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AD13280AF Datasheet(PDF) 11 Page - Analog Devices |
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AD13280AF Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page REV. 0 AD13280 –11– Power Supplies Care should be taken when selecting a power source. Linear sup- plies are strongly recommended. Switching supplies tend to have radiated components that may be “received” by the AD13280. Each of the power supply pins should be decoupled as closely as possible to the package, using 0.1 µF chip capacitors. The AD13280 has separate digital and analog power supply pins. The analog supplies are denoted AVCC and the digital supply pins are denoted DVCC. AVCC and DVCC should be separate power supplies because the fast digital output swings can couple switching current back into the analog supplies. Note that AVCC must be held within 5% of 5 V. The AD13280 is specified for DVCC = 3.3 V as this is a common supply for digital ASICs. Output Loading Care must be taken when designing the data receivers for the AD13280. The digital outputs drive an internal series resistor (e.g., 100 Ω) followed by a gate like 75LCX574. To minimize capacitive loading, there should be only one gate on each output pin. An example of this is shown in the evaluation board sche- matic shown in Figure 9. The digital outputs of the AD13280 have a constant output slew rate of 1 V/ns. A typical CMOS gate combined with a PCB trace will have a load of approxi- mately 10 pF. Therefore, as each bit switches, 10 mA (10 pF × 1 V ÷ 1 ns) of dynamic current per bit will flow in or out of the device. A full-scale transition can cause up to 120 mA (12 bits × 10 mA/bit) of transient current through the output stages. These switching currents are confined between ground and the DVCC pin. Standard TTL gates should be avoided since they can appreciably add to the dynamic switching currents of the AD13280. It should also be noted that extra capacitive loading will increase output timing and invalidate timing specifications. Digital output timing is guaranteed with 10 pF loads. LAYOUT INFORMATION The schematic of the evaluation board (Figure 10) represents a typical implementation of the AD13280. The pinout of the AD13280 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 can be standard high-quality ceramic chip capacitors. Care should be taken when placing the digital output runs. Because the digital outputs have such a high slew rate, the capacitive loading on the digital outputs should be minimized. Circuit traces for the digital outputs should be kept short and connect directly to the receiving gate. Internal circuitry buffers the outputs of the ADC through a resistor network to eliminate the need to externally isolate the device from the receiving gate. EVALUATION BOARD The AD13280 evaluation board (Figure 9) is designed to provide optimal performance for evaluation of the AD13280 analog-to-digital converter. The board encompasses everything needed to ensure the highest level of performance for evaluating the AD13280. The board requires an analog input signal, encode clock, and power supply inputs. The clock is buffered on-board to provide clocks for the latches. The digital outputs and out clocks are available at the standard 40-pin connectors J1 and J2. Power to the analog supply pins is connected via banana jacks. The analog supply powers the associated components and the analog section of the AD13280. The digital outputs of the AD13280 are powered via banana jacks with 3.3 V. Contact the factory if additional layout or applications assistance is required. Figure 9. Evaluation Board Mechanical Layout |
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