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AD8571AR bảng dữ liệu(PDF) 10 Page - Analog Devices |
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AD8571AR bảng dữ liệu(HTML) 10 Page - Analog Devices |
10 / 19 page AD8571/AD8572/AD8574 –10– REV. 0 FUNCTIONAL DESCRIPTION The AD857x family are CMOS amplifiers that achieve their high degree of precision through random frequency autozero stabilization. The autocorrection topology allows the AD857x to maintain its low offset voltage over a wide temperature range, and the randomized autozero clock eliminates any intermodulation distortion (IMD) errors at the amplifier’s output. The AD857x can be run from a single supply voltage as low as 2.7 V. The extremely low offset voltage of 1 µV and no IMD products allows the amplifier to be easily configured for high gains without risk of excessive output voltage errors. This makes the AD857x an ideal amplifier for applications requiring both dc precision and low distortion for ac signals. The extremely small temperature drift of 5 nV/ °C ensures a minimum of offset voltage error over its entire temperature range of –40 °C to +125°C. These combined features make the AD857x an excellent choice for a variety of sensitive measurement and automotive applications. Amplifier Architecture Each AD857x op amp consists of two amplifiers, a main amplifier and a secondary amplifier, used to correct the offset voltage of the main amplifier. Both consist of a rail-to-rail input stage, allowing the input common-mode voltage range to reach both supply rails. The input stage consists of an NMOS differential pair operating concurrently with a parallel PMOS differential pair. The outputs from the differential input stages are combined in another gain stage whose output is used to drive a rail-to-rail output stage. The wide voltage swing of the amplifier is achieved by using two output transistors in a common-source configuration. The output voltage range is limited by the drain-to-source resistance of these transistors. As the amplifier is required to source or sink more output current, the voltage drop across these transistors increases due to their rds. Simply put, the output voltage will not swing as close to the rail under heavy output current conditions as it will with light output current. This is a characteristic of all rail-to-rail output amplifiers. Figures 6 and 7 show how close the output voltage can get to the rails with a given output current. The out- put of the AD857x is short circuit protected to approximately 50 mA of current. The AD857x amplifiers have exceptional gain, yielding greater than 120 dB of open-loop gain with a load of 2 k Ω. Because the output transistors are configured in a common-source configu- ration, the gain of the output stage, and thus the open-loop gain of the amplifier, is dependent on the load resistance. Open-loop gain will decrease with smaller load resistances. This is another characteristic of rail-to-rail output amplifiers. Basic Autozero Amplifier Theory Autocorrection amplifiers are not a new technology. Various IC implementations have been available for over 15 years and some improvements have been made over time. The AD857x design offers a number of significant performance improvements over older versions while attaining a very substantial reduction in device cost. This section offers a simplified explanation of how the AD857x is able to offer extremely low offset voltages and high open-loop gains. As noted in the previous section on amplifier architecture, each AD857x op amp contains two internal amplifiers. One is used as the primary amplifier, the other as an autocorrection, or nulling, amplifier. Each amplifier has an associated input offset voltage that can be modeled as a dc voltage source in series with the noninverting input. In Figures 44 and 45 these are labeled as VOSX, where x denotes the amplifier associated with the offset; A for the nulling amplifier, B for the primary amplifier. The open- loop gain for the +IN and –IN inputs of each amplifier is given as AX. Both amplifiers also have a third voltage input with an associated open-loop gain of BX. There are two modes of operation determined by the action of two sets of switches in the amplifier: An autozero phase and an amplification phase. Autozero Phase In this phase, all φA switches are closed and all φB switches are opened. Here, the nulling amplifier is taken out of the gain loop by shorting its two inputs together. Of course, there is a degree of offset voltage, shown as VOSA, inherent in the nulling amplifier, which maintains a potential difference between the +IN and –IN inputs. The nulling amplifier feedback loop is closed through φA 2 and VOSA appears at the output of the nulling amp and on CM1, an internal capacitor in the AD857x. Mathematically, we can express this in the time domain as: Vt A V t B Vt OA A OSA A OA [] = []− [] (1) which can be expressed as, Vt AV t B OA A OSA A [] = [] + 1 (2) This shows us that the offset voltage of the nulling amplifier times a gain factor appears at the output of the nulling amplifier and thus on the CM1 capacitor. VIN+ VIN VOUT AB AA A B VOSA + BB CM2 CM1 A B VNB VNA BA VOA Figure 44. Autozero Phase of the AD857x Amplification Phase When the φB switches close and the φA switches open for the amplification phase, this offset voltage remains on CM1 and essentially corrects any error from the nulling amplifier. The voltage across CM1 is designated as VNA. Let us also designate VIN as the potential difference between the two inputs to the primary amplifier, or VIN = (VIN+ – VIN–). Now the output of the nulling amplifier can be expressed as: Vt A V t V t B V t OA A IN OSA A NA [] = []− [] ()− [] (3) |
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