Purposes of signal conditioning:
1. protection - voltage limiting, current limiting, fuses, polarity protection, isolation
2. right type of signal - convert from ac to dc, convert to voltage or current (as in using a Wheatstone bridge to convert a resistance change into a voltage change) analog to digital, digital to analog
3. right level of signal - amplification
4. reduced noise - filters
5. manipulation - compensation for non-linearities
Inverting amplifier
+-----R2----+
| |
o-----R1---+--|- |
| >---+-----o
Vin +--|+ Vout Vout/Vin = -R2/R1
|
o----------+-------+---------o
Gr
Noninverting amplifier
+-----R2----+
| |
+--|- |
| | >---+-----o
o----------|--|+ Vout Vout/Vin = 1 + R2/R1
|
Vin R1
|
o----------+-------+---------o
Gr
Voltage follower
+-----------+
| |
+--|- |
| >---+-----o
o-------------|+ Vout
Vin
o------------------+---------o
Gr
Summing amplifier
o--Ra--+ +-----R-----+
| | |
o--Rb--+---+--|- |
| | >---+-----o
o--Rc--+ +--|+ Vout Vout=-((R/Ra)Va+(R/Rb)Vb+(R/Rc)Vc)
|
Vin |
|
o----------+-------+---------o
Gr
Includes integrating, differentiating amplifiers
Integrating amplifier
+-----C-----+
| |
o----R-----+--|- |
| >---+-----o t2
+--|+ Vout Vout(t2)-Vout(t1)=-(1/(RC))integral Vin dt
| t1
Vin |
|
o----------+-------+---------o
Gr
A differentiating amplifier can be made by interchanging R and C.
Differential amplifier
+-----R2----+
V1 | |
o----R1----+--|- |
| >---+-----o
o----R1----+--|+ Vout Vout = (R2/R1)(V2 - V1)
V2 |
R2
|
o----------+-------+---------o
Gr
Above amp is especially useful with thermocouples: V1- --+--- -V2
\/ Gr \/
hot cold
Logarithmic amplifier
+----->|----+
| |
o----R1----+--|- | If a sensor input t is exponential,
| >---+-----o i.e., Vin = Ae^(at), then
+--|+ Vout Vout=K ln(Vin)=K ln(Ae^(at))=K lnA + Kat
|
Vin | So Vout is a linear function of t
| (because V = C ln(I) for a diode)
o----------+-------+---------o
Gr
Comparator
V1
o-------------|-
| >---------o
o-------------|+ Vout Amplifier saturates
V2
o------------------+---------o
Gr
Example: temperature switch
V+
|
+-------+-----------------+
| | |
R1 | R relay
thermistor | |
| +--|- |/
| | | >---R3--| This circuit uses a Wheatstone bridge
+-------|--|+ |v
R2 R |
| | Gr
+---+---+
Gr
Amplifier error
Many op amps have two pins called "offset null". A 10K pot can be connected
across these pins with the slider connected to a negative voltage (for
741 type op amps) to adjust the output voltage when there is no input.
Protection
- - - - -
Zener diode
o---R----+----o
|
|___
^ |
---
+
|
o--------+----o
Optoisolator
a package of LED and phototransistor
Low-pass active filter
active filter near bottom of page
See also "various voltage amplifiers" above
+-----R-----+
| |
+-----C-----+
| |
o----R1----+--|- |
| >---+-----o
+--|+ Vout
|
Vin | time constant = RC
|
o----------+-------+---------o
Gr
Wheatstone bridge
Wheatstone bridge (historical Wheatstone bridges)
Vs
|
+-----+-----+
| |
R1 R3
| | Usually drawn as a diamond.
+--o Vo o--+ When Vo=0, (R1/R2) = (R3/R4).
| |
R2 R4 Approximately, delta Vo = Vs (delta R1 / (R1+R2))
| |
+-----+-----+
|
Gr
Digital Signals
- - - - - - - -
Analog to digital conversion
analog signal->sample and hold->analog-to-digital converter-> digital output
quantization levels - levels of analog signal corresponding to digital outputs
quantization interval - difference between two successive quantization levels
usually a constant, but can vary with levels in audio processors
quantization error - error between actual analog signal level and the level
indicated by digital output
Sampling theorem
more than you want to know about the sampling theorem a downloadable simulator for demonstrating the theorem
Called Nyquist criterion or Shannon's sampling theorem.
Sampling rate must be at least twice the highest frequency in the analog signal.
When higher frequencies are sampled, output appears to have sampled a lower frequency. This is called aliasing. An anti-aliasing (low pass) filter should be used on signal before input.
Digital to analog converter
Weighted resistor network
Vref
|
+--S--R----+ +-----R'----+ The Ss are electronic switches
| | | | corresponding to digital bits
+--S--R/2--+---+--|- |
| | | >---+-----o
+--S--R/4--+ +--|+ Vout
|
Gr
Must have accurate resistance values over a wide range, hence seldom
used for more than 4-bit conversions.
R-2R ladder network
Vref----+--R--+--R--+--R--+--2R--+
| | | | |
2R 2R 2R 2R |
| | | | Gr
S S S S
| | | | | | | |
+--|-+---|-+---|-+---|-+ +-----2R----+
| | | | | | |
| +-----+-----+-----+ ---+--|- |
Gr | >---+-----o
+--|+ Vout
|
Gr
Only needs two accurate resistor values.
Analog to digital converters
A-to-D converters downloadable simulation of A-to-D conversion
by successive approximations
Analog input comparator clock
o--| |
| >-----------gate-------binary register
+--| | | |
| ------------------------+ | |
+------DAC------------------------|--+ |
------------------------|--|--+ etc.
| | |
o o o digital output
Faster conversion is obtained by changing the most significant bit
first, etc. so that DAC output is <= analog input.
by ramping
Analog input comparator clock
o--| |
| >-----------gate-------binary register(counter)
+--| | | | |
| | | | |
+------ramp----------+ | | |
generator | | |
o o o digital output
There is also a dual ramp converter, but it is very slow. Voltage to be
measured drives an integrating amp for a fixed amount of time, then amp's
output voltage is brought back to zero by driving it with a negative
reference voltage. The voltage being measured is proportional to the
time it takes to discharge the capacitor in the amp. Fairly insensitive
to noise in signal being measured.
by flash conversion
For an n-bit converter, 2^n - 1 comparators are used, one for
each quantization level above 0. A resistor ladder is to provide
the reference voltages for comparison:
Vref-R-+-R-+-R-+-R-+-etc.
| | | |
Sample and hold amplifier
+-----------+
| |
control +--|- |
: | >---+-----o
o----S-----+--|+ Vout
|
Vin C
|
Gr
The switch S is a special device called an FET switch.
control voltage
|
|___ When control voltage is high, resistance is low.
------ When control voltage is 0, resistance is very high.
o________| |_____o
These notes are based in part on W. Bolton, Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering, Second edition, Addison Wesley Longman, 1999.