AD5686R datasheet(26/31 Pages) AD

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AD5686R/AD5685R/AD5684R

Data Sheet

Rev. D | Page 26 of 31

HARDWARE RESET (RESET)

RESET is an active low reset that allows the outputs to be

cleared to either zero scale or midscale. The clear code value is

user selectable via the RESET select pin. It is necessary to keep

RESET low for a minimum amount of time to complete the

operation (see Figure 2). When the RESET signal is returned

high, the output remains at the cleared value until a new value is

programmed. The outputs cannot be updated with a new value

while the RESET pin is low. There is also a software executable

reset function that resets the DAC to the power-on reset code.

Command 0110 is designated for this software reset function

(see Table 8). Any events on LDAC during a power-on reset are

ignored. If the RESET pin is pulled low at power-up, the device

does not initialize correctly until the pin is released.

RESET SELECT PIN (RSTSEL)

The AD5686R/AD5685R/AD5684R contain a power-on reset

circuit that controls the output voltage during power-up. By

connecting the RSTSEL pin low, the output powers up to zero

scale. Note that this is outside the linear region of the DAC; by

connecting the RSTSEL pin high, VOUT powers up to midscale.

The output remains powered up at this level until a valid write

sequence is made to the DAC.

INTERNAL REFERENCE SETUP

The on-chip reference is on at power-up by default. To reduce

the supply current, this reference can be turned off by setting

software programmable bit, DB0, in the control register.

Table 15 shows how the state of the bit corresponds to the mode

of operation. Command 0111 is reserved for setting up the

internal reference (see Figure 9). Table 15 shows how the state

of the bits in the input shift register corresponds to the mode of

operation of the device during internal reference setup.

Table 15. Reference Setup Register

Internal Reference

Setup Register (DB0)

Action

Reference on (default)

Reference off

SOLDER HEAT REFLOW

As with all IC reference voltage circuits, the reference value

experiences a shift induced by the soldering process. Analog

Devices, Inc., performs a reliability test called precondition to

mimic the effect of soldering a device to a board. The output

voltage specification quoted previously includes the effect of

this reliability test.

Figure 56 shows the effect of solder heat reflow (SHR) as

measured through the reliability test (precondition).

Figure 56. SHR Reference Voltage Shift

2.498

2.499

2.500

2.501

2.502

VREF (V)

POSTSOLDER

HEAT REFLOW

PRESOLDER

HEAT REFLOW

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