AD5689R datasheet(24/28 Pages) AD

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AD5689R/AD5687R

Data Sheet

Rev. B | Page 24 of 28

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 power-on reset select pin (RSTSEL). RESET

must be kept 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 9). 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 AD5689R/AD5687R contain a power-on reset circuit that

controls the output voltage during power-up. When the RSTSEL

pin is connected low (to GND), the output powers up to zero

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

When the RSTSEL pin is connected high (to VLOGIC), VOUTX

powers up to midscale. The output remains powered up at this

level until a valid write sequence is sent to the DAC.

To ensure that the RSTSEL pin functions correctly, a power

sequence must be followed. Power up VLOGIC first, before VDD;

otherwise, VOUT remains at zero scale.

INTERNAL REFERENCE SETUP

Command 0111 is reserved for setting up the internal reference

(see Table 9). By default, the on-chip reference is on at power-up.

To reduce the supply current, this reference can be turned off by

setting the software-programmable bit, DB0, as shown in Table 17.

Table 16 shows how the state of the bit corresponds to the mode

of operation.

Table 16. 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,

that mimics the effect of soldering a device to a board. The

output voltage specification that is listed in Table 2 includes the

effect of this reliability test.

Figure 48 shows the effect of solder heat reflow (SHR) as

measured through the reliability test (precondition).

2.498

2.499

2.500

2.501

2.502

VREF (V)

POSTSOLDER

HEAT REFLOW

PRESOLDER

HEAT REFLOW

Figure 48. SHR Reference Voltage Shift

LONG-TERM TEMPERATURE DRIFT

Figure 49 shows the change in the VREF (ppm) value after

1000 hours at 25°C ambient temperature.

140

120

100

200

300

400

500

600

700

800

900 1000

–20

Figure 49. Reference Drift Through to 1000 Hours

Table 17. 24-Bit Input Shift Register Contents for Internal Reference Setup Command1

DB23

(MSB)

DB22

DB21

DB20

DB19

DB18

DB17

DB16

DB15 to DB1

DB0 (LSB)

1 or 0

Command bits (C3 to C0)

Address bits (A3 to A0)

Don’t care

Reference setup register

1 X = don’t care.

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