10

FN6078.1

November 12, 2004

(nominally +3.3V) to maintain optimum SFDR. However,

operation down to +2.7V is possible with some degradation

in SFDR. Reducing the analog output current can help the

SFDR at +2.7V. The SFDR values stated in the table of

specifications were obtained with a +3.3V supply.

Ground Planes

Separate digital and analog ground planes should be used.

All of the digital functions of the device and their

corresponding components should be located over the

digital ground plane and terminated to the digital ground

plane. The same is true for the analog components and the

analog ground plane.

Noise Reduction

To minimize power supply noise, 0.1

µF capacitors should be

placed as close as possible to the converter’s power supply

using separate digital and analog ground planes and these

capacitors should be terminated to the digital ground for

of the power supplies on the board is recommended.

Voltage Reference

The internal voltage reference of the device has a nominal

value of +1.23V with a

±40ppm/°C drift coefficient over the

full temperature range of the converter. It is recommended

that a 0.1

µF capacitor be placed as close as possible to the

REFIO pin, connected to the analog ground. The REFLO pin

(16) selects the reference. The internal reference can be

selected if pin 16 is tied low (ground). If an external

reference is desired, then pin 16 should be tied high (the

analog supply voltage) and the external reference driven into

REFIO, pin 17. The full scale output current of the converter

is a function of the voltage reference used and the value of

though operation below 2mA is possible, with performance

degradation.

approximately 1.2V (pin 18). If an external reference is used,

If the full scale output current is set to 20mA by using the

internal voltage reference (1.2V) and a 1.91k

resistor, then the input coding to output current will resemble

the following:

Analog Output

IOUTA and IOUTB are complementary current outputs. The

sum of the two currents is always equal to the full scale

output current minus one LSB. If single ended use is

desired, a load resistor can be used to convert the output

current to a voltage. It is recommended that the unused

output be either grounded or equally terminated. The voltage

developed at the output must not violate the output voltage

loading each current output) should be chosen so that the

desired output voltage is produced in conjunction with the

output full scale current. If a known line impedance is to be

driven, then the output load resistor should be chosen to

match this impedance. The output voltage equation is:

The most effective method for reducing the power

consumption is to reduce the analog output current, which

dominates the supply current. The maximum recommended

output current is 20mA.

Differential Output

IOUTA and IOUTB can be used in a differential-to-single-

ended arrangement to achieve better harmonic rejection.

will provide a 500mV (-2.5dBm) signal at the output of the

transformer if the full scale output current of the DAC is set

to 20mA (used for the electrical specifications table). Values

performance curves. The center tap in Figure 13 must be

grounded.

In the circuit in Figure 14, the user is left with the option to

ground or float the center tap. The DC voltage that will exist

at either IOUTA or IOUTB if the center tap is floating is

transformer. If the center tap is grounded, the DC voltage is

0V. Recommended values for the circuit in Figure 14 are

performance of Figure 13 and Figure 14 is basically the

same, however leaving the center tap of Figure 14 floating

allows the circuit to find a more balanced virtual ground,

theoretically improving the even order harmonic rejection,

but likely reducing the signal swing available due to the

output voltage compliance range limitations.

TABLE 1. INPUT CODING vs OUTPUT CURRENT WITH

INTERNAL REFERENCE AND RSET = 1.91k

Ω

INPUT CODE (D9-D0)

IOUTA (mA)

IOUTB (mA)

11111 11111

20

0

10000 00000

10

10

00000 00000

0

20

ISL5757