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AD7870LP 数据表(PDF) 7 Page - Analog Devices |
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AD7870LP 数据表(HTML) 7 Page - Analog Devices |
7 / 20 page REV. B 2 AD7870/AD7875/AD7876 –7– to the conversion time plus the track/hold amplifier acquisition time. For a 2.5 MHz input clock the throughput rate is 10 µs max. The operation of the track/hold is essentially transparent to the user. The track/hold amplifier goes from its tracking mode to its hold mode at the start of conversion. If the CONVST input is used to start conversion then the track to hold transition occurs on the rising edge of CONVST. If CS starts conversion, this transition occurs on the falling edge of CS. ANALOG INPUT The three parts differ from each other in the analog input volt- age range that they can handle. The AD7870 accepts ±3 V input signals, the AD7876 accepts a ±10 V input range, while the input range for the AD7875 is 0 V to +5 V. Figure 5a shows the AD7870 analog input. The analog input range is ±3 V into an input resistance of typically 15 kΩ. The designed code transitions occur midway between successive integer LSB values (i.e., 1/2 LSB, 3/2 LSBs, 5/2 LSBs . . . FS–3/2 LSBs). The output code is twos complement binary with 1 LSB = FS/4096 = 6 V/4096 = 1.46 mV. The ideal input/ output transfer function is shown in Figure 6. Figure 5a. AD7870 Analog Input The AD7876 analog input structure is shown in Figure 5b. The analog input range is ±10 V into an input resistance of typically 33 k Ω. As before, the designed code transitions occur midway between successive integer LSB values. The output code is 2s complement with 1 LSB = FS/4096 = 20 V/4096 = 4.88 mV. The ideal input/output transfer function is shown in Figure 6. Figure 5b. AD7876 Analog Input Figure 5c shows the analog input for the AD7875. The input range is 0 V to +5 V into an input resistance of typically 25 k Ω. Once again, the designed code transitions occur midway between successive integer LSB values. The output code is CONVERTER DETAILS The AD7870/AD7875/AD7876 is a complete 12-bit A/D con- verter, requiring no external components apart from power supply decoupling capacitors. It is comprised of a 12-bit suc- cessive approximation ADC based on a fast settling voltage output DAC, a high speed comparator and SAR, a track/hold amplifier, a 3 V buried Zener reference, a clock oscillator and control logic. INTERNAL REFERENCE The AD7870/AD7875/AD7876 has an on-chip temperature compensated buried Zener reference that is factory trimmed to 3 V ±10 mV. Internally it provides both the DAC reference and the dc bias required for bipolar operation (AD7870 and AD7876). The reference output is available (REF OUT) and capable of providing up to 500 µA to an external load. The maximum recommended capacitance on REF OUT for normal operation is 50 pF. If the reference is required for use external to the ADC, it should be decoupled with a 200 Ω resistor in series with a parallel combination of a 10 µF tanta- lum capacitor and a 0.1 µF ceramic capacitor. These decoupling components are required to remove voltage spikes caused by the ADC’s internal operation. Figure 3. Reference Circuit The reference output voltage is 3 V. For applications using the AD7875 or AD7876, a 5 V or 10 V reference may be required. Figure 4 shows how to scale the 3 V REF OUT voltage to pro- vide either a 5 V or 10 V external reference. Figure 4. Generating a 5 V or 10 V Reference TRACK-AND-HOLD AMPLIFIER The track-and-hold amplifier on the analog input of the AD7870/ AD7875/AD7876 allows the ADC to accurately convert input frequencies to 12-bit accuracy. The input bandwidth of the track/hold amplifier is much greater than the Nyquist rate of the ADC even when the ADC is operated at its maximum through- put rate. The 0.1 dB cutoff frequency occurs typically at 500 kHz. The track/hold amplifier acquires an input signal to 12-bit accuracy in less than 2 µs. The overall throughput rate is equal |
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类似说明 - AD7870LP |
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