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Selecting analogue integrated circuits (Part 1)

12 Nov 2014  | James Bryant

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ICs are supplied in "commercial", "industrial" or "military" temperature versions. The definitions of "commercial" and "industrial" temperature ranges may vary slightly from device to device but the ranges are normally 0 to +70° Celsius for commercial, -25° (or sometimes -40°) to +85° (or sometimes +100°) Celsius for industrial, and -55° to +125° Celsius for military devices. Not all devices are available in all temperature ranges.

These temperatures refer to package temperatures—self heating will make the chip itself warmer. The maximum allowed chip temperature is given in the absolute maximum ratings of the device (but see RAQ 31[4]). Since wider range devices are more expensive you should choose the smallest temperature range compatible with the temperatures that the device will encounter during operation.

When we choose a precision IC we must choose one which remains sufficiently precise over its entire operating temperature range, not just at room temperature. Many devices are specified to remain within certain performance limits over their entire operating temperature range—and if these limits are adequate we have no problem.

But if a device is specified only at 25°C then we must use its temperature coefficient (TC) to calculate its worst case performance at the limits of its actual operating temperature range in the system we are designing—which may not be the same as the range permitted by its data sheet.

For example, consider a device with a commercial specification of 0-70°C with a precision output under certain conditions of 10 V ±1 mV at 25°C. If its TC is 25µV/°C then at +70°C it is 45°C away from room temperature and so it may have an additional output error of 1.125 mV—if the 25°C error is in fact 1 mV the 70°C error might be 2.125 mV—more than twice as large. At 0°C it is 25° away from room temperature, so the extra error may be 0.625 mV.

Of course, if the device is used in an application where the environment does not vary as much as the specification allows the error over temperature may be less. Many medical devices are unlikely to be used outside +10°C to +40°C, so in this case the maximum deviation will be ±15°C and the additional error for the device in our example only 0.375 mV.


Supply rejection
Some analogue ICs need a very specific supply, which is usually defined to ±10% although some even require ±5% supply precision. Many, though, work without much change of performance over a wide range of supply voltages—for example there are plenty of op-amps that meet their specifications with supplies varying from ±3V to ±18V, but although the devices work within their specifications over these supply ranges their performance may vary slightly as the supply varies.

It is necessary to consider the effects of these changes when designing with high performance analogue ICs. If the device is powered via a voltage regulator it is unlikely that the regulator's small changes of output with varying load, temperature or input supply will have a significant effect on performance, but if the device is supplied by an unregulated battery the variation may be enough to affect the system calibration. This must be checked.

Even more important is AC rejection.


AC supply rejection
AC ripple on a power supply is very likely to modulate the output of devices using that supply. The effect is usually small at DC and very low frequencies but may become significant at frequencies as low as a fewkHz. It is important to be aware of this problem and to choose ICs which are relatively immune—that is, where possible, choose ones which have a good AC power supply rejection ratio (PSRR) at any rate to medium audio frequencies (5-10kHz). Complete immunity is impossible, though, and sometimes otherwise ideal devices have poor AC PSRR.


typical PSRR response

Figure 2: Typical PSRR response.


Thus it is very important to design power supplies and board-level and device-level supply decoupling and filtering so that noise on supplies is minimised and any residual noise is eliminated by careful supply decoupling throughout the system.

noise

Figure 3: What an electronic engineer would say about noise.


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