Lecture 4 - Instrumentation amplifiers PDF

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LECTURE 4 – Instrumentation amplifiers Main aspects related to the signal we have to amplify. Common mode: - few volts above zero - usually caused by the patient body coupling with power lines or other instruments, - sinusoidal shape with 50/60 Hz frequency. Electrode offset: DC voltage difference between electrodes caused by the difference in the semi-element potentials, the input current in the amplifier that can goes through the impedance of the electrodes creating a voltage drop. Useful ECG signal: separated into 2 DC levels corresponding to the offset of the electrodes

ECG signal characteristics

causes: -electrodes half-cell potentials difference (different electrode-tissue coupling) -electrodes partially polarized by input currents of preamplifier cause: capacitive coupling of the patient to the power lines

Ref.: A user’s guide to IC instrumentation amplifiers, AN-244 Analog Devices Application note Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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Instrumentation amplifiers

• High input impedance, suitable for sources with high and unbalance impedances • High CMRR, necessary for large common-mode voltages Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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This is the more basic differential amplifier, that does not have a high input impedance.

Differential amplifier

if R4/R3 = R2/R1 Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

Usually, the signal is measured by an INA, that is a differential amplifier (measuring voltage difference between two electrodes). It has a high input impedance (beneficial because the couple of pole-zero can be more or less neglected), hence having a difference in the electrode impedance in series with the input of the amplifier can lower the effect of the mismatch of the electrode impedance. At the extreme point of an infinite impedance of the amplifier (open circuit) the two resistors can be even a lot different without drop of voltage. This is what is achieved in INA. High CMRR is another characteristic of INA, that is important since common mode in ECG measurement is large.

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LECTURE 4 – Instrumentation amplifiers Common mode of this amplifier is mainly due to the mismatch of the resistors, i.e. when they do not respect perfectly R4/R 3 = R2 /R1 . This can be also calculated supplying the same common mode V IN to both inputs: if in the formula all the resistors were equal to R, the terms inside the bracket would be 0, whatever is the common mode VIN. On the contrary if resistors are not the same or even only one of them has a mismatch with the other three, in the formula not all the factors cancel out hence the output voltage is a gain times VIN. CMRR is by definition the ratio between differential if source impedances are low and/or unbalance, CMRR=1/0.0005 66dB gain and common mode gain of the amplifier. CMRR worsens further Differential gain when all resistors are equal is 1 (R/R Electronics Design for Biomedical Instrumentation 4 Prof. Carlo Fiorini = 1). Common mode gain has been calculated. The result 66dB is not nice at all. If we consider not only the imbalance of resistors inside the network but also the possible imbalance of the sources, considering that the input impedance is not infinite, the situation gets even worse. The solution is given by an INA made of 3 operational Instrumentation amplifier with 3 OPAs amplifiers in the classical configuration: 2 OPA at the input with a common resistor R G, followed by a second stage which is the classical differential amplifier. I stage = differential-to-differential amplifier, at the output of the first stage we still have a voltage difference between VA and V B. Common mode rejection

II stage = provides a unipolar VOUT with respect to ground.

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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Common mode rejection Va = V b = VCM

→ G CM1 = 1 → GCMtot = GCM1 ·GCM2 = 1·GCM2

GD = GD1·G D2 CMRRtot = (GD1·G D2)/1·GCM2 = GD1 ·CMRR2  CMRRtot increases with GD1 (and does not worsen if R1 e R1’ are different)

notes:

CMRR is given by the CMRR of the second amplifier multiplied by the gain of the first stage. Hence, I am multiplying the 66dB CMRR of the differential amplifier by the gain of the first stage: in this way I am improving the CMRR of the second stage, this is why INA CMRR is better than differential amplifier CMRR. N.B. GCM of the first stage is 1 because applying the same voltage at the amplifier inputs means that there is no current across R G, hence no current across R 1 as well as no voltage drops. The common mode is transferred unchanged through the first stage.

•IA with FET inputs (vs. BJT) have larger input impedances and very low input bias currents •remember to provide a DC path to discharge input IBias, in particular in case with sources AC coupled

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

Input impedance is beautiful since is the input impedance of an operational amplifier: adopting mos transistor or junction field effect transistor we have a very high input impedance.

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LECTURE 4 – Instrumentation amplifiers A review of practical details The manufacturer provides an independent connection, called sense, to the output pin in order to allow us to close it ourselves externally. Why?

Role of the “sense” electrode parasitic components (R,C,L) outside the loop the effects of the parasitic are outside the loop load

A long cable connection between amplifier and the next electronic stage can have a lot of parasitic components (summarized in the red box), which together with the load may slow the signal, create oscillations, ecc…

sense the effects of the parasitic are reduced because are held inside the loop load

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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Problem: Imax

IA RL IA may have limited capability to drive current into the load

Current-booster output : the driver provides high current into the load, but being inside the loop it does not change the characteristics of the transfer function

How to combine precision with high driving capability? We use an additional stage (buffer), for which I can choose freely the driving capability, placed inside the loop in order to obtain both properties (sense closed directly to the load).

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Role of the “reference” electrode it could happen that the grounds are at a different potential load

V

in this way a unique ground for amplifier and load is granted load reference Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

The cons of this method is that I am duplicating the cables. Another smart use of the sense pin is made when you need to drive particularly current hungry loads. Having low impedance loads RL , for a given voltage the amplifier has to provide a large current. Since INA are precision components, they may not be defined as power components which have to provide large current, they may have a limited maximum current.

Another use of the “sense” electrode

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

If the INA can be closed after the red box, the parasitic components can be included in the loop, reducing their effect (N.B. The loop gain reduces the impedance of the output node of the amplifier).

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INA provides also an independent ground, that is useful when for example the amplifier is far away from the load hence obviously grounds in the room cannot be equal

LECTURE 4 – Instrumentation amplifiers

Use of the “reference” electrode to provide an offset on the load

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

The reference can be used also to shift the voltage of INA. The output voltage has a DC shift.

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It is usually a good practice to shield the cable in INA. Shielding input cables against external interferences

Vin

IA

Unshielded cables: coupling of interferences to the cable produces currents on the signal lines

VCM V in

IA GND Problems with V CM and shield connected to ground: effects of parasitics capacitances and resistances due to the shield and internal insulator

Shielded cables: interferences produce currents on the shield which are then discharged to ground Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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With unshielded cables, every interference in the room couples capacitively with the cables, that work as antennas, obtaining noise on the input line of the INA (antenna is a floating resistor that couples electromagnetic waves producing a voltage). Shielding is added by applying a Faraday cage around cables: it is a grounded conductor so that when the electromagnetic waves interact with the conductor, the resulting current is charged to the ground. Usually the cage is grounded, but this may not be the right solution in ECG because in the cables, the common mode is a sinusoidal wave at 50 Hz, hence there are 2 problems:

The isolator between external conductor and internal wire has an internal parasitic resistance that causes parasitic current. External metal and internal wire form the so-called cylindrical capacitor  parasitic capacitance for the common mode. This is a problem because if capacitances across the two cables are not equal, the transfer function of the common mode becomes different for the two branches, hence V CM could not be cancelled at all. Use of the common-mode voltage measured by the amplifier to drive guards for shielding

VCM

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

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INA provides a smarter solution for cable shielding. I extract the common mode thanks to the 2 resistors (N.B. I cannot extract the common mode directly at the output because of the superposed differential signal, while in this way the differential signal cancelled itself) Then I simply buffer it and I use it to drive the shielding. In this way the resistor of the insulator and the capacitor are between the same voltage, hence they are not existing. The capacitors can also be different since they do not play any role, they do not change the voltage across the capacitor plates.

LECTURE 4 – Instrumentation amplifiers

V CM+V D/2 V CM-V D/2 V CM

biasing the guards to V CM reduces the effects of capacitances (and therefore of their mismatch) at higher frequency for the CM and of parasitic currents between internal electrodes and shield

Electronics Design for Biomedical Instrumentation Prof. Carlo Fiorini

The role of the shield is to collect the current at low impedance. Is this common mode driving still providing a low impedance path for the current? Yes, because the impedance in the external node of the cage is the output impedance of the buffer, hence it is not a physical ground but a low impedance. Any shaking o the external shield due to the interference will be discharged to the output impedance of the buffer.

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Boot strap of power supply Bootstrap of power supplies (to reduce the effect of V CM)

The measured VCM is provided to the reference voltage of the power supplies, therefore:

VDD = VCM + 15V VSS = V CM -15V

It consists of biasing the power supplies of the operational amplifier (at least at input) through the common mode: it’s like a rigid translation of the voltage reference in such a way that the amplifier does not feel any more the common mode. Generally, we extract the common mode and drive the reference electrode (common electrode) of the power supply.

In 95% of the cases, we plug the common electrode to ground, this means that ±15V (that are in this case) Electronics Design for Biomedical Instrumentation are referred to the ground of the building. 14 Prof. Carlo Fiorini Nevertheless, we can apply any voltage we like to the common electrode, like the common mode, in order to refer ±15 V to it. (U 3,U 4 biased as usual as CMRR has lower impact than for U1,U2)

For instance, when V CM=5V, it means we provide 5+15= 20V and 5-15= -10V to the power supply: we obtain a translation of the voltage supplied to opamp, that in this way is floating exactly with the common mode. The main motivation is that in this way we can eliminate the problem of common mode hence we have less critical requirement for CMRR of input of the amplifier (N.B. CMRR of the input amplifier dominates the entire amplifier; the second stage is less critical since is amplified by the gain of the first stage hence we have no need to apply the bootstrap)....