DAC2904

I

OUT

R

L

50W

R

L

50W

R

S

50W

R

DIFF

100W

I

OUT

ADTT1-1

(Mini-Circuits)

1:1

DAC2904

www.ti.com.............................................................................................................................................. SBAS198C – AUGUST 2001 – REVISED OCTOBER 2009

OUTPUT CONFIGURATIONS

DIFFERENTIAL WITH TRANSFORMER

The current outputs of the DAC2904 allow for a

Using an RF transformer provides a convenient way

variety of configurations, some of which are illustrated

of converting the differential output signal into a

in Table 1. As mentioned previously, utilizing the

single-ended

signal

while

achieving

excellent

converter differential outputs will yield the best

dynamic

performance

(see

Figure

20).

The

dynamic performance. Such a differential output

appropriate transformer should be carefully selected

circuit may consist of an RF transformer or a

based

on

the

output

frequency

spectrum

and

differential amplifier configuration. The transformer

impedance requirements. The differential transformer

configuration is ideal for most applications with ac

configuration has the benefit of significantly reducing

coupling, while op amps will be suitable for a

common-mode signals, thus improving the dynamic

dc-coupled configuration.

performance over a wide range of frequencies.

Furthermore, by selecting a suitable impedance ratio

Table 1. Input Coding vs Analog Output Current

(winding ratio), the transformer can be used to

provide

optimum

impedance

matching

while

INPUT CODE (D13 - D0)

IOUT

I OUT

controlling the compliance voltage for the converter

11 1111 1111 1111

20mA

0mA

outputs.

The

model

shown,

ADTT1-1

(by

10 0000 0000 0000

10mA

10mA

Mini-Circuits), has a 1:1 ratio and may be used to

00 0000 0000 0000

0mA

20mA

interface the DAC2904 to a 50

Ω load. This results in

a 25

Ω load for each of the outputs, IOUT and I OUT .

The single-ended configuration may be considered

The output signals are ac-coupled and inherently

for applications requiring a unipolar output voltage.

isolated because of its magnetic coupling.

Connecting a resistor from either one of the outputs

As shown in Figure 20, the transformer center tap is

to ground will convert the output current into a

connected to ground. This forces the voltage swing

ground-referenced voltage signal. To improve on the

on IOUT and I OUT to be centered at 0V. In this case

dc linearity by maintaining a virtual ground, an I-to-V

the two resistors, RL, may be replaced with one,

or op amp configuration may be considered.

RDIFF, or omitted altogether. This approach should

space

only be used if all components are close to each

other, and if the VSWR is not important. A complete

space

power transfer from the DAC output to the load can

space

be realized, but the output compliance range should

be observed. Alternatively, if the center tap is not

space

connected, the signal swing will be centered at (RL ×

IOUTFS/2). However, in this case, the two load

space

resistors, RL, must be used to enable the necessary

dc-current flow for both outputs.

Figure 20. Differential Output Configuration Using an RF Transformer

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