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A
B
Address
Decoder
ShiftRegister
EN
ToInputRegister
CS
CLK
SDI
POWER ON RESET
ESD Protection Circuits
250 W
DGND
DIGITAL
INPUTS
V
DD
PCB LAYOUT
DAC8812
SBAS349B – AUGUST 2005 – REVISED FEBRUARY 2007
Figure 33 shows the equivalent logic interface for the key digital control pins for the DAC8812.
Figure 33. DAC8812 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 V
DD
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 V
DD
power supply should have a smooth
positive ramp without drooping, in order to have consistent results, especially in the region of V
DD
= 1.5 V to
2.3 V. The DAC register data stays at 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 V
DD
as
shown in Figure 34 .
Figure 34. Equivalent ESD Protection Circuits
The DAC8812 is a high-accuracy DAC that can have its performance compromised by grounding and printed
circuit board (PCB) lead trace resistance. The 16-bit DAC8812 with a 10-V full-scale range has an LSB value of
153 mV. 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 76 m Ω will cause 1 LSB of
voltage drop. The preferred PCB layout for the DAC8812 is to have all A
GND
X 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 A
GND
X pin connection. The
feedback resistor trace to the I/V converter should also be kept short and have low resistance in order to prevent
IR drops from contributing to gain error. This attention to wiring ensures the optimal performance of the
DAC8812.
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