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2

Circuit Description

2.1

Schematic Diagram

2.2

Circuit Function

2.2.1

Power

2.2.2

Clock Input

2.2.3

External References

Circuit Description

The schematic diagram for the EVM is in Section 6.3.

The following sections describe the function of individual circuits. Refer to the relevant data sheet for

device operating characteristics.

Power is supplied to the EVM via banana jack sockets. By default, the EVM is configured to use a power

management solution to supply the ADC and analog and digital power supplies, allowing users to power

the board with a single 5-V power supply. The ADC power management solution is based on TI's

TPS77533 and TPS73218, which supply 3.3 V and 1.8 V, respectively. In addition, the EVM offers the

capability to supply to the ADC independent 3.3-V analog and 1.8-V digital supplies. The circuit board

uses only one ground plane, and the heat slug is tied to ground with multiple vias to provide for thermal

dissipation. Table 2 offers a snapshot of the power-supply options.

Table 2. EVM Power-Supply Options

EVM Banana Jack

DESCRIPTION

J1

Auxiliary circuit 5-V digital supply: PECL driver and USB circuitry

J2

Single ground plane

J4

ADS528x 3.3-V analog supply (only active when JP8 = 2–3)

J5

ADS528x 1.8-V digital supply (only active when JP9 = 2–3)

A single-ended square or sinusoidal clock input should be applied to J26. The clock frequency should not

exceed the maximum speed rating found in the data sheet. Several different clocking options exist to allow

flexible evaluation of the ADC.

In the default case, a single-ended clock is converted to a differential clock using a Mini-Circuits TC1-1T

transformer. When using this option, the ADC should be configured in differential clock mode by writing

0x8001 to ADC register address 0x42. By default, after a software reset this option is asserted to coincide

with the EVM default.

A second EVM option allows the ADC to be configured in single-ended mode. This is provided to the ADC

using an On Semiconductor™ MC100EPT21 amplifier, which provides for sine-wave to square-wave

conversion. This configuration can be used for both ADS528X and ADS527X devices. To use this mode,

use the surface-mount jumpers JP3, JP4, and JP5, and configure each one of them to have positions 2–3

shorted.

The EVM offers the ability to force external references to the ADC. By default, the ADC is configured to

use the references generated internal to the ADC. To force the ADC to use external references, users

must short JP2 pads 2–3, which in turn grounds the INT/EXT pin of the ADC. Users can then use J6 pin 1

to force a REFB and pin 3 to force a REFT voltage. GND should be connected to J6 pin 2.

8

SLAU205 – January 2008

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