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Auto-zero operational amplifiers in portable applications in signal conditioning

Print View , by: iSee ,Total views: 9 ,Word Count: 1932 ,Date: Tue, 5 May 2009 Time: 10:44 AM

Chopper amplifier has been used for decades, and its history can be traced back to the sixties of the last century. The invention of chopper amplifier is used to meet the needs for ultra-low offset and low drift op amp needs, such amplifiers than bipolar op amp was excellent. The chopper amplifier in the first place, the amplifier input and output for the switch (or intermittent)-type, input modulation signal to be the purpose of error compensation bias, and in no modulation output. Although this technology to solve a low offset voltage and low drift, but there are also other constraints. Owing to the amplifier input by sampling, the frequency of the input signal must be less than half of the chopper frequency in order to prevent aliasing. In addition to bandwidth constraints, the chopper also interfere with many of the larger cause, they need the output of these ripple smoothing filter.

Later, improvements on the chopper amplifier, through the formation of self-calibration of a chopper amplifier stability. This architecture uses two amplifiers, that is, a main amplifier, a zero amplifier, as shown in Figure 1. Amplifier against short-circuit through the input and in the imposition of a calibration factor to its client to zero bias correction of their error, and then to monitor and calibrate the main amplifier bias. Compared with the old chopper amplifier, such a structure has a great advantage, because the main amplifier can always connect to the IC input and output. So the main amplifier input signal bandwidth of the bandwidth of the decision. Therefore, the input bandwidth is no longer rely on the chopper frequency. However, switching from the charge injection remains a problem, there would be transient and coupled with the input signal, causing intermodulation distortion.

Auto-zero operational amplifiers in portable applications in signal conditioning

Figure 1: simplified chopper stabilized functional structure of Fig.

Auto-zero structure is similar to the concept of zero, respectively, with a main amplifier and a chopper amplifier amplifier stability. However, vis-à-vis the stability of chopper amplifier to reduce noise, charge injection as well as other properties, the later has made great improvements. The various manufacturers use different terminology to define such a structure, such as' auto-zero ',' zero self-calibration 'and' zero drift 'and so on. No matter how terminology is called, the basic concept behind is the same.

The advantages of auto-zero structure

As mentioned above, the auto-zero structure of the offset voltage of amplifier self-calibration errors. Compared to traditional amplifier, which produced the following significant advantages.

Low offset voltage: zero amplifiers as a result of the elimination of constant offset voltage of its own, and subsequently imposed on the main amplifier of a correction factor. The frequency of calibration and the actual design, but usually there are several thousand times per second. For example, Microchip's auto-zero amplifier MCP6V01 every 100 μs on a main amplifier calibration, or 10,000 times per second. As a result of continuous calibration, the offset voltage than traditional op amp is much lower. In addition, voltage offset calibration process for other indicators of DC calibration, such as power supply rejection and common-mode rejection. Therefore, the auto-zero amplifier than traditional amplifiers can achieve a better suppression performance.

Low temperature and time drift: All amplifiers, regardless of how the process technology and structure, the offset voltage with temperature and time will change. The vast majority of operational amplifier using V / ℃ to define the temperature drift. The drift may be different amplifiers with significant differences exist, but for a traditional amplifier, usually changes permicro-volts to tens of volts. The temperature drift for high-precision applications is a serious problem, and not the same as the initial drift error, the drift can not use a one-time calibration system calibration techniques.

In addition to temperature drift, the amplifier offset voltage will change over time. Traditional operational amplifier, the time shift (sometimes referred to as aging) in the data page is usually not specified, but with the aging of components will have a significant error.

Voltage drift due to continuous self-calibration, automatic zero from the nature of the structure of the temperature drift and time will minimize drift. Thus, the auto-zero amplifier can be achieved than the conventional op-amp drift performance is much higher. For example, MCP6V01 operational amplifiers the maximum temperature drift of only 50 nV / ℃.

No 1 / f noise: 1 / f noise or flicker noise, is due to irregular conduction channel and the transistor bias current in the low-frequency noise caused by the phenomenon. In high-frequency band, 1 / f noise can be ignored, because the noise from other sources of white noise will begin to dominate. If the input signal close to DC, such as from stress, the pressure sensor and the thermocouple output signal, then the low-frequency noise at this time is a big problem.

The introduction of auto-zero amplifier, as a part of calibration drift, the elimination of the 1 / f noise. Since the noise source present in the input, and the relatively slow action, it showed a part of amplifier drift off and compensation.

Low offset current: offset current is flowing into the input amplifier and input offset current of transistors produced. Changes in the magnitude of the current range from the slightly microamp security, and with the amplifier input circuit is closely related to. The input parameters in the amplifier when connecting high impedance sensors is very important. Current flows as a result of the migration of high resistance, resulting in impedance on the voltage drop, leading to voltage error. For these applications, the need to consider a low offset current.

In fact the market today all zero input stage amplifiers are used in CMOS, and therefore a very small offset current. However, from the internal switch can lead to the current injection than the traditional CMOS op amp input offset a slightly higher current.

Quiescent current: The battery-powered applications, quiescent current is a key parameter. Zero as a result of amplifiers and other circuits need to support the structure of auto-zero self-calibration for specific bandwidth and slew rate, auto-zero amplifiers are usually quiescent current consumption of greater than traditional amplifiers. But while increasing the efficiency of the structure has been greatly improved. Some operational amplifiers, such as Microchip's MCP6V03, in order to reduce the component of the quiescent current standby to provide a chip select pin, or cut off. [next]

Application: portable pocket balance

Above pointed out the auto-zero amplifier amplifiers help to improve the performance of several parameters. Will be discussed here include the use of the application of stress, to emphasize some of auto-zero amplifier advantages.

Balance is a special portable equipment for weighing small items such as precious metals, jewelry and drugs. These battery-powered devices, usually up to one-tenth of gram precision. Therefore, the application needs high precision for weighing the stress of low-power signal conditioning.

Terms used in the stress resistance caused by external force should be to quantify the strength. There are several different types of stress, but it is the most common of metal stress. Stress of this kind by a metal wire or a metal foil composition. When a powerful role, the stress of the stress change (or positive or negative), thus leading to the stress of resistance changes. By measuring changes in resistance, to determine the amount of added stress. Typically, the stress of the structure are Whiston Bridge, because the circuit structure can provide very high sensitivity. Changes in resistance value due to the smaller, and they Whiston total bridge circuit output voltage is also relatively small. For example, we can assume that full-scale output of 10mV.

Figure 2 is a simplified analysis for the application circuit. Please note that the circuit is not used for the characterization of complete, but has been simplified to show the advantages of auto-zero structure. For example, Whiston bridge circuit output should be buffered to provide a high input impedance, but in simplifying the circuit does not draw on. In the circuit, the differential amplifier gain of 500, so that, from Whiston bridge will be made full-scale output amplifier output to 5V.

Auto-zero operational amplifiers in portable applications in signal conditioning

Figure 2: Using auto-zero amplifier circuit to simplify the application.

Due to the need for applications in the high-gain, amplifier offset voltage drop becomes critical. Any voltage offset will be to enlarge the high-gain amplifier. For example, MCP606 is an amendment to the realization of the input offset voltage for the non-volatile memory, CMOS op amp, the room temperature, the maximum offset for the 250 μV. If the application, the maximum offset error amplifier output will make the error of up to 125mV, or 2.5% of full scale. However, if the use of auto-zero amplifier MCP6V01, the room temperature, the maximum deviation of only 2 μV. While the maximum error amplifier output is only 1mV, which means that only 0.02% of full scale.

As mentioned above, the structure of auto-zero Another advantage is its low time drift and temperature drift. For example, assuming that the portable weighing scales operating temperature range of 0 ℃ to 50 ℃. MCP606 for the temperature drift 1.8 μV / ℃. Generated by the temperature change of the error can be as high as 90 μV, amplifier gain, after re-amplification, the amplifier output will have a 45mV error. And MCP6V01, the largest of the temperature drift of only 50 nV / ℃. So the amplifier output caused by the offset error of only 1.25 mV, the performance of more than 30 times the MCP606 amplifier.

As mentioned above, 1 / f low-frequency applications is a limiting factor, such as discussed here weighing scales. MCP606 operational amplifier 1 / f noise spectrum of angular frequency hearing is generally 200Hz. In this regard, 1 / f noise began to dominate, in 1Hz below resulting voltage - noise density as high as 200 nV / Hz. As for the op amp MCP6V01, due to the structure with automatic zeroing, there is no 1 / f noise, thus maintaining a constant low-frequency band. Balance for weighing applications, as a result of the load cell output is a very slow change in the signal, so at this time 1 / f is a very crucial factor.

Summary of this article

Although today the concept of auto-zero structure dates back to the early chopper amplifier, but compared with the earlier products have been greatly improved. Old chopper amplifier can cause a great many system-level design shortcomings. The new auto-zero structure should be much more useful and provide much better performance. Such as the application of the above, the auto-zero structure in this kind of high-precision applications can provide than traditional op amp is much better performance.

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