DAC7678SPW datasheet(23/52 Pages) TI

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DAC

REF(+)

Resistor String

REF( )

REFIN/VREFOUT

OUTX

178kW

150kW

OUT

´ REFOUT

4096

OUT

´ REFIN

4096

REF

DIVIDER

To Output Amplifier

(2x Gain)

DAC7678

www.ti.com

SBAS493A – FEBRUARY 2010 – REVISED AUGUST 2010

THEORY OF OPERATION

will be un-shorted if external reference is used. Thus

the overall gain will be one and allows the user to

DIGITAL-TO-ANALOG CONVERTER (DAC)

provide an external reference value of 0 to AVDD. If

The DAC7678 architecture consists of eight string

internal reference is used RDIVIDER is shorted and the

DACs each followed by an output buffer amplifier.

overall gain will be two.

The

DAC7678

also

includes

internal

2.5V

reference with a maximum 25ppm/°C temperature

drift performance, offering a 5V, full-scale output

voltage. Figure 86 shows a principal block diagram of

the DAC architecture.

Figure 86. Device Architecture

For the TSSOP package, the input coding to the

DAC7678 is straight binary. For the QFN package,

the TWOC pin controls the code format.

When using the internal reference, the ideal output

voltage is given by Equation 1:

(1)

When using an external reference, the ideal output

voltage is given by Equation 2:

(2)

Where:

DIN = decimal equivalent of the binary code that

Figure 87. Resistor String

is loaded to the DAC register. It can range from 0

to 4095.

VREFOUT = internal reference voltage of 2.5V (typ),

OUTPUT AMPLIFIER

supplied at the VREFIN/VREFOUT pin.

The output buffer amplifier is capable of generating

VREFIN = external reference voltage of 0V to 5V

rail-to-rail voltages on its output, giving a maximum

(typ), supplied at the VREFIN/VREFOUT pin.

output range of 0V to AVDD. It is capable of driving a

load of 2k

Ω in parallel with 1000pF to GND. The

RESISTOR STRING

source and sink capabilities of the output amplifier

The resistor string circuitry is shown in Figure 87. It is

can be seen in the Typical Characteristics. The

a string of resistors, each of value R. The code

typical slew rate is 0.75V/ms, with a typical full-scale

loaded into the DAC register determines at which

settling time of 7ms with the output unloaded.

node on the string the voltage is tapped off to be fed

into the output amplifier by closing one of the

switches connecting the string to the amplifier. It is

monotonic because it is a string of resistors. RDIVIDER

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