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Recap
•RTOS
•Debugging/verification
Lab 4 Application of RTOS
• Input sound, analog filter
• Digital filter, FFT
• Display amplitude versus freq on the oLED
Noise Reduction
2048
1536
Output
1024
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
512
0
-512 0
1000
2000
3000
4000
-1024
-1536
-2048
Tim e
5000
6000
7000
Objectives
• Analog circuit design with single supply
– MAX494CPD/TLC2274ACN rail to rail op amp
• Instrumentation amps (EE445L)
– INA122
• Noise measurements and reduction
• Electret microphones
• IR distance sensor
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Convert to single supply
• Vcc = 3.3V, start with design using +Vs -Vs
• Assume ADC range is 0 to Vmax (0 to 3V)
• Add an analog reference, Vref = ½ Vmax
• Map
-Vs (-12)
to
digital ground
Analog ground
to
Vref reference
+Vs (+12)
to
Vcc supply
Reference EE345L book, chapter 5
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Inverting amp
R3
Iy
Ix
Vin I in
R1
+Vs
Vy
Vout
Vx
I2
0.1F
0.1F
R2
-Vs
R1 * R2
R3 =
R1 + R2
Use a rail-to-rail opamp and map
-Vs
to
digital ground
Analog ground
to
Vref reference
+Vs
to
Vcc supply
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
R2
=Vin
R1
Original design
Vout = 2Vin
with some LPF
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
New design
Vout = 2Vin-1.23
with some LPF
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Instrumentation amp
• Necessary conditions (must be true)
– Differential input
• Motivation (at least one must be true)
– Large gain
– Large CMRR
– Low noise
– Small package
– Large input impedance
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
INA122
V1
Vout
Vout = Gain*(V1-V2) + Vpin5
V2
Vpin5
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Final exam 2006, Q5
Magnetic field noise
Motor
Magnetic field
Vm= KBS
Our
instrument
Im
S
I1
V1
Vs
Vs
Mutual inductance
V1
AC
power
How do you reduce S?
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Electric field noise
Motor
Electric field
Our
instrument
Id
C
I1
V1
Vs
Vs
Stray capacitance
V1
AC
power
How do you reduce C?
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Measure noise
• DVM (AC mode)
• Oscilloscope (line trigger)
Voltage
White or 1/f noise
Voltage
Periodic noise
Peaktopeak
Peaktopeak
Time
Time
1000
• Spectrum analyzer
Voltage
Noise
100
60Hz noise
1/f noise
bandwidth limits
white noise
10
1
1
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
10
100
1k
Frequency (Hz)
10k
100k
Reduce noise
1) Reducing noise from the source
– enclose noisy sources in a grounded metal box
– filter noisy signals
– limit the rise/fall times of noisy signals.
– limiting the dI/dt in the coil.
Noisy
April 30, 2017
Less noisy
Jonathan Valvano
EE445M/EE380L.6
Reduce noise
2) limiting the coupling between the noise source and your
instrument.
– Maximize the distance from source to instrument
– Cables with noisy signals should be twisted together,
– Cables should also be shielded.
– For high frequency signals, use coaxial
– Reduce the length of a cable
– Place the delicate electronics in a grounded case
– Optical or transformer isolation circuits
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Reduce noise
2) limiting the coupling between the noise source
and your instrument.
Transducer
Instrument
Twisted wires
Zin
Rout
Shielded cable
Instrument
Twisted wires
Zin
Rout
Shielded cable
April 30, 2017
Instrument
Jonathan Valvano
EE445M/EE380L.6
Reduce noise
– 3) reduce noise at the receiver.
– bandwidth should be as small as possible.
– add frequency-reject digital filters
– use power supply decoupling capacitors on each
– twisted wires then Id1 should equal Id2.
I
– V1-V2 = Rs1 Id1 - Rs2 Id2.
Amp
R
d1
s1
Vs
V1
R s2
V2
Rm
Vm
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Id2
R diff R in1
R in2
Electret microphone
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Typical cross section of an ECM
Data sheet says 2k
•
JFET buffer
•
Phantom Biasing
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Gain = 1+R13/R12
Electret interface
HPF fc = 1/(2p(R9||R10)*C5)
Bias
LPF fc = 1/(2pR13*C6)
Use max494
HPF
LPF
Offset for single supply operation
April 30, 2017
Balance Z to subtract bias current
Jonathan Valvano
EE445M/EE380L.6
Show Measurements
• Noise
o DVM
o AC mode on scope
o Spectrum analyzer
• Signal
o Peak to peak
o Spectrum
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Sharp GP2Y0A21YK
•
•
•
•
•
•
You will need 5V to power IR sensor
Needs analog LPF
Reduces noise
Analog input protection
Needs digital median filter
Needs 10 F or larger +5V to gnd cap
for each sensor
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
Sharp GP2Y0A21YK
d (cm)
10
15
20
30
1/d
0.100
0.067
0.050
0.033
ADC
703
484
380
260
10-bit ADC
Calibration
800
700
600
500
400
300
200
100
0
0.000
ADC = 6706.7/d + 39.643
2
R = 0.9988
0.020
0.040
0.060
1/d (1/cm)
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6
0.080
0.100
0.120
Sharp GP2Y0A21YK
• Accuracy => calibration
• Resolution => noise
ADC = 6707/d+40
d = 6707/(ADC-40)
d (0.01cm) = 6706700/(ADC-40)
April 30, 2017
Jonathan Valvano
EE445M/EE380L.6