DAC8564 Datasheet, PDF(26/46 Page) Texas Instruments – 16-Bit, Quad Channel, Ultra-Low Glitch, Voltage Output DIGITAL-TO-ANALOG CONVERTER with 2.5V, 2ppm/°C Internal Reference

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DAC8564 Datasheet, PDF (26/46 Pages) Texas Instruments – 16-Bit, Quad Channel, Ultra-Low Glitch, Voltage Output DIGITAL-TO-ANALOG CONVERTER with 2.5V, 2ppm/°C Internal Reference

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DAC8564

SBAS403A â JUNE 2007 â REVISED NOVEMBER 2007

www.ti.com

INTERNAL REFERENCE

The DAC8564 includes a 2.5V internal reference that

is enabled by default. The internal reference is

externally available at the VREFH / VREFOUT pin. A

minimum 100nF capacitor is recommended between

the reference output and GND for noise filtering.

The internal reference of the DAC8564 is a bipolar

transistor-based, precision bandgap voltage

reference. Figure 94 shows the basic bandgap

topology. Transistors Q1 and Q2 are biased such that

the current density of Q1 is greater than that of Q2.

The difference of the two base-emitter voltages

(VBE1 â VBE2) has a positive temperature coefficient

and is forced across resistor R1. This voltage is

gained up and added to the base-emitter voltage of

Q2, which has a negative temperature coefficient. The

resulting output voltage is virtually independent of

temperature. The short-circuit current is limited by

design to approximately 100mA.

VREF

Reference

Disable

Figure 94. Simplified Schematic of the Bandgap

Reference

Enable/Disable Internal Reference

The internal reference in the DAC8564 is enabled by

default and operates in automatic mode; however, the

reference can be disabled for debugging, evaluation

purposes, or when using an external reference. A

serial command that requires a 24-bit write sequence

(see the Serial Interface section) must be used to

disable the internal reference, as shown in Table 1.

During the time that the internal reference is disabled,

the DAC functions normally using an external

reference. At this point, the internal reference is

disconnected from the VREF pin (3-state output). Do

not attempt to drive the VREF pin externally and

internally at the same time indefinitely.

To then enable the internal reference, either perform

a power-cycle to reset the device, or write the 24-bit

serial command shown in Table 2. These actions put

the internal reference back into the default mode. In

the default mode, the internal reference powers down

automatically when all DACs power down in any of

the power-down modes (see the Power-Down Modes

section); the internal reference powers up

automatically when any DAC is powered up.

The DAC8564 also provides the option of keeping the

internal reference powered on all the time, regardless

of the DAC(s) state (powered up or down). To keep

the internal reference powered on, regardless of the

DAC(s) state, write the 24-bit serial command shown

in Table 3.

Table 1. Write Sequence for Disabling Internal Reference

(internal reference always powered downâ012000h)

DB23

DB16

DB13

DB0

000000010010000000000000

Table 2. Write Sequence for Enabling Internal Reference

(internal reference powered up to default modeâ010000h)

DB23

DB16

DB0

000000010000000000000000

Table 3. Write Sequence for Enabling Internal Reference

(internal reference always powered upâ011000h)

DB23

DB16

DB12

DB0

000000010001000000000000

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