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AD7575BQ Datasheet(PDF) 10 Page - Analog Devices |
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AD7575BQ Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD7575 –10– REV. B APPLICATION HINTS 1. NOISE: Both the input signal lead to AIN and the signal return lead from AGND should be kept as short as possible to minimize input-noise coupling. In applications where this is not possible, either a shielded cable or a twisted pair transmis- sion line between source and ADC is recommended. Also, since any potential difference in grounds between the signal source and ADC appears as an error voltage in series with the input signal, attention should be paid to reducing the ground circuit impedance as much as possible. In general, the source resistance should be kept below 2 k Ω. Larger values of source resistance can cause undesired system noise pickup. 2. PROPER LAYOUT: Layout for a printed circuit board should ensure that digital and analog lines are kept separated as much as possible. In particular, care should be taken not to run any digital track alongside an analog signal track. Both the analog input and the reference input should be screened by AGND. A single point analog ground separate from the logic system ground, should be established at or near the AD7575. This single point analog ground subsystem should be con- nected to the digital system ground by a single-track connec- tion only. Any reference bypass capacitors, analog input filter capacitors or input signal shielding should be returned to the analog ground point. AD7575 WITH AD589 REFERENCE The AD7575 8-bit A/D converter features a total unadjusted error specification over its entire operating temperature range. This total unadjusted error includes all errors in the A/D con- verter—offset, full scale and linearity. The one feature not pro- vided on the AD7575 is a voltage reference. This section discusses the use of the AD589 bandgap reference with the AD7575, and gives the combined reference and ADC error budget over the full operating temperature range. This allows the user to compare the combined AD589/AD7575 errors to ADCs whose specifications include on-chip references. Two distinct application areas exist. The first is where the refer- ence voltage and the analog input voltage are derived from the same source. In other words, if the reference voltage varies, the analog input voltage range varies by a ratioed amount. In this case, the user is not worried about the absolute value of the reference voltage. The second case is where changes in the refer- ence voltage are not matched by changes in the analog input voltage range. Here, the absolute value of the reference voltage, and its drift over temperature, are of prime importance. Both applications are discussed below. If the analog input range varies with the reference voltage, the part is said to be operating ratiometrically. This is representative of many applications. If the reference is on-chip, and the user does not have access to it, it is not possible to get ratiometric operation. Since the AD7575 uses an external reference, it can be used in ratiometric applications. However, because the part is specified with a reference of +1.23 V ± 5%, then the voltage range for ratiometric operation is limited. The error analysis over temperature of ratiometric applications is different from nonratiometric ones. Since the reference and analog input voltage range are ratioed to each other, tempera- ture variations in the reference are matched by variations in the analog input range. Therefore, the AD589 contributes no addi- tional errors over temperature to the system errors, and the combined total unadjusted error specification for the AD589 and AD7575 is as per the total unadjusted error specification in this data sheet. With nonratiometric applications, however, the analog input range stays the same if the reference varies and a full-scale error is introduced. The amount by which the reference varies deter- mines the amount of error introduced. The AD589 is graded on temperature coefficient; therefore, selection of different grades allows the user to tailor the amount of error introduced to suit the system requirements. The reference voltage from the AD589 can lie between 1.2 V and 1.25 V. This reference voltage can be adjusted for the desired full-scale voltage range using the circuit outlined in Figure 19. For example, if an analog input voltage range of 0 V to +2.46 V is required, the reference should be adjusted to +1.23 V. Once the reference is adjusted to the de- sired value at 25 °C, the total error is as per the total unadjusted error specification on the AD7575 specification pages. (To reduce this still further, offset and full-scale errors of the AD7575 can be adjusted out using the calibration procedure outlined in this data sheet.) TLC271* +5V + – 6.8k +5V 10k *1k * 10k * AD589 *ONLY REQUIRED IF IT IS NECESSARY TO ADJUST THE ABSOLUTE VALUE OF REFERENCE VOLTAGE. Figure 19. Reference Adjust Circuit However, it is as the temperature varies from 25 °C that the AD589 starts to introduce errors. The typical temperature char- acteristics of the AD589 are shown in Figure 20. The tempera- ture coefficients (TCs) represent the slopes of the diagonals of the error band from +25 °C to T MIN and +25 °C to T MAX. The AD589 TC is specified in ppm/ °C max and is offered in four different grades. |
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