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AD774BAR Datasheet(PDF) 8 Page - Analog Devices |
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AD774BAR Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page REV. C –8– AD674B/AD774B Circuit layout should attempt to locate the ADC, associated analog input circuitry, and interconnections as far as possible from logic circuitry. For this reason, the use of wire-wrap circuit construction is not recommended. Careful printed-circuit layout and manufacturing is preferred. UNIPOLAR RANGE CONNECTIONS FOR THE AD674B AND AD774B The AD674B and AD774B contain all the active components required to perform a complete 12-bit A/D conversion. Thus, for most situations, all that is necessary is connection of the power supplies (+5 V, +12/+15 V, and –12/–15 V), the analog input, and the conversion initiation command, as discussed on the next page. 2 3 4 5 6 8 12 13 10 14 9 AD674B/AD774B STS 28 HIGH BITS 24–27 MIDDLE BITS 20–23 LOW BITS 16–19 +15V 7 –15V 11 DIG COM 15 +5V 1 100 R2 GAIN R1 100k OFFSET +12V/ +15V –12V/ –15V 100k 100 0 TO 10V ANALOG INPUTS 0 TO 20V 12/ 8 CS A0 R/ C CE REF OUT BIP OFF 10VIN REF IN 20VIN ANA COM Figure 7. Unipolar Input Connections All of the thin-film application resistors of the AD674B and AD774B are factory trimmed for absolute calibration. Therefore, in many applications, no calibration trimming will be required. The absolute accuracy for each grade is given in the specification tables. For example, if no trims are used, ±2 LSB max zero offset error and ±0.25% (10 LSB) max full-scale error are guaranteed. If the offset trim is not required, Pin 12 can be connected directly to Pin 9; the two resistors and trimmer for Pin 12 are then not needed. If the full-scale trim is not required, a 50 Ω 1% metal film resistor should be connected between Pin 8 and Pin 10. The analog input is connected between Pins 13 and 9 for a 0 V to 10 V input range, between Pins 14 and 9 for a 0 V to 20 V input range. Input signals beyond the supplies are easily accommo- dated. For the 10 V span input, the LSB has a nominal value of 2.44 mV; for the 20 V span, 4.88 mV. If a 10.24 V range is desired (nominal 2.5 mV/bit), the gain trimmer (R2) should be replaced by a 50 Ω resistor and a 200 Ω trimmer inserted in series with the analog input to Pin 13 (for a full-scale range of 20.48 V [5 mV/bit] use a 500 Ω trimmer into Pin 14). The gain trim described below is now done with these trimmers. The nominal input impedance into Pin 13 is 5 k Ω, and into Pin 14 is 10 k Ω. UNIPOLAR CALIBRATION The connections for unipolar ranges are shown in Figure 7. The AD674B or AD774B is trimmed to a nominal 1/2 LSB offset so that the exact analog input for a given code will be in the middle of that code (halfway between the transitions to the codes above and below it). Thus, when properly calibrated, the first transition (from 0000 0000 0000 to 0000 0000 0001) will occur for an input level of +1/2 LSB (1.22 mV for 10 V range). If Pin 12 is connected to Pin 9, the unit will behave in this manner, within specifications. If the offset trim (R1) is used, it should be trimmed as above, although a different offset can be set for a particular system requirement. This circuit will give approximately ±15 mV of offset trim range. The full-scale trim is done by applying a signal 1 1/2 LSB below the nominal full scale (9.9963 for a 10 V range). Trim R2 to give the last transition (1111 1111 1110 to 1111 1111 1111). BIPOLAR OPERATION The connections for bipolar ranges are shown in Figure 8. Again, as for the unipolar ranges, if the offset and gain specifica- tions are sufficient, one or both of the trimmers shown can be replaced by a 50 Ω ±1% fixed resistor. The analog input is applied as for the unipolar ranges. Bipolar calibration is similar to unipolar calibration. First, a signal 1/2 LSB above negative full scale (–4.9988 V for the ±5 V range) is applied and R1 is trimmed to give the first transition (0000 0000 0000 to 0000 0000 0001). Then a signal 1 1/2 LSB below positive full scale (+4.9963 V for the ±5 V range) is applied and R2 trimmed to give the last transition (1111 1111 1110 to 1111 1111 1111). AD674B/AD774B HIGH BITS 24–27 MIDDLE BITS 20–23 LOW BITS 16–19 100 R2 GAIN ANALOG INPUTS 10V R1 100 OFFSET 5V 2 3 4 5 6 8 12 13 10 14 9 STS 28 +15V 7 –15V 11 DIG COM 15 +5V 1 12/ 8 CS A0 R/ C CE REF OUT BIP OFF 10VIN REF IN 20VIN ANA COM Figure 8. Bipolar Input Connections GROUNDING CONSIDERATIONS The analog common at Pin 9 is the ground reference point for the internal reference and is thus the “high quality” ground for the ADC; it should be connected directly to the analog reference point of the system. To achieve the high-accuracy performance available from the ADC in an environment of high digital noise content, the analog and digital commons must be connected together at the package. In some situations, the digital common at Pin 15 can be connected to the most convenient ground ref- erence point; digital power return is preferred. |
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