养殖技术资料12位ad574a转换器中英文翻译资料.docx

1、养殖技术资料12位ad574a转换器中英文翻译资料英文原文12-Bit A/D ConverterCIRCUIT OPERATIONThe AD574A is a complete 12-bit A/D converter which requires no external components to provide the complete successive approximation analog-to-digital conversion function. A block diagram of the AD574A is shown in Figure 1.Figure 1. B

2、lock Diagram of AD574A 12-Bit A-to-D ConverterWhen the control section is commanded to initiate a conversion (as described later), it enables the clock and resets the successiveapproximation register (SAR) to all zeros. Once a conversion cycle has begun, it cannot be stopped or restarted and data is

3、 not available from the output buffers. The SAR, timed by the clock, will sequence through the conversion cycle and return an end-of-convert flag to the control section. The control section will then disable the clock, bring the output status flag low, and enable control functions to allow data read

4、 functions by external command. During the conversion cycle, the internal 12-bit current output DAC is sequenced by the SAR from the most significant bit (MSB) to least significant bit (LSB) to provide an output current which accurately balances the input signal current through the 5k(or10k) input r

5、esistor. The comparator determines whether the addition of each successively-weighted bit current causes the DAC current sum to be greater or less than the input current; if the sum is less, the bit is left on; if more, the bit is turned off. After testing all the bits, the SAR contains a 12-bit bin

6、ary code which accurately represents the input signal to within 1/2 LSB. The temperature-compensated buried Zener reference provides the primary voltage reference to the DAC and guarantees excellent stability with both time and temperature. The reference is trimmed to 10.00 volts 0.2%; it can supply

7、 up to 1.5 mA to an external load in addition to the requirements of the reference input resistor (0.5 mA) and bipolar offset resistor (1 mA) when the AD574A is powered from 15 V supplies. If the AD574A is used with 12 V supplies, or if external current must be supplied over the full temperature ran

8、ge, an external buffer amplifier is recommended. Any external load on the AD574A reference must remain constant during conversion. The thin-film application resistors are trimmed to match the full-scale output current of the DAC. There are two 5 k input scaling resistors to allow either a 10 volt or

9、 20 volt span. The 10 k bipolar offset resistor is grounded for unipolar operation and connected to the 10 volt reference for bipolar operation.DRIVING THE AD574 ANALOG INPUTFigure 2. Op Amp AD574A InterfaceThe output impedance of an op amp has an open-loop value which, in a closed loop, is divided

10、by the loop gain available at the frequency of interest. The amplifier should have acceptable loop gain at 500 kHz for use with the AD574A. To check whether the output properties of a signal source are suitable, monitor the AD574s input with an oscilloscope while a conversion is in progress. Each of

11、 the 12 disturbances should subside in sorless. For applications involving the use of a sample-and-hold amplifier, the AD585 is recommended. The AD711 or AD544 op amps are recommended for dc applications. SAMPLE-AND-HOLD AMPLIFIERSAlthough the conversion time of the AD574A is a maximum of 35 s, to a

12、chieve accurate 12-bit conversions of frequencies greater than a few Hz requires the use of a sample-and-hold amplifier (SHA). If the voltage of the analog input signal driving the AD574A changes by more than 1/2 LSB over the time interval needed to make a conversion, then the input requires a SHA.

13、The AD585 is a high linearity SHA capable of directly driving the analog input of the AD574A. The AD585s fast acquisition time, low aperture and low aperture jitter are ideally suited for high-speed data acquisition systems. Consider the AD574A converter with a 35 s conversion time and an input sign

14、al of 10 V p-p: the maximum frequency which may be applied to achieve rated accuracy is 1.5 Hz. However, with the addition of an AD585, as shown in Figure 3, the maximum frequency increases to 26 kHz.The AD585s low output impedance, fast-loop response, and low droop maintain 12-bits of accuracy unde

15、r the changing load conditions that occur during a conversion, making it suitable for use in high accuracy conversion systems. Many other SHAs cannot achieve 12-bits of accuracy and can thus compromise a system. The AD585 is recommended for AD574A applications requiring a sample and hold.Figure 3. A

16、D574A 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 recommended fo

17、r 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.Circuit layout should attempt to locate the AD574A, associated analog input c

18、ircuitry, 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 construction is preferred.UNIPOLAR RANGE CONNECTIONS FOR THE AD574AThe AD574A contains all the active components required to

19、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 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 easil

20、y accomplished; the unipolar operating mode is shown in Figure 4.Figure 4. Unipolar Input ConnectionsAll of the thin-film application resistors of the AD574A are trimmed for absolute calibration. Therefore, in many applications, no calibration trimming will be required. The absolute accuracy for eac

21、h 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 res

22、istors and trimmer for Pin 12 are then not needed. If the full-scale trim is not needed, a 50 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 rang

23、e. 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 50esistor, and a 200 trimmer inse

24、rted in series with the analog input to Pin 13 for a full-scale range of 20.48 V (5 mV/bit), use a 500 trimmer into Pin 14. The gain trim described below is now done with these trimmers. The nominal input impedance into Pin 13 is 5k, and 10k into Pin 14.UNIPOLAR CALIBRATIONThe AD574A is intended to

25、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 fo

26、r 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

27、trim range.The full-scale trim is done by applying a signal 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 OPERATIONThe connections for bipolar ranges are shown in Figure 5. Again, as for the unipolar ran

28、ges, if the offset and gain specifications are sufficient, one or both of the trimmers shown can be replaced by a 50 1% fixed resistor. Bipolar calibration is similar to unipolar calibration. Figure 5. Bipolar Input ConnectionsCONTROL LOGICThe AD574A contains on-chip logic to provide conversion init

29、iation and data read operations from signals commonly available in microprocessor systems. Figure 6 shows the internal logic circuitry of the AD574A.The control signals CE, CS, and R/C control the operation of the converter. The state of R/C when CE and CS are both asserted determines whether a data

30、 read (R/C = 1) or a convert (R/C = 0) is in progress. The register control inputs AO and 12/8 control conversion length and data format. The AO line is usually tied to the least significant bit of the address bus. If a conversion is started with AO low, a full 12-bit conversion cycleis initiated. I

31、f AO is high during a convert start, a shorter 8-bit conversion cycle results. During data read operations, AO determines whether the three-state buffers containing the 8 MSBs of the conversion result (AO = 0) or the 4 LSBs (AO = 1) are enabled. The 12/8 pin determines whether the output data is to

32、be organized as two 8-bit words (12/8 tied to DIGITAL COMMON) or a single 12-bit word (12/8 tied to VLOGIC). The 12/8 pin is not TTL-compatible and must be hard-wired to either VLOGIC or DIGITAL COMMON. In the 8-bit mode, the byte addressed when AO is high contains the 4 LSBs from the conversion followed by four trailing zeroes. This organization allows the data lines to be overlapped for direct interface to 8-bit buses without the need for external three-state buffers. It is not recommended that AO change state during a data read operation. Asymmetrical enable and disable ti