DAC8802IPWR datasheet(14/18 Pages) TI

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Address

Decoder

Shift Register

To Input Register

CLK

SDI

POWER ON RESET

ESD Protection Circuits

VDD

250 W

DGND

DIGITAL

INPUTS

PCB LAYOUT

DAC8802

SBAS351A – AUGUST 2005 – REVISED DECEMBER 2005

Figure 33 shows the equivalent logic interface for the key digital control pins for the DAC8802.

Figure 33. DAC8802 Equivalent Logic Interface

Two additional pins RS and MSB provide hardware control over the preset function and DAC register loading. If

these functions are not needed, the RS pin can be tied to logic high. The asynchronous input RS pin forces all

input and DAC registers to either the zero-code state (MSB = 0), or the half-scale state (MSB = 1).

When the VDD power supply is turned on, an internal reset strobe forces all the Input and DAC registers to the

zero-code state or half-scale, depending on the MSB pin voltage. The VDD power supply should have a smooth

positive ramp without drooping, in order to have consistent results, especially in the region of VDD = 1.5 V to

2.3 V. The DAC register data stays at the zero or half-scale setting until a valid serial register data load takes

place.

All logic-input pins contain back-biased ESD protection zener diodes connected to ground (DGND) and VDD, as

shown in Figure 34.

Figure 34. Equivalent ESD Protection Circuits

The DAC8802 is a high-accuracy DAC that can have its performance compromised by grounding and printed

circuit board (PCB) lead trace resistance. The 14-bit DAC8802 with a 10-V full-scale range has an LSB value of

610 µV. The ladder and associated reference and analog ground currents for a given channel can be as high as

2 mA. With this 2 mA current level, a series wiring and connector resistance of only 305 m

Ω will cause 1 LSB of

voltage drop. The preferred PCB layout for the DAC8802 is to have all AGNDX pins connected directly to an

analog ground plane at the unit. The noninverting input of each channel I/V converter should also either connect

directly to the analog ground plane or have an individual sense trace back to the AGNDX pin connection. The

feedback resistor trace to the I/V converter should also be kept short and low resistance to prevent IR drops from

contributing to gain error. This attention to wiring ensures the optimal performance of the DAC8802.

DAC8802IPWR Datasheet(PDF) 14 Page - Texas Instruments