DAC902 datasheet(12/25 Pages) TI2

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DAC902

SBAS094B

The value of the load resistance is limited by the output

compliance specification of the DAC902. To maintain speci-

fied linearity performance, the voltage for IOUT and IOUT

should not exceed the maximum allowable compliance range.

The two single-ended output voltages can be combined to

find the total differential output swing:

(7)

ANALOG OUTPUTS

The DAC902 provides two complementary current outputs,

IOUT and IOUT. The simplified circuit of the analog output

stage representing the differential topology is shown in

Figure 2. The output impedance of 200k

Ω || 12pF for I

OUT

and IOUT results from the parallel combination of the differ-

ential switches, along with the current sources and associ-

ated parasitic capacitances.

IOUT and IOUT. Furthermore, using the differential output

configuration in combination with a transformer will be

instrumental for achieving excellent distortion performance.

Common-mode errors, such as even-order harmonics or

noise, can be substantially reduced. This is particularly the

case with high output frequencies and/or output amplitudes

below full-scale.

For those applications requiring the optimum distortion and

noise performance, it is recommended to select a full-scale

output of 20mA. A lower full-scale range down to 2mA may

be considered for applications that require a low power

consumption, but can tolerate a reduced performance level.

FIGURE 2. Equivalent Analog Output.

The signal voltage swing that may develop at the two

outputs, IOUT and IOUT, is limited by a negative and positive

compliance. The negative limit of –1V is given by the

breakdown voltage of the CMOS process, and exceeding it

will compromise the reliability of the DAC902, or even

cause permanent damage. With the full-scale output set to

20mA, the positive compliance equals 1.25V, operating with

+VD = 5V. Note that the compliance range decreases to

about 1V for a selected output current of IOUTFS = 2mA.

Care should be taken that the configuration of DAC902 does

not exceed the compliance range to avoid degradation of the

distortion performance and integral linearity.

Best distortion performance is typically achieved with the

maximum full-scale output signal limited to approximately

0.5V. This is the case for a 50

Ω doubly-terminated load and

a 20mA full-scale output current. A variety of loads can be

adapted to the output of the DAC902 by selecting a suitable

transformer while maintaining optimum voltage levels at

OUTPUT CONFIGURATIONS

The current output of the DAC902 allows for a variety of

configurations, some of which are illustrated below. As men-

tioned previously, utilizing the converter’s differential out-

puts will yield the best dynamic performance. Such a differ-

ential output circuit may consist of an RF transformer or a

differential amplifier configuration. The transformer configu-

ration is ideal for most applications with ac coupling, while

op amps will be suitable for a DC-coupled configuration.

The single-ended configuration may be considered for appli-

cations requiring a unipolar output voltage. Connecting a

resistor from either one of the outputs to ground will convert

the output current into a ground-referenced voltage signal.

To improve on the DC linearity, an I-to-V converter can be

used instead. This will result in a negative signal excursion

and, therefore, requires a dual supply amplifier.

DIFFERENTIAL WITH TRANSFORMER

Using an RF transformer provides a convenient way of

converting the differential output signal into a single-ended

signal while achieving excellent dynamic performance (see

Figure 3). The appropriate transformer should be carefully

selected based on the output frequency spectrum and imped-

ance requirements. The differential transformer configura-

tion has the benefit of significantly reducing common-mode

signals, thus improving the dynamic performance over a

wide range of frequencies. Furthermore, by selecting a

suitable impedance ratio (winding ratio), the transformer can

be used to provide optimum impedance matching while

controlling the compliance voltage for the converter outputs.

The model shown in Figure 3 has a 1:1 ratio and may be used

to interface the DAC902 to a 50

Ω load. This results in a 25Ω

load for each of the outputs, IOUT and IOUT. The output

signals are ac coupled and inherently isolated because of its

magnetic coupling.

INPUT CODE (D11 - D0)

IOUT

1111 1111 1111

20mA

0mA

1000 0000 0000

10mA

0000 0000 0000

0mA

20mA

TABLE I. Input Coding vs Analog Output Current.

Code

OUTDIFF

OUT

OUTFS

LOAD

== •

••

–

(–

)

4095

4096

OUT

DAC902

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