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AD7870LP-REEL Datasheet(PDF) 11 Page - Analog Devices |
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AD7870LP-REEL Datasheet(HTML) 11 Page - Analog Devices |
11 / 28 page AD7870/AD7875/AD7876 Rev. C | Page 11 of 28 CONVERTER DETAILS The AD7870/AD7875/AD7876 is a complete 12-bit ADC, requiring no external components apart from power supply decoupling capacitors. It is comprised of a 12-bit successive approximation ADC based on a fast settling voltage output DAC, a high speed comparator and SAR, a track-and-hold amplifier, a 3 V buried Zener reference, a clock oscillator, and control logic. INTERNAL REFERENCE The AD7870/AD7875/AD7876 have 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 tantalum capacitor and a 0.1 μF ceramic capacitor. These decoupling components are required to remove voltage spikes caused by the ADC’s internal operation. TEMPERATURE COMPENSATION AD7870/AD7875/AD7876 REF OUT VSS VDD Figure 6. 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 7 shows how to scale the 3 V REF OUT voltage to provide either a 5 V or 10 V external reference. INTERNAL 3V REFERENCE AD7870/AD7875/AD7876 REF OUT 15kΩ (3.9kΩ) 10kΩ (9.1kΩ) VOUT = 5V (10V) Figure 7. 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-and-hold amplifier is much greater than the Nyquist rate of the ADC even when the ADC is operated at its maximum throughput rate. The 0.1 dB cutoff frequency occurs typically at 500 kHz. The track-and-hold amplifier acquires an input signal to 12-bit accuracy in less than 2 μs. The overall throughput rate is equal to the conversion time plus the track-and-hold amplifier acquisition time. For a 2.5 MHz input clock the throughput rate is 10 μs max. The operation of the track-and-hold is essentially transparent to the user. The track-and-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 voltage 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 8 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 (that is, 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 11. AD7870 TRACK-AND-HOLD AMPLIFIER TO INTERNAL 3V REFERENCE R R TO INTERNAL COMPARATOR Figure 8. AD7970 Analog Input The AD7876 analog input structure is shown in Figure 9. The analog input range is ±10 V into an input resistance of typically 33 kΩ. As before, the designed code transitions occur midway between successive integer LSB values. The output code is twos complement with 1 LSB = FS/4096 = 20 V/4096 = 4.88 mV. The ideal input/output transfer function is shown in Figure 11. |
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