10.1 Power Supply Design

The ADC14X250 device is a very-high dynamic range device and therefore requires very-low noise power supplies. LDO-type regulators, capacitive decoupling, and series isolation devices like ferrite beads are all recommended.

LDO-type low noise regulators should be used to generate the 1.2-, 1.8-, and 3-V supplies used by the device. To improve power efficiency, a switching-type regulator may precede the LDO to efficiently drop a supply to an intermediate voltage that satisfies the drop-out requirements of the LDO. TI recommends to follow a switching-type regulator with an LDO to provide the best filtering of the switching noise. Additional ferrite beads and LC filters may be used to further suppress noise. Supplying power to multiple devices in a system from one regulator may result in noise coupling between the multiple devices; therefore, series isolation devices and additional capacitive decoupling is recommended to improve the isolation.

The power supplies must be applied to the ADC14X250 device in this specific order:

1. VA3.0
2. VA1.8
3. VA1.2

First, the VA3.0 (+3.0 V) must be applied to provide the bias for the ESD diodes. The VA1.8 (+1.8-V) supply should be applied next, followed by the VA1.2 (+1.2-V) supply. As a guideline, each supply should stabilize to within 20% of the final value within 10 ms and before enabling the next supply in the sequence. If the stabilization time is longer than 10 ms, then the system should perform the calibration procedure after the supplies have stabilized. Turning power supplies off should occur in the reverse order.

In the case of a DC coupled interface with driving amplifier, the ADC supplies should be enabled and allowed to stabilize at least 1 ms before enabling the supply of driving amplifier. The sequencing delay allows the capacitors in the common-mode control loop to charge and avoids reliability concerns related to driving the ADC input outside the VIN+/- absolute maximum range for an extended time.

10.2 Decoupling

Decoupling capacitors must be used at each supply pin to prevent supply or ground noise from degrading the dynamic performance of the ADC and to provide the ADC with a well of charge to minimize voltage ripple caused by current transients. The recommended supply decoupling scheme is to have a ceramic X7R 0201 0.1-μF capacitor at each supply pin. The 0201 capacitor must be placed on the same layer as the device as close to the pin as possible to minimize the AC decoupling path length from the supply pin, through the capacitor, to the nearest adjacent ground pin. The 0402 capacitor should also be close to the pins. TI does not recommend placing the capacitor on the opposite board side. Each voltage supply should also have a single 10-μF decoupling capacitor near the device but the proximity to the supply pins is less critical.

The BP2.5 pin is an external bypass pin used for stabilizing an internal 2.5-V regulator and must have a ceramic or tantalum 10-μF capacitor and a ceramic 0402 0.1-μF capacitor. The 0.1-μF capacitor should be placed as close to the BP2.5 pin as possible.