Several new high performance electrical amplifiers

Overview

Bio-electrical signals are very weak in detecting biological signal both strong interference, therefore, the design of high-quality electrical amplifiers has numerous technical difficulties.

This article describes the use of ADI produced integrated instrument amplifier and amplifier, the design of several new structure forms of bioelectricity preamplifier.

Figure 1-electrical design application a preamplifier

Figure 2 electrical preamplifier design application 2

Figure 3 electrical preamplifier design application 3

Several new high performance electrical amplifiers

A design application

The amplifier circuit consists of four parts: instrument amplifier A5 form of front-stage amplifier, operational amplifier A4 constitutes differential amplifiers after, DC compensation amplifiers A3 and A1, A2 form right leg drive circuit, circuit structure as shown in Figure 1.

The advantage of this circuit highlight is the introduction of the DC circuit, so that the original DC-coupled amplifier become AC coupled amplifier, to remove the effect of polarization voltage.

Because maximum polarization voltage can reach 300mV, AC coupled to reduce the effect of polarization voltage is required.

In this circuit, DC compensation amplifiers to offset DC offset. To ECG collection, for example, if the left arm DC offset + 300mV, right arm to 0V, differential input voltage-300mV. Assuming that the former class instrument amplifier gain is 5, then the instrument amplifier output amplitude will reach 1.5V, if after the class magnification of 50 or higher, the output does not appear to 75V and saturation. Because in this circuit, feedback loop provides a reverse voltage equal to the reference point, as this kind of linear and impact, polarization voltage being eliminated, output saturation does not appear.

However, the vast majority of integrated instrument amplifier gain and common-mode rejection ratio: the higher the gain, the more common-mode rejection ratio.

And integrated instrument amplifier as bioelectrical preamplifier, due to the presence of polarization voltage preamplifier gain only in dozens of times, this enables integrated instrument amplifier as preamplifiers of cmrr is not possible.

For the circuit, pay attention to when choosing devices: as the front-stage amplifier instrument amplifier low gain requires high common-mode rejection ratio, as a result of subsequent DC compensation circuit and the magnification level is minimal, so the instrument amplifier input offset voltage is too high.

For positive and negative power supply system, you can use analog instrument amplifier AD620. It has the following features: adjustable gain (1 to 1000); power supply range (2.3V ~ 18V); maximum input offset voltage 50 V; input bias current up to 1nA; gain low has a large common-mode rejection ratio (G = 10, cmrr 100dB minimum), meet the design requirements. For rear differential amplifiers operational amplifier A4, magnification mainly in this level, so there are required operational amplifier low input offset voltage. You can choose the ADI OP747 (four operational amplifier), OP2177 (dual operational amplifier), have a micro-v magnitude of offset voltage and good performance.

Design application 2

The circuit consists of four parts: parallel dual operational amplifier consisting of front-stage amplifier, RC coupling circuits, integrated instrument amplifiers back-level amplifiers and common-mode signal sampling driver circuit, circuit design, as shown in Figure 2.

The advantage of parallel dual operational amplifier is not sophisticated matching resistor, theoretically it's cmrr is infinite, and with its peripheral resistance matches.

But parallel dual operational amplifier output is the double-differential output signal, if only with single-ended output signal will no longer have this advantage. So this circuit in after the class uses an integrated instrument amplifier A5, will double-differential output signal is converted to a common single-ended output signal. Integrated instrument amplifier has a good performance, but because the cmrr proportional gain in differential mode, while devices exist high offset voltage and usually signal source in large DC offset voltage (such as the detection of polarized when physiological signal voltage and sensor zero offset voltage), in direct application of integrated instrument amplifier as preamplifier is not able to achieve the maximum of cmrr performance. So this circuit in after the class uses an integrated instrument amplifier, and uses the RC-coupled circuit isolation DC signals, thus making integrated instrument amplifiers achieve higher differential-mode gain, resulting in very high cmrr performance. Common mode sample driver circuit consists of two equal resistor R4, R5, and one by the operational amplifier A3 composition of follower. A3 of the input signal is taken from the a1 and a2 outputs two series resistance of the voltage Vc, i.e. when only the differential mode signal output V01 = V02, VC = 0, the operational amplifier A3 output voltage of 0 is equivalent to the Earth; and when both common-mode voltage and differential-mode signal input, the total output A3 contains only common mode input signal part VC = 1/2 (Vi1 + Vi2). Making common mode signal without RC coupling circuit pressure directly on the integrated amplifier input, avoids due to capacitance coupled circuit does not match the overall reduction circuit cmrr.

After the actual measured, as shown in Figure 2 uses the circuit diagram the parameter, the circuit cmrr in 120dB above.

Use this circuit to note selected devices: as after the class amplifier instrument amplifier input offset voltage to as small as possible, because after the class has a major role of amplification. Can the AD620 Adi, the input offset voltage of up to 50 V.

Design application 3

The circuit consists of four parts: Qualcomm network, and

With dual op amp amplifier with integral feedback circuit of Qualcomm differential amplifiers and common-mode signal sampling driver circuit, circuit design, as shown in Figure 3. The amplifier has two different from past other biological preamplifier features: 1. Qualcomm Network on the front-end amplifiers; 2. Amplifying circuit magnification is done in amplification that parallel dual operational amplifier.

In this amplifier design, the signal input of the Qualcomm network is not grounded, if you enter a common-mode voltage, there is no current flowing through the network (no common-mode current pathway), high-pass network points potential equal, won't change the common mode signal signal for differential mode, you can achieve a high common-mode rejection ratio.

In addition, because the internal resistance of the organism signal source is generally higher, so the design-time selection of great resistance as possible R1 and R2 (M class), you can better meet the needs of the circuit.

For a multistage amplifier system, the first level of magnification for large, as much as possible to reduce the noise amplifiers behind, so that the entire system of noise reduction.

But because polarization voltage and input offset voltage operational amplifier itself, fore gain not may be too large. Qualcomm Network better solution to the DC input voltage (polarization voltage), however, the operational amplifier itself by input offset voltage is magnified, still obviously affect the output of dynamic range. For example, the input offset voltage 1mV, 1000 times magnification, the amplifier output will reach 1V. Qualcomm Network resistor-capacitor will bring the noise impact on the system, this is past bioelectrical preamplifier design not the capacitance is placed in front of the system. The circuit uses Qualcomm differential amplifiers solve this problem. In the latter part of the differential amplifier feedback loop by adding an integrator, it has no effect on the AC signal, only on DC and extremely low frequency signal integration, offset by the impact. Right leg drive circuit through a follower access circuits, you can avoid right leg signal effect of the circuit, power frequency interference suppression.

After the actual measured, as shown in Figure 3 uses the circuit diagram the parameter, taking into account the problem of resistance, capacitance match the circuit cmrr can reach 123dB.

Design and application of the three, though the magnification is in the fore parallel dual operational amplifier, but former Qualcomm network, after integrating feedback circuit, dual operational amplifier requirements. As a back-level differential amplification of operational amplifier, because they do not make amplification, so do not specifically require a low offset voltage. In this circuit to make resistance as possible, so that the capacitor matching the best system performance.

Several comparison of bioelectrical preamplifier

Integrated instrument amplifier cmrr and gain.

Gain higher, cmrr. And integrated instrument amplifier as bioelectrical preamplifier, due to the presence of polarization voltage preamplifier gain only in dozens of times, this enables integrated instrument amplifier as preamplifiers of cmrr is not possible.

Combined with common mode driving technology of RC coupling circuit and integral feedback circuit, whether on the front or in the middle of the circuit are aimed at removing a magnified impact on biological signals of polarization voltage and devices in high offset voltage.

In the design of application a, front-stage amplifier by instrument amplifiers, gain lower, in order to achieve a high common-mode rejection ratio, instrument amplifier low gain with high common-mode rejection ratio.

The amplification signal DC components (including polarization voltage and instrument amplifier input offset voltage) by eliminating DC compensation circuit. Back level amplifier has a major task of amplification, operational amplifier input offset voltage has a high demand, not too large, high-gain amplifier, affect the output signal. In the design and application of the second, parallel dual operational amplifier as a top level amplifiers, gain low. It does not need sophisticated matching resistor, theoretically it's cmrr is infinite, and with its peripheral resistance matches. The amplification of the signal after common mode sample driver removal DC component. Instrument amplifier as after the class amplifier, the primary task of enlargement, since the cmrr proportional gain in differential mode, so you can achieve a high common-mode rejection ratio, but also require instrument amplifier input offset voltage is not too large, otherwise high-gain amplifier the signal output after impact.

In the design of the application, the front-end of Qualcomm Network filter addition polarization voltage, led as the front-stage amplifier parallel double amplifier can assume major enlargement task without causing the output saturation, and high-gain amplification can reduce system noise, and the operational amplifier itself by input offset voltage with high-gain amplifier by after the class Qualcomm differential amplifier feedback loop Integrator elimination.

However, it is necessary to pay attention to the design resistance, capacitance of matching, to achieve a high common-mode rejection ratio.

Conclusion

This article on several high-performance, low-cost electrical preamplifier provides an analysis of comparison, they skillfully use instrument amplifier gain and common-mode rejection ratio, combined with RC coupling circuit, integral feedback circuits and common-mode driver technology enables high performance amplifiers, suitable for detection of biological signals.