Lecture-4: Biopotential Amplifiers & Filters

Biomedical Instrumentation I

Lecture-4: Biopotential Amplifiers & Filters

Dr Muhammad Arif, PhD

m.arif@faculty.muet.edu.pk

Lecture Outline

• Introduction to Biopotential Amplifiers

• Low, Medium, and High Gain Biopotential Amplifiers • Typical Biopotential Amplifier Requirements

• Operational Amplifiers

• Basic Operational Amplifier Configurations • Summing Amplifier

• Differential Amplifier

• Main Stages of a Biopotential Amplifier • Instrumentation Amplifier

• Isolation Amplifiers

• Chopper Stabilized Amplifier • Integrators

• Differentiators

• Log-Antilog Amplifiers

• Active and Passive Filters

Biopotential Amplifiers

• Biopotential amplifier is a term given to amplifiers used to process biopotential signals (e.g., ECG, EMG, EEG, EOG, … etc.).

• The designation applies to a large number of different types of amplifiers (i.e., instrumentation amplifier, isolation amplifier, etc.).

• The basic function of biopotential amplifier is to increase the amplitude of a weak electric signal of biological origin.

• Biopotential amplifiers typically process voltages, but in some cases they also process currents.

Biopotential Amplifiers

• Some biopotential amplifiers are ac-coupled, while some are dc-coupled.

• The dc-coupling is required where input signals are clearly dc or changes very slowly.

• At frequencies as low as 0.05Hz, the ac-coupling should be used instead of dc-coupling.

• This is to overcome the electrode offset potential.

• Also, the skin-electrode interface generates dc offsets.

Biopotential Amplifiers

Low Gain Biopotential Amplifiers

i.Gain factors x1 and x10.

ii.The unity-gain amplifier is mainly for isolation, buffering and possibly impedance transformation between signal source and readout device.

Biopotential Amplifiers

Medium Gain Biopotential Amplifiers

i.Gain factors x100 and x1000.

Biopotential Amplifiers

High Gain Biopotential Amplifiers

i.Gain factors over x1000.

Typical Biopotential Amplifier Requirements

The basic requirements that a biopotential amplifier has to satisfy are:

1.Biopotential amplifiers should have high input impedance i.e., greater than 10 MΩ.

2. Safety: the amplifier should protect the organism being studied. •Careful design to prevent macro and micro shocks.

•Isolation and protection circuitry to limit the current through the electrode to safe level.

3.Output impedance of the amplifier should be low to drive any external load with minimal distortion.

Typical Biopotential Amplifier Requirements

6. Most biopotential amplifiers are differential amplifier as signals are recorded using a bipolar electrodes which are symmetrically located.

7. High common mode rejection ratio (CMMR): biopotentials ride on a large offset signals or noise.

8. Rapid calibration of the amplifier in laboratory conditions.

9. Adjustable gains:

• Often the change in scale is automatic.

Typical Biopotential Amplifier Requirements

10. The physiological process to be monitored should not be influenced in any way by the amplifier.

11. The measured signal should not be distorted.

12. The amplifier should provide the best possible separation of signal and interferences.

13. The amplifier has to offer protection of the patient from any hazard of electrical shock.

Operational Amplifiers

Operational Amplifiers

Operational Amplifiers

Operational Amplifiers

Operational Amplifiers

Differential and Common Mode Inputs for Operational Amplifiers

Ideally the differential gain should be, A_d = ∞ The common mode gain should be, A_cm = 0

Operational Amplifiers

The properties of Ideal Operational Amplifiers

1. Infinite open-loop voltage gain (A_vol = ∞)

2. Zero output impedance (Z_o = 0)

3. Infinite input impedance (Z_i = ∞)

Operational Amplifiers

Operational Amplifiers

Open Loop Vs. Feedback

Operational Amplifiers

Comparator Operational Amplifier (Open-Loop or No Feedback)

Operational Amplifiers

Basic Operational Amplifier Configurations

There are many circuit configurations using op amps as the active device, but only three basic classes of voltage amplifiers exist:

1. Inverting Amplifier

2. Non-inverting Amplifier

Operational Amplifiers

Basic Operational Amplifier Configurations

Operational Amplifiers

Basic Operational Amplifier Configurations

Operational Amplifiers

Basic Operational Amplifier Configurations

Operational Amplifiers

Basic Operational Amplifier Configurations

Operational Amplifiers

Basic Operational Amplifier Configurations

3. Unity Gain Non-inverting Amplifier

•Also called voltage follower or non-inverting follower. •A 100% negative feedback is provided.

Operational Amplifiers

Operational Amplifiers

Summing Amplifier or Multiple-Input Inverting Follower

Example: Find the output voltage in a circuit of summing amplifier, if R₁, =

R₂, = R₃, = 10 kΩ, R₄, = 22 kΩ, E₁, = 100 mV, E₂, = 500 mV, and E₃, = 75 mV.

Differential Amplifier

Differential Amplifier

• The voltage gain for the differential signals is the same as for the inverting followers, provided the ratio equality of R2/R1 = R4/R3 is maintained.

• Differential amplifiers are useful because it rejects common voltages while amplifying the differential signal of interest.

Example:

• Suppose equal 50 Hz supply noise is present on each input of the differential amplifier, and one input is at 5 Vdc while the other is at 1 Vdc.

Main Stages of a Biopotential Amplifier

• Three electrodes connect the patient to a preamplifier stage.

Instrumentation Amplifier

• A solution to both high-gain and high-input impedance problem. • Uses three operational amplifiers.

Instrumentation Amplifier

Advantages of Instrumentation Amplifier

i.Ability to obtain high gain with low resistor values.

ii.Extremely high input impedance.

Instrumentation Amplifier

Isolation Amplifier

• Some hospital patients are extraordinarily susceptible to electrical shock hazards.

• To prevent accidental internal cardiac shock, the manufacturers of modem bioelectric amplifiers, especially those used in ECG recording, use isolation amplifiers (iso-amps) for the direct patient connection.

Isolation Amplifier

Isolation Barrier

Isolation Amplifier

The isolation amplifier usually composed of the following;

1.Input Amplifier 2.Modulator

3.Isolation Barrier 4.Demodulator 5.Output amplifier

•Modulation schemes include amplitude, voltage-to-frequency, duty cycle, pulse width, and others.

•Barrier can be optical, magnetic transformer, capacitive, or even heat transfer.

Isolation Amplifier

Isolation Amplifier Vs. Instrumentation Amplifier

Isolation Amplifier

The modern isolation amplifiers serve three purposes:

1.They break ground loops to permit incompatible circuits to be interfaced together while reducing noise.

2.They amplify signals while passing only low leakage current to prevent shock to people or damage to equipment.

Isolation Amplifier

Isolation Amplifier

Single Package Transformer Coupled or Carrier Type Isolation Amplifier

Isolation Amplifier

Single Package Optical Coupled Isolation Amplifier

Isolation Amplifier

Single Package Capacitive Coupled Isolation Amplifier

Isolation Amplifier

Fiber-optic isolation amplifier using voltage-to frequency converter eliminates power supply feed through and noise coupling.

Analog channel-to-channel Isolation

Analog Isolation

Digital Isolation

Chopper Stabilized Amplifier

Integrators

• Integrators are function as low-pass filters.

• Integration is a mathematical process that allows us to find the area under the curve defined by a function.

Integrators

Integrators

Integrators

Example: An analog integrator uses a 1 MΩ resistor and a 0.2 μF capacitor Find the output voltage after 1 sec if the input voltage is a constant 0.5 V.

Differentiators

• Differentiators are function as high-pass filters.

• The differentiator circuit produces a voltage output proportional to the time rate of change of the input signal voltage.

• Differentiation is the inverse process of integration.

Differentiators

Differentiators

Differentiators

Example: Find the output voltage produced by an operational amplifier differentiator, if R₁, = 100 kΩ, C₁, = 0.5 μF, and E_in has a constant slope (i.e., a ramp function) of 400 V/s.

Filters

Filters may be classified as either digital or analog.

Digital Filters

Digital filters are implemented using a digital computer or special purpose digital hardware, i.e., FIR and IIR filters.

Analog filters

Analog filters may be classified as either passive or active and are usually implemented with R, L, and C components and operational amplifiers.

Filters

An active filter is one that, along with R, L, and C components, also contains an energy source, such as that derived from an operational amplifier.

Passive Filter

Analog Filters

Analog Filter Response Characteristics

Analog Filters

• Very flat amplitude,

A

,

response in the passband.

• Role-off rate is

20dB

/decade/pole.

• Phase response is not

linear.

• Used when all frequencies

in the passband must have

the same gain.

• Often referred to as a

maximally flat response

.

Av

Analog Filters

Chebyshev Characteristics

Frequency (f) M ag n it u d e (d B ) Av f

• Overshoot or ripples in

the passband.

• Role-off rate greater than

20dB

/decade/pole.

• Phase response is not

linear - worse than

Butterworth.

Analog Filters

• Flat response in the passband.

• Role-off rate less than

20dB

/decade/pole.

• Phase response is linear.

• Used for filtering pulse

Analog Filters

Pole of the Filter

•A pole is nothing more than an RC circuit.

Filters

Analog Filters

Single Pole or First Order Low-pass Filter

A first order low pass Butter-worth filter using an RC network for filtering, operational amplifier is used in non-inverting configuration, R₁ and R_f decides the gain of the filter.

Analog Filters

Single Pole or First Order Low-pass Filter

Analog Filters

Two Pole (Sallen-Key) Low-pass Filter

Analog Filters

-+ +V -V R₁ R_f1 R_f2 C₁ v_in C₂ R₂ -+ +V -V R₃ R_f3 R_f4

C₃ v_out

Analog Filters

Analog Filters

Single Pole or First Order High-pass Filter

Analog Filters

Analog Filters

Two Pole (Sallen-Key) High-pass Filter

Analog Filters

Analog Filters

Analog Filters

R₂ R₁

v_in

C₁

C₂

R_f1

R_f2

C₄ C₃

Analog Filters

Two Pole Band-pass Filter Response

BW

f

f

f

A

Stage 2

response

Stage 1

response

f

Analog Filters

Analog Filters

Band-reject or Band Stop or Notch Filter

•The band reject filter is designed to block all frequencies that fall within its bandwidth.

•The band reject filter circuit is made up of a high pass filter, a low-pass filter

and a summing amplifier.

•The summing amplifier will have an output that is equal to the sum of the filter output voltages.

f₁

f₂

v _in v _out

Analog Filters

Analog Filters