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AD10242 데이터시트(PDF) 10 Page - Analog Devices |
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AD10242 데이터시트(HTML) 10 Page - Analog Devices |
10 / 16 page ANALOG INPUT FREQUENCY – MHz 60 0 510 40 20 ENCODE = 40 MSPS AIN = 1dBFs SNR (dB) 30 50 10 20 29.2 34.5 52.5 60.95 70 80 SFDR (dBFS) Figure 18. SNR/Harmonics to AIN > Nyquist MSPS –10– REV. A AD10242–Typical Performance Characteristics INPUT FREQUENCY – MHz 0.5 3 05 2 1.5 1 2.5 10 15 20 25 30 40 0 –0.5 45 50 55 35 ENCODE = 40 MSPS Figure 19. Gain Flatness vs. Input Frequency THEORY OF OPERATION Refer to the block diagram. The AD10242 employs three monolithic ADI components per channel (AD9631, OP279, and AD9042), along with multiple passive resistor networks and de- coupling capacitors to fully integrate a complete 12-bit analog- to-digital converter. The input signal is first passed through a precision laser trimmed resistor divider allowing the user to externally select operation with a full scale signal of ±0.5 V, ±1.0 V, or ±2.0 V by choosing the proper input terminal for the application. The result of the resistor divider is to apply a full-scale input of approxi- mately 0.4 V to the noninverting input of the internal AD9631 amplifier. The AD9631 provides the dc coupled level shift circuit required for operation with the AD9042 ADC. Configuring the amplifier in a noninverting mode the ac signal gain can be trimmed to provide a constant input to the ADC centered around the inter- nal reference voltage of the AD9042. This allows the converter to be used in multiple system applications without the need for external gain and level shift circuitry normally requiring trim. The AD9631 was chosen for its superior ac performance and in- put drive capabilities which have limited the ability of many am- plifiers to drive high performance ADCs. As new amplifiers are developed, pin compatible improvements are planned to incor- porate the latest operational amplifier technology. The OP279 provides the buffer and inversion of the internal ref- erence of the AD9042 in order to supply the summing node of the AD9631 input amplifier. This dc voltage is then summed with the input voltage and applied to the input of the AD9042 ADC. The reference voltage of the AD9042 is designed to track internal offsets and drifts of the ADC and is used to ensure matching over an extended temperature range of operation. APPLYING THE AD10242 Encoding the AD10242 The AD10242 is designed to interface with TTL and CMOS logic families. The source used to drive the ENCODE pin(s) must be clean and free from jitter. Sources with excessive jitter will limit SNR and overall performance. 0.01µF TTL OR CMOS SOURCE ENCODE ENCODE AD10242 Figure 20. Single-Ended TTL/CMOS Encode The AD10242 encode inputs are connected to a differential in- put stage (see Figure 4 under EQUIVALENT CIRCUITS). With no input connected to either the ENCODE or input, the voltage divider bias the inputs to 1.6 volts. For TTL or CMOS usage, the encode source should be connected to ENCODE (Pins 29 and/or 51). ENCODE (Pins 28 and/or 52) should be decoupled using a low inductance or microwave chip capacitor to ground. Devices such as AVX 05085C103MA15, a 0.01 µF capacitor, work well. Performance Improvements It is possible to improve the performance of the AD10242 slightly by taking advantage of the internal characteristics of the amplifier and converter combination. By increasing the +5 V supply slightly, the user may be able to gain up to a 5 dB improve- ment in SFDR over the entire frequency range of the converter. It is not recommended to exceed +5.5 V on the analog supplies as there are no performance benefits beyond that range and care should be taken to avoid the absolute maximum ratings. |
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