DAC7741YCSLASH2K datasheet(17/23 Pages) TI2

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DAC7741

SBAS248B

www.ti.com

LAYOUT

A precision analog component requires careful layout, adequate

bypassing, and clean, well-regulated power supplies. The

DAC7741 offers separate digital and analog supplies, as it will

often be used in close proximity with digital logic, microcontrollers,

microprocessors, and digital signal processors. The more digital

logic present in the design and the higher the switching speed,

the more important it will become to separate the analog and

digital ground and supply planes at the device.

Since the DAC7741 has both analog and digital ground pins,

return currents can be better controlled and have less effect

on the DAC output error. Ideally, AGND would be connected

directly to an analog ground plane and DGND to the digital

ground plane. The analog ground plane would be separate

from the ground connection for the digital components until

they were connected at the power entry point of the system.

The voltages applied to VCC and VSS should be well regulated

and low noise. Switching power supplies and dc/dc convert-

ers will often have high-frequency glitches or spikes riding on

the output voltage. In addition, digital components can create

similar high-frequency spikes as their internal logic switches

states. This noise can easily couple into the DAC output

voltage through various paths between the power connec-

tions and analog output.

In addition, a 1

µF to 10µF bypass capacitor in parallel with a

0.1

µF bypass capacitor is strongly recommended for each

supply input. In some situations, additional bypassing may

be required, such as a 100

µF electrolytic capacitor or even

a "Pi" filter made up of inductors and capacitors–all designed

to essentially low-pass filter the analog supplies, removing

any high frequency noise components.

NOISE PERFORMANCE

Increased noise performance of the DAC output can be

achieved by filtering the voltage reference input to the

DAC7741. Figure 10 shows a typical internal reference filter

schematic. A low-pass filter applied between the REFOUT and

REFIN pins can increase noise immunity at the DAC and

output amplifier. The REFOUT pin can source a maximum of

µA so care should be taken in order to avoid overloading

the internal reference output

(Other Connections

Omitted for Clarity)

RSTSEL

REFEN

REF

OUT

REFADJ

REF

100k

Ω

µF

FIGURE 10. Internal Reference Filter.

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