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AD7827BRZ 데이터시트(PDF) 7 Page - Analog Devices |
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AD7827BRZ 데이터시트(HTML) 7 Page - Analog Devices |
7 / 12 page AD7827 –7– REV. 0 ADC TRANSFER FUNCTION The output coding of the AD7827 is straight binary. The designed code transitions occur at successive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size is = VREF/256 (VDD = 5 V) or the LSB size = (0.8 VREF)/256 (VDD = 3 V). The ideal transfer characteristic for the AD7827 is shown in Figure 6 below. (VDD = 5V) 1LSB = VREF/256 (VDD = 3V) 1LSB = 0.8VREF/256 1LSB (VDD = 5V) VREF/2 (VDD = 3V) 0.8VREF/2 VREF/2+1.25V – 1LSB 0.8VREF/2+1V – 1LSB (VDD = 5V) VREF/2 – 1.25V (VDD = 3V) 0.8VREF/2 – 1V 11111111 111....110 111....000 10000000 000....111 000....010 000....001 00000000 Figure 6. Transfer Characteristic ANALOG INPUT The AD7827 has a single input channel with an input range of 0 V to 2.5 V or 0 V to 2.0 V, depending on the supply voltage (VDD). This input range is automatically set up by an on-chip “VDD detector” circuit. 5 V operation of the ADC is detected when VDD exceeds 4.1 V and 3 V operation is detected when VDD falls below 3.8 V. This circuit also possesses a degree of glitch rejection; for example, a glitch from 5.5 V to 2.7 V up to 60 ns wide will not trip the VDD detector. Note: Although there is a VREF pin from which a voltage refer- ence of 2.5 V may be sourced, or to which an external reference may be applied, this does not provide an option of varying the value of the voltage reference. As stated in the specifications for the AD7827, the input voltage range at this pin is 2.5 V ± 2%. Analog Input Structure Figure 7 shows an equivalent circuit of the analog input struc- ture of the AD7827. The two diodes, D1 and D2, provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 200 mV. This will cause these diodes to become forward biased and start conducting current into the substrate. The maximum current these diodes can conduct without caus- ing irreversible damage to the part is 20 mA. The capacitor C2 in Figure 7 is typically about 4 pF and can mostly be attributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of several components including that of the multiplexer and the track-and-hold. This resistor is typically about 310 Ω. The capacitor C1 is the track-and-hold capacitor and has a capacitance of 0.5 pF. Switch 1 is the track- and-hold switch, while Switch 2 is that of the sampling capaci- tor as shown in Figures 2 and 3. When in track phase, Switch 1 is closed and Switch 2 is in Position A. When in hold mode, Switch 1 opens while Switch 2 remains in Position A. The track-and-hold remains in hold mode for 120 ns—see Circuit Description, after which it returns to track mode and the ADC enters its conversion phase. At this point Switch 1 opens and Switch 2 moves to Position B. At the end of the conversion Switch 2 moves back to Position A. VDD VIN D1 R1 310 D2 SW1 C1 0.5pF A B SW2 C2 4pF Figure 7. Equivalent Analog Input Circuit The on-chip track-and-hold can accommodate input frequen- cies to 10 MHz, making the AD7827 ideal for subsampling applications. When the AD7827 is converting a 10 MHz input signal at a sampling rate of 1 MSPS, the effective number of bits typically remains above seven corresponding to a signal-to- noise ratio of 42 dBs as shown in Figure 8. INPUT FREQUENCY – MHz 50 38 0.2 10 134 5 6 8 48 46 44 42 40 FSAMPLE = 1MHz Figure 8. SNR vs. Input Frequency On the AD7827 |
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