Analog Signal Conditioning
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Transcript of Analog Signal Conditioning
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Instrumentation: Transducers
Environmental
Changes e.g.
Heat, pressure,
Light etc.
Changes in
Resistance,
Inductance,
Capacitance,
Shape, colour
Etc.
Conditioning
system
Available output:
4 – 20 mA or
1 – 5 volt electrical
and/ or
0.2 – 1.0 kg/cm2
pneumatic
Sensor (primary)
Transfer characteristics:
1. Transfer function, f = ������������� �
������������� �
Sensitivity, G = �������������������� �
�������������������� �= Gain
2. Error
- Scale error, e.g. zero (offset), sensitivity, non-conformity, hysteresis.
- Dynamic error, e.g. due to capacitor
- Due to noise, e.g. DC & AC , voltage or current, signal level & range
(4 – 20) mA ≡ (0.25 – 1.25) V across 62.5Ω, output impedance
- Noises, e.g. pickup, crosstalk, interference.
- Frequency change
3. Effect due to environmental changes, e.g. temperature, pressure,
humidity,
4. Linearity
(Secondary)
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(a) Offset (zero) (b) Sensitivity (c) Combined
(d) Non comformity (e) Hysterisis (f) Dead space
Types of error
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Sensor time responses:
a. Zero order b. First order
c. 2nd order oscillatory
b(t) = bi + (bf – bi)[1 – e-t/T]
R(t) = Roe-atsin(2πfnt)
4. Linearity
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Analog signal conditioning
- Convert the signal from sensor, etc. , suitable for process control.
- Sensor produces change in voltage/ current/ pulse (frequency).
(electrical signals)
Types:
1. Offset (bias), also called zero error.
E.g.
Both potentiometers are used for full scale
deflection (FSD) adjustment on the
Ammeter.
So that, 0.1 V ≈ 1 mA range.
2. Linearization – provides output that varies linearly with physical
Parameter.
3. Conversion
e.g. sensitivity 20 mV/oC
i) mV to V or current [4, 20] mA or [0.25, 1.25] V via 62.5 Ω.
ii) freq. to V/I
iii) Digital interface (ADC & DAC).
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d. Loading effect
- Open circuit – no load, Vy = Vc
- With load, Vy = ��
�����Vc
e. Noises – unwanted signal
- Eliminate using - a discriminator or comparator.
- filters
f. Wheatstone bridge
Va = ��
�����Vs
Vb= ��
�����Vs
∆V = Va – Vb= = ���������
��������������Vs
Eg. R1 is RTD from Pt100, with conversion range
[95, 105]Ω ≡ [-100, 100]oC
Then, choose R2, R3 and R4 = 100Ω, Vs = 5V.
Using MATLAB, R1= [95:0.1:105]
dv=5*(100*100-R1.*100)./(R1+100)/(100+100);
plot(R1,dv)
If R3 is Pt100 and other R’s = 100Ω
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Band reject RC filter (notch filter):
fL= 0.187fc and fH = 4.57fc, fc = 1/(2πRC)
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Important features:
- Open loop gain
- Freq. responses
- Offset
- Slew rate
- CMRR
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Id = Is exp(Vd/Vt)
Id
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Power, PL = i2ZL
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Matched: no reflection, λ & Amplitude
not changed.
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Amplifier with impedance of 500 Ω drives 8 Ω speaker
70V line matching transformer:
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