Almost all electronic measuring instruments have a "heart" - the reference voltage source. It has a major impact on the performance and accuracy of the simulation system of the entire instrument. We are both excited and anxious when selecting it. We can't blindly select the device parameter file provided by the manufacturer. This often has some pits. When selecting the reference voltage source, we should refer to the device parameter file provided by the manufacturer, which usually includes various indicators of the device, electrical parameters, physical parameters, material description, working environment and usage suggestions, etc., generally in explanatory text. Characteristic curves, charts and data sheets are used, and the so-called "pits" are often found in characteristic curves, working environments and special notes. Here, the reference voltage chip is used for explanation. The reference voltage selection is nothing more than the word "quasi-, stable". The "quasi" can be roughly understood as the initial absolute error, but modern instruments are usually equipped with a perfect calibration system. Therefore, the absolute accuracy requirements can be relatively relaxed, and the basic impact on the "quasi" is mostly noise, the noise is often less pits, as long as more attention to 1 / f noise and broadband noise levels combined with the corresponding application bandwidth, in line with system requirements Basically there is not much problem. Good at digging is often "stable", stable as the name suggests is stable, with stable power supply, stable with load, stable with time, with temperature stability, any slight negligence, the former work is abandoned. With temperature stability, that is, temperature drift performance, this is often the first parameter emphasized by the excellent benchmark, but the emphasis is emphasized, what they say is often the typical value, what is the typical value? As the saying goes, "in general, this is the case." In fact, typical values ​​are not derived from direct measurements, but are based on representative statistical values ​​of a large number of measurements, but in any case, there will be "two things". Inexperienced manufacturers often only give typical values, what is the maximum value? In general, chip manufacturers tend to provide typical values, maximum values, and minimum values ​​at the same time. After the trade-off, it is basically determined whether the chip is available. There is also a "specially authentic" manufacturer that only provides the maximum value. It is a special joke to say that this chip may have problems with partial dispersion of parameters, and sometimes it is also a hidden danger. The important ones are not exactly the two values ​​of the typical value and the maximum value, but rather, but the relationship curve. Below are the output and temperature curves of the three reference chips. Did you find out? Different forms. Which one is good? Not necessarily, different applications may correspond to different temperature ranges, and the most stable in the corresponding temperature range is the best, but this most stable is only the most stable in the sense of reference. After all, the curve provided by the manufacturer is also a "typical curve". The A device is a 2.5V reference curve of a manufacturer, and the guaranteed maximum temperature drift is 3ppm/°C. B is the curve of another manufacturer's 2.048V reference. The guaranteed maximum temperature drift is 2ppm/°C, while C is the top in the industry. One of the benchmarks, the typical temperature drift is 0.5ppm / ° C, the maximum is only 1ppm / ° C. However, it is important to note that the temperature drift calculations used by various manufacturers are generally box method, that is, the output of the maximum output difference in the corresponding temperature range is divided by the temperature difference and divided by the reference point (typically 25 ° C). , that is, according to the following formula, Where T1 is the lowest temperature, T2 is the reference point temperature, and T3 is the highest temperature. So what happens? Since the temperature drift curve is generally non-linear and may be "extremely non-linear", when the range is narrowed down to a certain temperature sub-interval, the temperature drift may be much larger or much smaller than the maximum value of the numerical value. Aggregated in the interval of 20-50 °C, it is found that the difference of the numerical index A has a smoother performance in the interval of 20-50 °C, and the relatively good B device is squatting in the corresponding interval, and the C device performs better. (After all, it is the top in the industry), so the choice of which device to choose may be the result of comprehensive consideration of various factors. The numerical index can only be used as a reference. It depends on the curve, and it depends on the curve of the corresponding interval. In fact, it is not enough. Because the curve given in the data sheet is usually also a "typical" curve, what else to do? ...measured, try to evaluate as many devices as possible on a conditional basis. The above is only a case of temperature drift as an example to illustrate the selection and evaluation of a certain parameter of a device. A chip cannot be selected only by one index. It is necessary to evaluate each parameter to make a decision. After the evaluation, it may be found that the parameter difference is The chip is better, in fact, it should also be the sentence, "There is no best chip, only the most suitable." PA8000 series power analyzers require high precision and stability at higher bandwidths. The selection of the benchmarks is of course strict and prudent. We evaluate the chips of each candidate model in the closest real application environment. The actual performance, consider the most suitable one, but not the indicator.
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