analog signal conditioning
DESCRIPTION
anaTRANSCRIPT
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)
(a) Offset (zero) (b) Sensitivity (c) Combined
(d) Non comformity (e) Hysterisis (f) Dead space
Types of error
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
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).
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
Band reject RC filter (notch filter):
fL= 0.187fc and fH = 4.57fc, fc = 1/(2πRC)
Important features:
- Open loop gain
- Freq. responses
- Offset
- Slew rate
- CMRR
Id = Is exp(Vd/Vt)
Id
Power, PL = i2ZL
Matched: no reflection, λ & Amplitude
not changed.
Amplifier with impedance of 500 Ω drives 8 Ω speaker
70V line matching transformer:
Insert a buffer here!