Restriction of the hottest three operational ampli

Constraints of three operational amplifiers on instrument amplifiers

Abstract: three operational amplifier instrument amplifiers have been used as an industrial standard in precision applications requiring high gain and/or high CMRR. However, when this kind of amplifier works in the single power supply system required by most current applications, it has great limitations. This paper expounds the limitations of the traditional three op amp instrument amplifier, and introduces Maxim's patented indirect current feedback structure. This structure of instrument amplifier has obvious advantages when working with a single power supply. This paper also gives the test waveform to support the specific analysis structure

application of instrument amplifier

under the condition of large common mode voltage, instrument amplifier can amplify very weak differential voltage signal and has high input impedance. These characteristics make it popular in many applications, such as strain gauge bridge interface for measuring pressure and temperature, thermocouple temperature detection and various low side and high side current detection

three op amp instrument amplifier

a typical three op amp instrument amplifier (see Figure 1) can provide excellent common mode suppression, and the differential gain can be accurately set through a single resistor. Its structure consists of two-stage circuits: the first stage provides unit common mode gain and overall (or most) differential gain, and the second stage provides unit (or less) differential mode gain and overall common mode suppression (see Figure 2)

Figure 1 Internal structure diagram of max4194 max4197 series three op amp instrument amplifier

Figure 2 In this two-stage amplification architecture of the input signal, the input common mode voltage is brought into the intermediate stage (within the circle)

at present, most low-voltage amplifiers provide full swing output, but do not necessarily have full swing input characteristics. Nevertheless, here we take the single power supply (VCC) three op amp instrument amplifier as an example, assuming that the instrument amplifier has high gain, full swing input and output, as shown in Figure 1

because Vout = gain vdiff + VREF, it can be obtained:

in practical applications, VREF = 0 (for unipolar input signals) or VREF = vcc/2 (for bipolar input signals) is often set

when VREF = 0, the inequality is simplified as:

0 VCM vout/2 vcc

vref = vcc/2, the inequality is simplified as:

0 VCM vout/2 vcc/4 vcc

it is easier to understand the above conditions through charts, as shown in Figure 3

Figure 3 (a) When VREF = 0 and (b) VREF = vcc/2, the VCM allowed by the single power supply three op amp instrument amplifier under different input differential voltage. The horizontal axis is the amplified input differential voltage (VOUT)

the gray area in Figure 3 indicates the range of input common mode voltage (related to input differential voltage), in which the output of the amplifier (A1, A2) in Figure 1 will not reach the saturation state of power supply voltage swing, and this range depends on Vout and VREF. Because Vout - VREF is the input differential voltage part after real amplification, the allowable common mode input range changes with the change of input differential voltage

of course, the most ideal situation is to make full use of the circuit gain to obtain full swing output (VOUT) when the input reaches the expected maximum differential voltage. The black area in Figure 4 represents the input common mode voltage range when the instrument amplifier amplifies the maximum input differential voltage, and the output is Vout = 0 or Vout = VCC

Figure 4 The black area indicates the input common mode voltage range corresponding to the maximum output voltage (i.e. the maximum input differential voltage) when (a) VREF = 0 and (b) VREF = vcc/2, the traditional three op amp instrument amplifier amplifies the signal

it can be seen that the input common mode voltage is strictly limited in both cases, especially:

if you want to fully amplify the unipolar differential input signal (assuming VREF = 0 and you can get a full swing output from 0 to VCC), the common mode voltage of the signal should be VCC. Under other common mode voltages, the output voltage will not reach the full swing of VCC (the maximum value of input differential voltage must be reduced). For bipolar input differential signals (VREF = VCC), the input common mode voltage range corresponding to the full swing output voltage from 0 to VCC is only VCC to VCC

in both cases, if the input common mode voltage reaches or approaches the ground potential (0V), the amplifier will not be able to amplify the input differential voltage signal. Therefore, it is assumed that the input differential voltage (required) is independent of the input common mode voltage (unnecessary), and the black area represents the minimum and maximum VCM values corresponding to the full-scale output voltage Vout. Outside this area, improper coordination between vdiff and VCM will produce unacceptable VCM voltage. Note that in Figure 4a, if you need to get a full-scale VCM change, the tolerance of the input common mode voltage is 0. In other words, the input signal is not allowed to have common mode change

it can be seen that in a single power supply system, the application range of three operational amplifiers is very limited. We need to have an in-depth discussion to answer the following two questions:

What if the internal amplifier (A1 and A2) saturates when the output reaches the power swing

what will happen if the input cannot reach full swing

effect of input amplifier saturation

assume that saturation occurs when the output of amplifier A1 reaches the ground potential, that is, VIN + vin-, and the common mode voltage is in the X region in Figure 4 (vdiff is greater than the allowable value in the gray region)

because A1 is saturated (Vout1 = 0), it enters the comparator (nonlinear) working mode, and the voltage of the inverting pin is no longer equal to that of the in-phase pin (vin-). Amplifier A2 will be equivalent to an in-phase amplifier, amplifying the voltage at the same phase terminal (vin+) with a gain of 1 + R1/(R1 + RG). For the high gain amplifier, RG R1, so amplifier A2 becomes an amplifier with in-phase gain of 2

this is a potential hazard in the working mode of the three operational amplifier instrument amplifier. At this time, the instrument amplifier not only does not amplify the input differential voltage, resulting in the decline of device performance, but also amplifies the input common mode voltage. Worst of all, common mode voltage is usually uncontrolled and is noise harmful to useful signals. This is a very serious problem, because the original intention of choosing the instrument amplifier is to eliminate this kind of noise

influence of non full swing input structure

as mentioned above, most amplifiers have full swing output, but do not have full swing input. In precision applications, the full swing input stage is often difficult to design, because when the common mode voltage is close to VCC and GND, the crossover characteristics are not ideal. In the process of transformation, the offset voltage will be generated between the n-type and p-type pairs of input differential classification. The key parameters of excellent precision instrument amplifier design are low Vos and high CMRR. When changing the common mode voltage in the crossing area, because CMRR = Vos/VCM, the change of Vos will greatly reduce the performance of CMRR

therefore, precision instrument amplifiers mostly adopt non full swing input structure, although the input common mode voltage range of such amplifiers still includes the negative swing (0V) of the power supply. If we look back at Figure 3 below and consider its limitation on the input common mode voltage, the redrawn figure is shown in Figure 5

Figure 5 When the non full swing input stage is used, (a) VREF = 0 and (b) VREF = vcc/2, the input common mode voltage that can be accepted by the three op amp instrument amplifier powered by a single power supply under different input differential voltage

indirect current feedback structure

indirect current feedback structure is a new scheme for designing instrument amplifiers. Because of its many advantages, this structure is more and more favored by people. Figure 6 shows the indirect current feedback structure used by max4462 and max4209 instrument amplifiers. Individually or due to lack of 12% Goods, or manufacturer operation, or high temperature and weak mentality pessimistic narrow range adjustment

Figure 6 The indirect current feedback structure adopted by max4462 and max4209 instrument amplifiers

this new structure includes a high gain amplifier (c) and two transconductance amplifiers (A and b). Each transconductance amplifier converts the input differential voltage into the output current and suppresses all input common mode voltages. When the amplifier works stably, the output current of GM class a source is equal to the input current absorbed by GM class B. This current matching is achieved by the feedback of high gain amplifier C, and the differential voltage at the input of feedback amplifier B is the same as that at the input of amplifier a. The design then establishes a specified current (equal to vdiff/R1) in the output resistor network, which also flows through R2. Therefore, the out output voltage amplifies only the input differential voltage (gain = 1 + R2/R1). An arbitrary reference voltage can be added to ref to provide bias for the output voltage. The principle is similar to that of a standard three op amp instrument amplifier

convert the device block diagram into an equivalent circuit, as shown in Figure 7. Comparing this figure with figure 2, we can see an important advantage. The intermediate signal of the three op amp instrument amplifier contains not only the amplified differential voltage, but also the input common mode voltage. The indirect current feedback structure only contains the amplified input differential voltage, and the first stage circuit provides all common mode suppression. Then, the second stage gives all differential gains and further suppresses the common mode signal, so the reference voltage can be used to provide bias for the output. It can be seen that the input common mode voltage in the three op amp instrument amplifier is completely suppressed in the indirect current feedback structure

Figure 7 There is no common mode voltage in the first stage output of the indirect current feedback instrument amplifier

considering the limitation of input common mode voltage (for example, a non full swing input stage), the transmission characteristics of the device are shown in Figure 8. The black area indicates the limit range of the input common mode voltage corresponding to the full-scale output voltage. The gray area shows the range of input common mode voltage when the instrument amplifier works normally according to the assumption. The output voltage is proportional to the amplified input differential voltage, and all input common mode voltages are suppressed at the same time

figure 8 The acceptable input common mode voltage range of the indirect current feedback instrument amplifier is shown in the gray and black parts in the figure. In (a) and (b), the black area is a subset of the gray area, in which the full-scale output voltage can be obtained

the following experimental results provide strong support for the discussion of the indirect current feedback architecture. Assuming that max4197 and Ma fixtures are used with only one set of x4209h, the gain of both instrument amplifiers is 100. Max4197 is a three op amp structure, and max4209h is an indirect current feedback instrument amplifier. Both use VCC = 5V power supply and VREF = 2.5V to provide zero output bias of the device

in this experiment, two signal waveforms are input into the instrument amplifier

example 1 is a 1kHz differential signal with a large common mode voltage of 100Hz. The ideal instrument amplifier output does not contain 100Hz signal components, but only 1kHz signals. The signal waveform can be approximated as:

vin+ = sine wave amplitude = 2Vp-p,

offset = 1V, frequency = 100Hz

(vin+ - vin-) = sine wave amplitude = 30mvp-p,

offset = 0, frequency = 1kHz

example 2 is a 100Hz differential voltage with a larger common mode voltage of 1kHz. The ideal instrument amplifier output does not contain 1kHz signal components, only 100Hz signal. The input signal waveform can be approximated as:

vin+ = sine wave amplitude = 2Vp-p,

offset = 1V, frequency = 1kHz

(vin+ - vin-) = sine wave amplitude = 30mvp-p,

offset = 0, frequency = 100Hz

the experimental results are as follows, in which channel 1 is vin+, channel 2 is vin-, and channel 3 is the output of the instrument amplifier