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AD7934 Datasheet(PDF) 19 Page - Analog Devices |
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AD7934 Datasheet(HTML) 19 Page - Analog Devices |
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19 / 32 page  AD7933/AD7934 Rev. B | Page 19 of 32 RSOURCE (Ω) –40 –45 –50 –55 –80 –75 –70 –65 –60 –90 –85 10 100 1k FIN = 50kHz VDD =5V VDD =3V Figure 20. THD vs. Source Impedance in Single-Ended Mode RSOURCE (Ω) –60 –80 –75 –70 –65 –100 –85 –90 –95 10 100 1k FIN = 50kHz VDD =5V VDD =3V Figure 21. THD vs. Source Impedance in Fully Differential Mode Figure 22 shows a graph of the THD vs. the analog input fre- quency for various supplies, while sampling at 1.5 MHz with an SCLK of 25.5 MHz. In this case, the source impedance is 10 Ω. INPUT FREQUENCY (kHz) –50 –60 –70 –80 –110 –100 –90 –120 0 100 400 200 300 600 500 700 FSAMPLE = 1.5MSPS RANGE = 0 TO VREF VDD =3V SINGLE-ENDED MODE VDD =5V/3V DIFFERENTIAL MODE VDD =5V SINGLE-ENDED MODE Figure 22. THD vs. Analog Input Frequency for Various Supply Voltages ANALOG INPUTS The AD7933/AD7934 have software selectable analog input configurations. Users can choose from among the following configurations: four single-ended inputs, two fully differential pairs, or two pseudo differential pairs. The analog input configuration is chosen by setting the MODE0/MODE1 bits in the internal control register (see Table 10). Single-Ended Mode The AD7933/AD7934 can have four single-ended analog input channels by setting the MODE0 and MODE1 bits in the control register to 0. In applications where the signal source has a high impedance, it is recommended to buffer the analog input before applying it to the ADC. An amplifier suitable for this function is the AD8021. The analog input range of the AD7933/AD7934 can be programmed to be either 0 V to VREF, or 0 V to 2 × VREF. If the analog input signal to be sampled is bipolar, the internal reference of the ADC can be used to externally bias up this signal to make it the correct format for the ADC. Figure 23 shows a typical connection diagram when operating the ADC in single-ended mode. This diagram shows a bipolar signal of amplitude ±1.25 V being preconditioned before it is applied to the AD7933/AD7934. In cases where the analog input amplitude is ±2.5 V, the 3R resistor can be replaced with a resistor of value R. The resultant voltage on the analog input of the AD7933/AD7934 is a signal ranging from 0 V to 5 V. In this case, the 2 × VREF mode can be used. 0.47µF +1.25V VIN R R R 3R 0V –1.25V +2.5V 0V VIN0 VIN3 VREFOUT AD7933/ AD7934* *ADDITIONAL PINS OMITTED FOR CLARITY. Figure 23. Single-Ended Mode Connection Diagram Differential Mode The AD7933/AD7934 can have two differential analog input pairs by setting the MODE0 and MODE1 bits in the control register to 0 and 1, respectively. Differential signals have some benefits over single-ended signals, including noise immunity based on the device’s common-mode rejection and improvements in distortion performance. Figure 24 defines the fully differential analog input of the AD7933/AD7934. |
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