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AD7453BRT-R2 数据表(PDF) 11 Page - Analog Devices |
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AD7453BRT-R2 数据表(HTML) 11 Page - Analog Devices |
11 / 20 page REV. 0 –11– AD7453 THE ANALOG INPUT The AD7453 has a pseudo differential analog input. The VIN+ input is coupled to the signal source and must have an ampli- tude of VREF p-p to make use of the full dynamic range of the part. A dc input is applied to the VIN–. The voltage applied to this input provides an offset from ground or a pseudo ground for the VIN+ input. The main benefit of pseudo differential inputs is that they separate the analog input signal ground from the ADC’s ground, allowing dc common-mode voltages to be cancelled. Because the ADC operates from a single supply, it is necessary to level shift ground based bipolar signals to comply with the input requirements. An op amp (for example, the AD8021) can be configured to rescale and level shift a ground based (bipolar) signal so that it is compatible with the input range of the AD7453. See Figure 7. When a conversion takes place, the pseudo ground corresponds to 0 and the maximum analog input corresponds to 4096. EXTERNAL VREF (2.5V) R VIN+ VIN– AD7453 5V 2.5V 0V VREF +2.5V 0V –2.5V VIN R R 0.1 F R Figure 7. Op Amp Configuration to Level Shift a Bipolar Input Signal Analog Input Structure Figure 8 shows the equivalent circuit of the analog input structure of the AD7453. The four diodes provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signals never exceed the supply rails by more than 300 mV. This will cause these diodes to become forward biased and start conducting into the substrate. These diodes can conduct up to 10 mA without causing irreversible damage to the part. The capacitors, C1 in Figure 8, are typically 4 pF and can be attributed primarily to pin capacitance. The resistors are lumped components made up of the on resistance of the switches. The value of these resistors is typically about 100 W. The capacitors, C2, are the ADC’s sampling capacitors and have a capacitance of 16 pF typically. For ac applications, removing high frequency components from the analog input signal through the use of an RC low-pass filter on the relevant analog input pins is recommended. In applica- tions where harmonic distortion and the signal-to-noise ratio are critical, the analog input should be driven from a low imped- ance source. Large source impedances will significantly affect the ac performance of the ADC, which may necessitate the use of an input buffer amplifier. The choice of the op amp will be a function of the particular application. VIN+ C1 C2 R1 D D VDD VIN– C1 C2 R1 D D VDD Figure 8. Equivalent Analog Input Circuit. Conversion Phase—Switches Open; Track Phase—Switches Closed When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance depends on the amount of total harmonic distortion (THD) that can be tolerated. The THD increases as the source impedance increases and performance degrades. Figure 9 shows a graph of the THD versus analog input signal frequency for different source impedances. INPUT FREQUENCY (kHz) –100 10 100 277 –80 –50 –40 0 200 –10 –20 –30 –60 –70 –90 10 62 100 Figure 9. THD vs. Analog Input Frequency for Various Source Impedances |
类似零件编号 - AD7453BRT-R2 |
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类似说明 - AD7453BRT-R2 |
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