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AD7872 Datasheet(PDF) 6 Page - Analog Devices |
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AD7872 Datasheet(HTML) 6 Page - Analog Devices |
6 / 16 page AD7871/AD7872 –6– REV. D CONVERTER DETAILS The AD7871/AD7872 is a complete 14-bit A/D converter, re- quiring no external components apart from power supply decoupling capacitors. It is comprised of a 14-bit successive ap- proximation ADC based on a fast settling voltage-output DAC, a high speed comparator and CMOS SAR, a track/hold ampli- fier, a 3 V buried Zener reference, a clock oscillator and control logic. INTERNAL REFERENCE The AD7871/AD7872 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. Reference noise is minimized by connecting a capacitor between CREF and AGND. For specified operation this capacitor should be 10 nF. 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 AD7871/AD7872, 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 AD7871/AD7872’s internal operation. Figure 3. Reference Circuit TRACK-AND-HOLD AMPLIFIER The track-and-hold amplifier on the analog input of the AD7871/AD7872 allows the ADC to accurately convert an in- put sine wave of 6 V peak-peak amplitude to 14-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 oper- ated at its maximum throughput rate. The 0.1 dB cutoff fre- quency occurs typically at 500 kHz. The track/hold amplifier acquires an input signal to 14-bit accuracy in less than 2 µs. The overall throughput rate is determined by the conversion time plus the track/hold amplifier acquisition time. For a 2 MHz input clock the throughput time is 12 µs maximum. Figure 4. Analog Input The operation of the track/hold amplifier is essentially transpar- ent 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 on the AD7871 starts conversion, this transition occurs on the fall- ing edge of CS. ANALOG INPUT Figure 4 shows the AD7871/AD7872 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/16384 = 6 V/16384 = 366 µV. The ideal input/out- put transfer function is shown in Figure 5. Figure 5. Bipolar Input/Output Transfer Function BIPOLAR OFFSET AND FULL-SCALE ADJUSTMENT When the AD7871/AD7872’s offset and full-scale errors need to be adjusted, offset error must be adjusted first. This is achieved by trimming the offset of the op amp driving the analog input of the AD7871/AD7872 while the input voltage is 1/2 LSB below AGND. The trim procedure is as follows: apply a voltage of –0.183 mV (–1/2 LSB) at V1 in Figure 6 and adjust the op amp offset voltage until the ADC output code flickers between 11 1111 1111 1111 and 00 0000 0000 0000. Figure 6. Bipolar Adjust Circuit |
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