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AD7476SRTZ-REEL7 Datasheet(PDF) 11 Page - Analog Devices |
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AD7476SRTZ-REEL7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page REV. D AD7476/AD7477/AD7478 –11– Alternatively, because the supply current required by the AD7476/AD7477/AD7478 is so low, a precision reference can be used as the supply source to the AD7476/AD7477/AD7478. A REF19x voltage reference (REF195 for 5 V, or REF193 for 3 V) can be used to supply the required voltage to the ADC (see Figure 6). This configuration is especially useful if the power supply is quite noisy or if the system supply voltages are at some value other than 5 V or 3 V (e.g., 15 V). The REF19x will output a steady voltage to the AD7476/AD7477/AD7478. If the low dropout REF193 is used, the current it typically needs to supply to the AD7476/AD7477/AD7478 is 1 mA. When the ADC is converting at a rate of 1 MSPS, the REF193 will need to supply a maximum of 1.6 mA to the AD7476/AD7477/AD7478. The load regulation of the REF193 is typically 10 ppm/mA (REF193, VS = 5 V), which results in an error of 16 ppm (48 µV) for the 1.6 mA drawn from it. This corresponds to a 0.065 LSB error for the AD7476 with VDD = 3 V from the REF193, a 0.016 LSB error for the AD7477, and a 0.004 LSB error for the AD7478. For applica- tions where power consumption is of concern, the Power-Down mode of the ADC and the Sleep mode of the REF19x reference should be used to improve power performance. See the Modes of Operation section. AD7476/ AD7477/ AD7478 VDD VIN GND 0V TO VDD INPUT 1 F TANT 0.1 F REF193 10 F 0.1 F 5V SUPPLY SCLK SDATA CS SERIAL INTERFACE C/ P 3V 1mA 680nF Figure 6. REF193 as Power Supply to AD7476/AD7477/ AD7478 Table I provides some typical performance data with various references used as a VDD source with a low frequency analog input. Under the same setup conditions, the references were compared and the AD780 proved the optimum reference. Table I. Reference Tied AD7476 SNR Performance to VDD 1 kHz Input (dB) AD780 @ 3 V 71.17 REF193 70.4 AD780 @ 2.5 V 71.35 REF192 70.93 AD1582 70.05 Analog Input Figure 7 shows an equivalent circuit of the analog input structure of the AD7476/AD7477/AD7478. 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 300 mV. This will cause these diodes to become forward-biased and start conducting current into the substrate. These diodes can conduct a maximum of 10 mA without causing irreversible damage to the part. The capacitor C1 in Figure 7 is typically about 4 pF and can primarily be attributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of a switch. This resistor is typically about 100 Ω. The capacitor C2 is the ADC sampling capacitor and typically has a capacitance of 30 pF. For ac applications, removing high frequency components from the analog input signal is recommended by use of a band-pass filter on the relevant analog input pin. In applications where harmonic distortion and signal-to-noise ratio are critical, the analog input should be driven from a low impedance source. Large source impedances will significantly affect the ac performance of the ADC. This may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the particular application. VDD VIN C1 4pF D2 D1 R1 C2 30pF CONVERSION PHASE - SWITCH OPEN TRACK PHASE - SWITCH CLOSED Figure 7. Equivalent Analog Input Circuit When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance will depend on the amount of total harmonic distortion (THD) that can be tolerated. The THD will increase as the source impedance increases and performance will degrade. Figure 8 shows a graph of the total harmonic distortion versus source impedance for different analog input frequencies when using a supply voltage of 2.7 V and sampling at a rate of 605 kSPS. Figures 9 and 10 each show a graph of the total harmonic distortion versus analog input signal frequency for various supply voltages while sampling at 993 kSPS with an SCLK frequency of 20 MHz and 605 kSPS with an SCLK frequency of 12 MHz, respectively. SOURCE IMPEDANCE – 0 10 –10 –20 –30 –40 –50 –60 –70 100 1k –80 –90 –100 VDD = 2.7V fS = 605kSPS fIN = 200kHz fIN = 300kHz fIN = 10kHz fIN = 100kHz 1 10k Figure 8. THD vs. Source Impedance for Various Analog Input Frequencies |
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