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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