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

separate power supplies because the fast digital output swings

can couple switching current back into the analog supplies.

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

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