【正文】 ng it suitable for use in high accuracy conversion systems. Many other SHAs cannot achieve 12bits of accuracy and can thus promise a system. The AD585 is remended for AD574A applications requiring a sample and hold.Figure 3. AD574A with AD585 Sample and HoldSUPPLY DECOUPLING AND LAYOUTCONSIDERATIONSIt is critically important that the AD574A power supplies be filtered, well regulated, and free from high frequency noise. Use of noisy supplies will cause unstable output codes. Switching power supplies are not remended for circuits attempting to achieve 12bit 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 12bit ADC.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 wirewrap circuit construction is not remended. Careful printed circuit construction is preferred.UNIPOLAR RANGE CONNECTIONS FOR THE AD574AThe AD574A contains all the active ponents required to perform a plete 12bit A/D conversion. Thus, for most situations, 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 mand, as discussed on the next page. Analog input connections and calibration are easily acplished。 the unipolar operating mode is shown in Figure 4.Figure 4. Unipolar Input ConnectionsAll of the thinfilm application resistors of the AD574A are 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, the AD574AK guarantees 177。1 LSB max zero offset error and 177。% (10 LSB) max fullscale error. (Typical fullscale error is 177。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 fullscale trim is not needed, a 50 W 177。 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 acmodates an input signal beyond the supplies. For the 10 volt span input, the LSB has a nominal value of mV。 for the 20 volt span, mV.If a V range is desired (nominal mV/bit), the gain trimmer (R2) should be replaced by a 50Ωesistor, and a 200Ω trimmer inserted in series with the analog input to Pin 13 for a fullscale range of V (5 mV/bit), use a 500 W trimmer into Pin 14. The gain trim described below is now done with these trimmers. The nominal input impedance into Pin 13 is 5kΩ, and 10kΩ into Pin 14.UNIPOLAR CALIBRATIONThe 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 ( 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 177。15 mV of offset trim range.The fullscale trim is done by applying a signal 1/2 LSB below the nominal full scale ( for a 10 V range). Trim R2 to give the last transition (1111 1111 1110 to 1111 1111 1111).BIPOLAR OPERATIONThe connections for bipolar ranges are shown in Figure 5. Again, as for the unipolar ranges, if the offset and gain specifications are sufficient, one or both of the trimmers shown can be replaced by a 50 W 177。 1% fixed resistor. Bipolar calibration is similar to unipolar calibration. Figure 5. Bipolar Input ConnectionsCONTROL LOGICThe AD574A contains onchip logic to provide conversion initiation and data read operations from signals monly available in microprocessor systems. Figure 6 shows the inter