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AD10242 데이터시트(PDF) 12 Page - Analog Devices |
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AD10242 데이터시트(HTML) 12 Page - Analog Devices |
12 / 16 page AD10242 –12– REV. A USING THE FLEXIBLE INPUT The AD10242 has been designed with the user’s ease of opera- tion in mind. Multiple input configurations have been included on board to allow the user to have a choice of input signal levels and input impedance. While the standard inputs are ±0.5 V, ±1.0 V, and ±2.0 V, the user can select the input impedance of the AD10242 on any input by using the other inputs as alternate locations for GND or an external resistor. The following chart summarizes the impedance options available at each input location: AIN1 = 100 Ω when A IN2 and AIN3 Are Open. AIN1 = 75 Ω when AIN3 Is Shorted to GND. AIN1 = 50 Ω when A IN2 Is Shorted to GND. AIN2 = 200 Ω when A IN3 Is Open. AIN2 = 100 Ω when AIN3 Is Shorted to GND. AIN2 = 75 Ω when A IN2 to AIN3 Has an External Resistor of AIN2 = 300 Ω, with A IN 3 Shorted to GND. AIN2 = 50 Ω when AIN2 to AIN3 Has an External Resistor of AIN2 = 100 Ω, with A IN3 Shorted to GND. AIN3 = 400 Ω. AIN3 = 100 Ω when AIN3 Has an External Resistor of 133 Ω to GND. AIN3 = 75 Ω when A IN3 Has an External Resistor of 92 Ω to GND. AIN3 = 50 Ω when A IN3 Has an External Resistor of 57 Ω to GND. While the Analog inputs of the AD10242 are designed for dc coupled bipolar inputs, the AD10242 has the ability to use uni- polar inputs in a user selectable mode through the addition of a external resistor. This allows for 1 V, 2 V, and 4 V full-scale unipolar signals to be applied to the various inputs (AIN1, AIN2, and AIN3 respectively). Placing a 2.43 k Ω resistor (typical, off- set calibration required) between UPOS and UCOM shifts the reference voltage setpoint to allow a unipolar positive voltage to be applied at the inputs of the device. To calibrate offset a midscale dc voltage should be applied to the converter while adjusting the unipolar resistor for a midscale output transition. AIN2 UPOS AD10242 2.43k Ω UCOM AIN3 AIN1 Figure 27. Unipolar Positive To operate with –1 V, –2 V, or –4 V full-scale unipolar signals place a 2.67 k Ω resistor (typical, offset calibration required) between UNEG and UCOM. This again shifts the reference voltage setpoint to allow a unipolar negative voltage to be applied at the inputs of the device. To calibrate offset a mid- scale dc voltage should be applied to the converter while adjust- ing the unipolar resistor for a midscale output transition. AIN2 UNEG AD10242 2.67k Ω UCOM AIN3 AIN1 Figure 28. Unipolar Negative GROUNDING AND DECOUPLING Analog and Digital Grounding Proper grounding is essential in any high speed, high resolution system. Multilayer printed circuit boards (PCBs) are recom- mended to provide optimal grounding and power schemes. The use of ground and power planes offers distinct advantages: 1. The minimization of the loop area encompassed by a signal and its return path. 2. The minimization of the impedance associated with ground and power paths. 3. The inherent distributed capacitor formed by the power plane, PCB insulation, and ground plane. These characteristics result in both a reduction of electro- magnetic interference (EMI) and an overall improvement in performance. It is important to design a layout that prevents noise from cou- pling to the input signal. Digital signals should not be run in parallel with input signal traces and should be routed away from the input circuitry. The AD10242 does not distinguish between analog and digital ground pins as the AD10242 should always be treated like an analog component. All ground pins should be connected together directly under the AD10242. The PCB should have a ground plane covering all unused portions of the component side of the board to provide a low impedance path and manage the power and ground currents. The ground plane should be removed from the area near the input pins to reduce stray capacitance. LAYOUT INFORMATION The schematic of the evaluation board (Figure 29) represents a typical implementation of the AD10242. The pinout of the AD10242 is very straightforward and facilitates ease of use and the implementation of high frequency/high resolution design practices. It is recommended that high quality ceramic chip capacitors be used to decouple each supply pin to ground directly at the device. All capacitors except the one placed on ENCODE can be standard high quality ceramic chip capacitors. The capacitor used on ENCODE pin must be a low inductance chip capacitor as referenced previously in the data sheet. |
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