英文原文
12-Bit A/D Converter
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CIRCUIT OPERATION
The 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. Block Diagram of AD574A 12-Bit A-to-D Converter
When the control ction is commanded to initiate a conversion (as described later), it enables the clock and rets the successiveapproximation register (SAR) to all zeros. Once a conversion cycle has begun, it cannot be stopped or restarted and data is not available from the output buffers. The SAR, timed by the clock, will quence through the conversion cycle and return an end-of-convert flag to the control ction. The control ction will then disable the clock, bring the output status flag low, and enable control functions to allow data read functions by external command.
王一虹 During the conversion cycle, the internal 12-bit current output DAC is quenced 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 5kΩ(or10kΩ) input resistor. The comparator determines whether the addition of each successively-weighted bit current caus 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 testi
ng all the bits, the SAR contains a 12-bit binary code which accurately reprents 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 up to 1.5 mA to an external load in addition to the requirements of the reference input resistor (0.5 mA) and bipolar offt resistor (1 mA) when the AD574A is powered from 15 V supplies. If the AD574A is ud with 12 V supplies, or if external current must be supplied over the full temperature range, 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 20 volt span. The 10 k bipolar offt resistor is grounded for unipolar operation and connected to the 10 volt reference for bipolar operation.王长喜英语
DRIVING THE AD574 ANALOG INPUT
Figure 2. Op Amp – AD574A Interface
全球免签护照The output impedance of an op amp has an open-loop value which, in a clod loop, is divided by the loop gain available at the frequency of interest. The amplifier should have acceptable loop gain at 500 kHz for u with the AD574A. To check whether the output properties of a signal source are suitable, monitor the AD574’s input with an oscilloscope while a conversion is in progress. Each of the 12 disturbances should subside in sorless.
For applications involving the u of a sample-and-hold amplifier, the AD585 is recommended. The AD711 or AD544 op amps are recommended for dc applications.
skid row waldeinsamkeitSAMPLE-AND-HOLD AMPLIFIERS
Although the conversion time of the AD574A is a maximum of 35 s, to achieve accurate 12-bit conversions of frequencies greater than a few Hz requires the u 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.
The AD585 is a high linearity SHA capable of directly driving the analog input of the AD574A. The AD585’s 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 signal 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 increas to 26 kHz.
The AD585’s low output impedance, fast-loop respon, and low droop maintain 12-bits of accuracy under the changing load conditions that occur during a conversion, making it
suitable for u in high accuracy conversion systems. Many other SHAs cannot achieve 12-bits of accuracy and can thus compromi a system. The AD585 is recommended for AD574A applications requiring a sample and hold.
Figure 3. AD574A with AD585 Sample and Hold
SUPPLY DECOUPLING AND LAYOUT
CONSIDERATIONS
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It is critically important that the AD574A power supplies be filtered, well regulated, and free from high frequency noi. U of noisy supplies will cau unstable output codes. Switching power supplies are not recommended for circuits attempting to achieve 12-bit accuracy unless great care is ud in filtering any switching spikes prent in the output. Remember that a few millivolts of noi reprents veral counts of error in a 12-bit ADC.影响英文
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 u of wire-wrap circuit construction is not recommended. Careful printed circuit construction is preferred.
