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AD574AT Datasheet(PDF) 7 Page - Analog Devices |
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AD574AT Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page AD574A REV. B –7– A 8 10 13 12 3 15 4 9 A2 AD574A 27 16 12-BIT 3-STATE DATA + C3 C1 C2 + + +15V +5V –15V +VS TO A1 –VS TO A1 14 13 12 11 10 9 8 12 345 6 7 A R4 100k +15V –15V R1 100k OFFSET R3 100 Ω 7404 OR EQ. CONVERT STATUS GAIN R2 100 Ω VREF AGND +VS –VS ANALOG INPUT 0V TO +10V 10k 10k 100pF A1 AD585 17 6 2 11 NOTE 1. C1, C2, C3 ARE 47 F TANTALUM, BYPASSED BY 0.1 F CERAMIC. LOCATE AT ASSOCIATED A2 PINS. µ µ 28 5 Figure 3. AD574A with AD585 Sample and Hold SUPPLY DECOUPLING AND LAYOUT CONSIDERATIONS It is critically important that the AD574A power supplies be fil- tered, well regulated, and free from high frequency noise. Use of noisy supplies will cause unstable output codes. Switching power supplies are not recommended for circuits attempting to achieve 12-bit accuracy unless great care is used in filtering any switching spikes present in the output. Remember that a few millivolts of noise represents several counts of error in a 12-bit ADC. Decoupling capacitors should be used on all power supply pins; the +5 V supply decoupling capacitor should be connected directly from Pin 1 to Pin 15 (digital common) and the +VCC and –VEE pins should be decoupled directly to analog common (Pin 9). A suitable decoupling capacitor is a 4.7 µF tantalum type in parallel with a 0.1 µF disc ceramic type. Circuit layout should attempt to locate the AD574A, 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 con- struction is preferred. 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 AD574A; it should be connected directly to the analog refer- ence point of the system. In order to achieve all of the high accuracy performance available from the AD574A in an envi- ronment of high digital noise content, the analog and digital commons should be connected together at the package. In some situations, the digital common at Pin 15 can be connected to the most convenient ground reference point; analog power return is preferred. UNIPOLAR RANGE CONNECTIONS FOR THE AD574A The AD574A contains all the active components required to perform a complete 12-bit A/D conversion. Thus, for most situ- ations, all that is necessary is connection of the power supplies (+5 V, +12 V/+15 V and –12 V/–15 V), the analog input, and the conversion initiation command, as discussed on the next page. Analog input connections and calibration are easily ac- complished; the unipolar operating mode is shown in Figure 4. 9 14 13 12 8 10 6 5 4 3 2 28 15 11 7 1 27 24 19 16 23 20 AD574A STS HIGH BIT MIDDLE BITS LOW BITS +5V +15V –15V DIG COM 12/8 CS AO R/C CE REF IN REF OUT BIP OFF 10VIN 20VIN ANA COM OFFSET R1 100k –12V/–15V +12V/+15V GAIN 100k R2 100 Ω 100 Ω ANALOG INPUTS 0 TO +10V 0 TO +20V Figure 4. Unipolar Input Connections All of the thin-film application resistors of the AD574A are trimmed for absolute calibration. Therefore, in many applica- tions, 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, the AD574AK guarantees ±1 LSB max zero offset error and ±0.25% (10 LSB) max full-scale error. (Typical full-scale error is ±2 LSB.) 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 needed, a 50 Ω ± 1% metal film resistor should be connected between Pin 8 and Pin 10. The analog input is connected between Pin 13 and Pin 9 for a 0 V to +10 V input range, between 14 and Pin 9 for a 0 V to +20 V input range. The AD574A easily accommodates an input signal beyond the supplies. For the 10 volt span input, the LSB has a nominal value of 2.44 mV; for the 20 volt 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 trim- mer 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 10 kΩ into Pin 14. UNIPOLAR CALIBRATION The AD574A is intended to have 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, 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 man- ner, 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 ap- proximately ±15 mV of offset trim range. |
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