification.

• The effective amplification is the product of attenuation and the gain factor of the amplifier.

• Amplifier gain affect the system’s sensitivity and calibration, therefore the high gain stability become and important criteria.

Gain… continued

Type of amplifier circuits

• Several amplifier circuits can be constructed using the operational amplifier (such as µA741). These are:– Non-Inverting Amplifier– Inverting Amplifier– Differential Amplifier– Instrumentation Amplifier

Filters

• Filter is the network used to attenuate certain frequencies but allow others without attenuation.

• Consist at least one pass band, which is a band of frequencies that the output is approximately equal to input and attenuation band that the output is equal to zero.

• Cut-off frequencies is the frequencies that separate the various pass and attenuation bands.

• Important characteristic of filter networks is its construction make use of purely reactive elements.

• Two types of filter:– Passive Filters – Active Filters

Types of Filters

• Passive filters only use passive circuit component such as resistors, capacitors and inductors.

• Active filters use active elements like operational amplifiers in addition to passive elements like resistance, capacitance and inductance.

• Both of passive and active filters can be classified as follows:– Low Pass Filter– High Pass Filter– Band Pass Filter– Band Stop Filter– All Pass Filter

Types of Filters… continued

a) Low Pass

b) High Pass

c) Band Pass

d) Band Stop

e) All Pass

The responses of various filters are shown in figure above: • Fig. (a)

– The gain is constant over a frequency range starting from zero to a cut off frequency fc

– The output of any signal having a frequency greater than fc will be attenuated, i.e: there will be no output voltage for frequencies greater than cutoff frequency fc. Hence output will be available faithfully from 0 to fc with constant gain and is zero from fc onward.

• Fig. (b)– The gain is zero starting from zero to a frequency fc (the cutoff

frequency)– Above the cutoff frequency, the gain is constant and equal to A.

Hence signal of any frequency beyond fc will be faithfully reproduced with a constant gain, and frequencies from 0 to fc will be attenuated.

The responses of various filters are shown in figure above: Cont…

• Fig. (c)– The band pass filter reproduces signals falling between fc1 and fc2,

while signals between 0 to fc1 and frequencies greater than fc2 are attenuated.

– There is an output corresponding to signals having frequencies between fc1 and fc2 but no output for signals having frequencies below fc1 and above fc2. Hence this filter passes a band of frequencies.

• Fig. (d)– The band stop filter attenuates a particular band of frequencies from fc1

to fc2, while passing all frequencies between 0 to fc1 and fc2 onwards.– This filter also called a notch filter.

• Fig. (e)– All pass filter is filter are passed all frequencies without attenuation. The

important feature of this filter is that provides predictable phase shift for frequencies of different input signals. Mostly used in communications.

Passive Filters

• Low Pass Filters (LPF)– A RC network.– At low frequencies, the capacitive reactance is very high,

therefore the capacitor circuit acts like an open circuit. These condition gives Vo = Vi and voltage gain equal to unity.

– At very high frequencies, the capacitive reactance is very low therefore Vo is very small compared to Vi. The gain fall and drops off gradually as the frequency is increased.

Passive Filters… continued

• High Pass Filters (HPF)– A RC network.– At low frequencies, the

gain is small, therefore Vo is small compared to Vi.

– As the frequencies goes high the gain approaches unity.

RCf

RC

RCA

c

HPF

2

1

1 2

Passive Filters… continued• Band Pass Filters (BPF)

– Can be constructed by cascading LPF and HPF.

– At frequencies below the pass band, BPF behave like HPF while above the pass band frequencies the BPF acts like LPF.

– In pass band, the BPF circuit is almost as a resistive network.

1122

21

2

2

1,

2

1

CRf

CRf

RR

RA

upperlower cc

BPF

Passive Filters… continued• Band Stop Filters (BSF)

– Simple RC filters.– Twin T BSF; At the very low

and high frequencies the gain is almost unity, but between the two there is a frequency where the gain become zero.

– The frequency is known as Notch Frequency, f0.

RCf

21

0

Active Filters• Generally the impedances are used in the inverting amplifiers using

operational amplifiers.• Advantages of Active Filter over Passive Filter:

– Gain and frequency adjustment flexibility• Since the OpAmp is capable of providing a gain, the input signal is not

attenuated, as in a passive filter. Active filter easy to tune.– No loading problem

• Because of the high input resistance and low output resistant of the OpAmp, the active filter does not cause loading of the source or load.

– Cost• Active filter more economical than passive filter because of the variety of

cheaper OpAmps available and the absence of inductors.

• Basic relationship can be used to obtain the desired filter sections is as follows (in the case of inverting amplifiers).

• The voltage can also be amplified.

Active Filters… continued

• Low Pass Filters (LPF)

• H refers to characteristic high frequency

11

0 1V

CRjR

RV

fHfHH

fH

fHfHH CR

1

Active Filters… continued

• High Pass Filters (HPF)

• L refers to characteristic low frequency

111

10 1

VCRj

CRjV

LL

LfL

LLL CR 11

1

Active Filters… continued

• Band Pass Filters (BPF)

112

11220 11

VCRjCRjRR

CRjRRRRV

HHLL

LLff