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Luis Romeral
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Power Supply Connections
Electronic Systems: Supplied from one or more DC Power Supplies
Instrumentation Systems: Symmetric Power Supply +Vcc y -Vcc, with central zero.
Digital system: Unipolar Power Supply
“Drivers” : Different Power Supplies, electrically isolated to avoid signal
coupling
Zero point of every Power Supply in the equipment must be connected to a
common point:
Zero Point of the equipment (Signal Ground)
To reduce noise, the zero point should be connected to a shield or envelope of the
equipment:
Shield Point of the equipment (Chassis Ground)
Every metallic part of electrical equipment must be connected to ground for
operator safety (electrical normative):
Ground Connection (Earth Ground)
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Power Supply Connections
In a real systems, ground points (signal, chassis, Earth) are connected to each other, to
avoid dangerous voltages between them.
However, a few milivolts can appear between them, which can modify the analogue signals
measured.
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The Importance of Isolation
Electrical current always seeks the path of least
resistance to ground. In this case, that path is
created by the single-ended configuration of the
amplifier, allowing current to flow from the
common mode voltage present on the shunt,
through the grounded amplifier, and to ground.
Differential amplifiers are not isolation
amplifiers, unless specifically designed to be.
Here, a non-isolated, differential amplifier is
applied in the presence of a high common
mode voltage with disastrous results
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The Importance of Isolation
Input-to-output isolation allows the front
end of the amplifier to float with respect
to its output in the presence of common
mode voltages. This allows a potential
difference to exist between input and
output.
Common mode voltages can be very
small, extremely large, or anywhere in
between.
An understanding of what they are and
how they can disrupt specific instrument
configurations is crucial for the success
of the measurement.
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Where are isolation amplifiers needed ?
• To accommodate different "earth" (beyond power supply)
• For safe applications
Isolation techniques: • Capacitive
• Inductive
• Optical
Example: Transformer – based
• Electrically isolated input and output
(and power !) stages
• Based on modulation-demodulation to
ac couple the signal
• Separated power supplies are used
• Filter is needed to remove modulation frequency
• The bandwidth is limited (typically < 100 kHz)
Isolation Amplifiers
Basis of isolation
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Galvanic Isolation Amplifiers
Galvanic isolated amplifiers present some
limitations due to intermediate signal
conversion, i.e., A/D or A/FM.
These limitations are:
- Shortening of Bandwidth
- Output ripple
- Independent (isolated) power supplies
are necessary for input and output stages
Capacitive coupling
Optical coupling
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Optocoupled Systems
TTL optocoupler
An Optocoupler, also known as an Opto-isolator or Photo-
coupler, is an electronic components that interconnects two
separate electrical circuits by means of a light sensitive optical
interface.
An optocoupler or opto-isolator consists of a light emitter, the LED
and a light sensitive receiver which can be a single photo-diode,
photo-transistor, photo-resistor, photo-SCR, or a photo-TRIAC
I1=Vin/R1; Vout=I2’·R2; I2’=I2 => Vout=I2·R2
I2=IF·K2; I1=IF·K1; I1=Vin/R1; K1=K2 => I2=I1
Vout = Vin · R2/R1
Linear optocoupler,
LOC
Galvanic isolation is required for many circuits
found in Telecommunication, Industrial, Medical
and Instrumentation systems. Unlike standard
optocouplers, the LOC operates in a servo mode
configuration which compensates for the LED’s
non-linear time and temperature characteristics.
In addition, the LOC can couple both AC and DC
signals.
Concept
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FM Modulation Used for Linear Signal Transfer
Due to noise, safety requirements, or distance, you may need to isolate a transducer from its
controlling circuitry. Thus, this circuit consists of a transducer and a VFC on the isolated side.
The transducer measures some physical quantity, such as temperature, weight, or
acceleration, and the VFC converts the transducer’s analog output into a pulse train. The
circuit feeds the pulse train to the host computer via an optocoupler, which eliminates any
ground-loop noise or common-mode-voltage effects.
A frequency-to-voltage converter can reconvert the pulse train to an analog voltage. The
result is a cheap, compact circuit in which the duty cycle of the pulse train at FOUT is directly
proportional to the measured quantity.
A disadvantage of these circuits is that the speed of the optocoupler determines the maximum
frequency of FOUT. Low-cost optocouplers have low switching speeds, and their high-speed
counterparts are often expensive.
A typical optical-isolation circuit
uses a VFC (voltage-to-frequency
converter) with an optocoupler.
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Voltage to Frequency converter
The capacitors are either connected to the
charging constant current, or are shorted to
ground.
The capacitor is charged linearly by the
constant current sources, the output of the
capacitor is connected to a comparator
where this voltage is compared with Vin.
When the capacitor voltage exceeds
Vin, the SR flipflop is triggered which
results in the output and the switches,
S1 and S2, changing state. This
alternates the charging and
discharging of the capacitors.
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PWM − Pulse-Width Modulation over Light Barrier
Pulse-width modulation (PWM) is a
modulation used to encode information
into a pulsing signal.
The average value of voltage (and
current) fed to the load is controlled by
turning the switch between supply and
load on and off at a fast rate. The longer
the switch is on compared to the off
periods, the higher the total power
supplied to the load.
Typically switching is about tens or
hundreds of kHz in audio amplifiers and
linear coupling.
Triangular
Oscillator
Filter
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Differential Isolation Amplifier with LOC
V03 = Ic1 ·R3; V03 = IR4 ·R4;
IR4 = Ic2 + IR5 ; V0 = - IR5 · R5 ;
V0 = R5·IR4 - R5·Ic2 ; R3=K4 =>
Vout = R5 (Ic1 - Ic2)
Ic1 = K1· IF1 ; Ic2 = K2· IF2
K1 = K2 = K
V1
V2
2,7 K
2,7 K
IF1
IF2
-V2
V1 = Vi / 2; V2 = -Vi / 2;
IF1 = 1/R1 (0,5 Vi – V); IF2 = 1/R2 (-0,5 Vi – V);
R1 = R2 = R ; IF1 - IF2 = 1/R · Vi
V
Vout = R5 /R · K · Vi
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Current Sources
RL
R
2
3
R
The Zener diode voltage Vz produces a current in the load equal to IL = (Vz-0.7)/R3
(the voltage across the base-emitter junction is fixed at 0.7V and the
zener voltage is fixed to Vz).
I L
A current iin is generated as V1/R1 and is kept constant.
The collector current in the lower left transistor is virtually equal to iin.
The voltage across the base of Q1 keeps the current through the load equal to iin, hence the name current mirror.
As long as iin is constant, so will the current in the load.
iQ2=iQ; iQ2 = iout+ iout /1 – 2·iQ2 / ; (= Q2)
iin=iQ+iout/1; iin= iout + iout /1 – 2·iQ2 / + iout /1 ; Iout iQ2 =>
iout = iini
Q
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Current Sources
The properties of the voltage follower based on an op-amp can be used to generate a constant current as shown in the figure.
The output of the voltage follower is V1 and the current is V1/R1.
The transistor is necessary to provide currents larger than those possible with an op-amp
Voltage References
The simplest voltage reference is the Zener diode, a reversed diode biased at the breakdown voltage for the junction.
The resistor limits this current so that the diode does not overheat.
As long as the maximum current of the Zener diode is not exceeded the voltage across the diode is kept at the breakdown voltage.
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Isolation Mode Rejection Ratio
IMRR
V)
CMRR
VV(·AV
ISOCM
DDOUT ++=
CMRR: Common Mode Rejection Ratio = AD / AMC.
IMRR: Isolation Mode Rejection Ratio = VISO / VOUT.
Note: Some times, isolated amplifiers are such as
trans-resistance ones, and the Transfer Function
is as follows:
( )
INPUTOUT0
IN2G1G0D
I/V)cetanresis-trans(A
RRR/AA
=
++=
Assuming infinite CMRR and IMRR,
AD= VOUT / VDIN = RX / RG
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Isolation Amplifiers
Inductive Coupling Optical Coupling
Amplitude Modulation PWM A / lx Modulation
Max non linearity (%) 0.03 - 0.3 0.005 - 0.025 0.05 - 0.2
Isolation voltage > 7.5 KV > 5 KV > 5 KV
CMRR (60 Hz) y (0 dB) >120dB >120dB 100dB
Bandwidth 2.5 KHz 2.5 KHz 10 - 30KHz
EMI (elec/mag. interferenc.) Low Low None
Susceptibility HF High Low Very Low
Size (cm3) 75-150 90 <1
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Example: how to break the ground loop by means of an isolation amplifier
Data adquisition whitout isolation
Isolation amplifier in a data adquisition system
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Galvanic Isolation Amplifiers: Example of Application
Voltage and Current Feedback for a Motor Control
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Galvanic Isolation Amplifiers: Example of Application
Current Measurement for a High Voltage Circuit
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Galvanic Isolation Amplifiers: Electrocardiograph (ECG) Application
Medical applications of ECG typically use conductive gels at sensor sites, which lower human body impedance
to the extent that a few milliamps of leakage current can cause injury or death. To mitigate this issue, ECGs
typically have multiple stages of isolation to prevent downstream leakage current from flowing into the patient.
The patient is connected to an instrumentation amplifier powered by a low-current floating supply, V1.
The ISOlinear circuit galvanically isolates the instrumentation amplifier input side from potential down-stream
leakage currents generated by the voltage source, V2. The lower voltage, higher-current DSP circuit is also
isolated from the ADC/filter circuit by a separate Si86xx digital isolator, again to ensure no leakage current
paths into the ECG inputs.
ECG requires safety
isolation to protect the
patient from dangerous
leakage currents
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Current Differential Amplifier
It is based on the Current
Mirror: The currents in both
inputs trend to be the same!
VBE is the input voltage, i.e.,
the base-emitter voltage of the
current mirror
There is an output offset voltage
It is used as CA amplifier
VBE
1
RF
VBE
IREF = IOUT + 2 IBE IOUT
iA = iB
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Isolation by means of a Current Differential Amplifier
Mutual transconductance can be usually adjusted.
For instance, for the CA 3080 Amplifier depends on IB,
which can be externally controlled:
jL « RA
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Filtering Concept
Usually, Low-Pass filter, but
also Band Reject, Band-Pass filter,..
Mux: Multiplexer concept
After amplification and eventually
isolation, a filtering process is done
A filter included in each channel reduces
noise, adjust bandwidth, and passes the
desired, measured frequency.
The signal is selected by the Multiplexer,
and sent to the Analogue to Digital
Converter
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