AD12400KWS datasheet(17/28 Pages) AD

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AD12400

Rev. 0 | Page 17 of 28

While this product has been designed to provide good PSRR

performance, systems designers need to be aware of the risks

associated with switching power supplies and consider using

linear regulators in their high speed ADC systems. Switching

power supplies typically produce both conducted and radiated

energy that result in common-/differential-mode EMI currents.

Any system that requires 12-bit performance has very little

room for errors associated with power supply EMI. For

example, a system goal of 74 dB dynamic range performance on

the AD12400 will require noise currents that are less than

4.5 µA and noise voltages of less than 225 µV in the analog

input path.

START-UP AND RESET

The AD12400’s FPGA configuration is stored in the on-board

EPROM and loaded into the FPGA when power is applied to

the device. The RESET pin (active low) allows the user to reload

the FPGA in case of a low digital supply voltage condition or a

power supply glitch. Pulling the RESET pin low will pull the

data ready and output bits high until the FPGA has been

reloaded. The RESET pin should remain low for a minimum of

200 ns. On the rising edge of the reset pulse, the AD12400 will

start loading the configuration into the FPGA. The reload

process requires a maximum of 600 ms to complete. Valid

signals on the data ready pins indicate that the reset process is

complete. Also, system designers need be aware of the thermal

conditions of the AD12400 at start-up. If large thermal

imbalances are present, the AD12400 may require additional

time to stabilize before providing specified image spur

performance.

LEAD/LAG

The LEAD/LAG pin is used to synchronize the collection of

data into external buffer memories. The LEAD/LAG pin can be

applied synchronously or asynchronously to the AD12400. If

applied asynchronously, LEAD/LAG must be held high for a

minimum of 5 ns to ensure correct operation. The function will

shut off DRA and DRB until the LEAD/LAG pin is released.

DRA and DRB will resume on the next valid DRA after

LEAD/LAG is released. If this feature is not required, tie this pin

to DGND.

THERMAL CONSIDERATIONS

The module is rated to operate over a case temperature of 0°C

to 60°C. In order to maintain the tight channel matching and

reliability of the AD12400, care must be taken to assure that

proper thermal and mechanical considerations have been made

and addressed to assure case temperature is kept within this

range. Each application will require evaluation of the thermal

management as applicable to the system design. The following

provides information that should be used in the evaluation of

AD12400 thermal management for each specific use.

In addition to the radiation of heat into its environment, the

AD12400 module enables flow of heat through the mounting

studs and standoffs as they contact the motherboard. As

described in the Package Integrity/Mounting Guidelines

section, the module should be secured to the motherboard

using 2-56 nuts (washer use is optional). The torque on the nuts

should not exceed 32 inch ounces. Use of a thermal grease at the

standoffs will result in better thermal coupling between the

board and module. Depending on the ambient conditions, air

flow may be necessary to ensure the components in the module

do not exceed their maximum operating temperature. In terms

of reliability, the most sensitive component has a maximum

junction temperature rating of 125°C.

Figures 21 and 22 provide a basic guideline for two key thermal

management decisions: the use of thermal interface material

between the module bottom cover/mother board and airflow.

Figure 21 characterizes the typical thermal profile of an

AD12400 that is not using thermal interface material. Figure 22

provides the same information for a configuration that uses

gap-filling thermal interface material (in this case Thermagon

T-Flex 600 series, 0.040” thickness was used). One can see from

these profiles that the maximum die temperature is reduced by

approximately 2°C when thermal interface material is used.

Figures 21 and 22 also provide a guideline for determining the

airflow requirements for given ambient conditions. For example,

a goal of 120°C die temperature in a 40°C ambient environment

without the use of thermal interface material would require an

air flow of 100 LFM. See the AD12400 Thermal Management

and Measurement Application Note for further details.

From a channel matching perspective, the most important

consideration will be external thermal influences. It is possible

for thermal imbalances in the end application to adversely affect

the dynamic performance. Due to the temperature dependence

of the image spur, substantial deviation from the factory

calibration conditions can have a detrimental effect.

Unbalanced thermal influences can cause gradients across the

module, and performance degradation may result. Examples of

unbalanced thermal influences may include large heat

dissipating elements near one side of the AD12400 or

obstructed air flow that does not flow uniformly across the

module. The thermal sensitivity of the module can be affected

by a change in thermal gradient across the module of 2°C.

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