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AD589UH Folha de dados(PDF) 11 Page - Analog Devices |
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AD589UH Folha de dados(HTML) 11 Page - Analog Devices |
11 / 12 page AD7575 –11– REV. B Taking the 25 °C measurement as the starting point, the full-scale error introduced is always in the negative direction whether the temperature goes to TMIN or TMAX. This can be seen from the AD589 temperature characteristic shown in Fig- ure 20. If the reference voltage is adjusted for 1.23 V at 45 °C (for the 0 °C to +70°C range) and 75°C (for the –55°C to +125 °C range) the magnitude of the error introduced is reduced since it is distributed in both the positive and negative direc- tions. Alternatively, this can be achieved not by adjusting at these temperatures, which would be impractical, but by adjust- ing the reference to 1.231 V instead of 1.23 V (for the extended temperature range) at 25 °C. This has the required effect of distributing the plot of Figure 20 more evenly about the desired value. An additional error source is the mismatch between the tem- perature coefficients (TCs) of the 10 k Ω and 1 kΩ resistors in the feedback loop of the TLC271. If these resistors have ±50 ppm/°C absolute TCs, the worst case difference in drift be- tween both resistors is 100 ppm/ °C. From +25°C to +125°C, this introduces a worst case shift of 1.22 mV, which results in an addi- tional full-scale error of 0.25 LSB. If ±25 ppm/°C resistors are used, then the worst case error is 0.13 LSB. Over the 0 °C to +70 °C range, the ±50 ppm/°C resistors introduce an additional full-scale error of 0.11 LSB. All these errors are worst case and assume that the resistance values drift in opposite directions. In practice, resistors of the same type, and from the same manufac- turer, would drift in the same direction and hence the above error would be considerably reduced. An additional error source is the offset drift of the TLC271. This is significant only over the –55 °C to +125°C range and, even in this case, it contrib- utes <0.1 LSB worth of full-scale error. The error outlined in the right-hand column of Table I is a total unadjusted error specification, excluding resistor and offset drift (the effect of these can be controlled by the user). It consists of errors from two error sources: a ±l LSB contribution from the AD7575 (including full-scale, offset and relative accuracy er- rors), and the remainder is a full-scale error introduced by the AD589. It is important to note that the variation of the AD589 voltage only introduces a full-scale error; the relative accuracy (or endpoint nonlinearity) of the system, with a top grade AD7575, is still ±1/2 LSB (i.e., 8-bits accurate). TEMPERATURE – C 1.2370 1.2365 1.2345 –50 125 –25 0 255075 100 1.2360 1.2355 1.2350 Figure 20. Typical AD589 Temperature Characteristics The effect the TC has on the system error is that it introduces a full-scale error in the ADC. This, in turn, affects the total unad- justed error specification. For example, using the AD589KH with a 50 ppm/ °C max TC the change in reference voltage from 25 °C to 70°C will be from 1.23 V to 1.22724 V, a change of – 2.76 mV. This results in a change in the full-scale range of the ADC of –5.52 mV, since the full-scale range on the AD7575 is 2 VREF. Because the LSB size for the AD7575 is 9.61 mV, the AD589 introduces an additional full-scale error of –0.57 LSBs on top of the existing full-scale error specification for the ADC. Since the total unadjusted error specification for the ADC includes the full-scale error, there is also a corresponding in- crease in the total unadjusted error of –0.57 LSBs. The change in reference voltage at 0 °C is –1.5 mV, resulting in a full-scale change of –3 mV or –0.31 LSBs worth of full-scale error. Table I shows the amount of additional total unadjusted error, which is introduced by the temperature variation of the AD589, for different grades and for different temperature ranges. This table applies only to nonratiometric applications, because the tem- perature variation of the reference does not affect the system error in ratiometric applications as outlined earlier. It shows the amount of error introduced over TMIN to TMAX for a system in which the reference has been adjusted to the desired value at 25 °C. The final or right-most column of the table gives the total combined error for the AD589 and the top grade AD7575. Table I. AD589/AD7575 Error over Temperature (Nonratiometric Applications) Full-Scale Error Introduced Combined Worst Case AD589 Temperature by AD589 @ TMAX AD589/AD7575 Grade Range (Worst Case) T.U.E. @ TMAX AD589JH 0 °C to +70°C –1.15 LSB –2.15 LSB AD589KH 0 °C to +70°C –0.57 LSB –1.57 LSB AD589LH 0 °C to +70°C –0.29 LSB –1.29 LSB AD589MH 0 °C to +70°C –0.115 LSB –1.115 LSB AD589SH –55 °C to +125°C –2.56 LSB –3.56 LSB AD589TH –55 °C to +125°C –1.28 LSB –2.28 LSB AD589UH –55 °C to +125°C –0.64 LSB –1.64 LSB *Excluding resistor and offset drift. |
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