9

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, 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 +1.2V with a

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 (15)

selects the reference. The internal reference can be selected if

pin 15 is tied low (ground). If an external reference is desired,

then pin 15 should be tied high (to the analog supply voltage)

and the external reference driven into REFIO, pin 23. The full

scale output current of the converter is a function of the voltage

the 2mA to 20mA range, through operation below 2mA is

possible, with performance degradation.

approximately 1.16V (pin 22). If an external reference is used,

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

internal voltage reference (1.16V) and a 1.86k

resistor, then the input coding to output current will resemble

the following:

Outputs

IOUTA and IOUTB (or QOUTA and QOUTB) 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 compliance range of

output voltage is produced in conjunction with the output full

scale current, which is described above in the ‘Reference’

section. 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:

These outputs can be used in a differential-to-single-ended

arrangement to achieve better harmonic rejection. The

SFDR measurements in this data sheet were performed with

a 1:1 transformer on the output of the DAC (see Figure 1).

With the center tap grounded, the output swing of pins 16

and 17 will be biased at zero volts. It is important to note

here that the negative voltage output compliance range limit

with this configuration. The loading as shown in Figure 1 will

result in a 500mV signal at the output of the transformer if

the full scale output current of the DAC is set to 20mA.

Allowing the center tap to float will result in identical

transformer output, however the output pins of the DAC will

have positive DC offset. The 50

Ω load on the output of the

transformer represents the spectrum analyzer’s input

impedance.

TABLE 1. INPUT CODING vs OUTPUT CURRENT

INPUT CODE (D7-D0)

IOUTA (mA)

IOUTB (mA)

111 11111

20

0

100 00000

10

10

000 00000

0

20

PIN 17 (20)

PIN 16 (21)

100

Ω

50

Ω

50

Ω

50

Ω

IOUTB (QOUTB)

IOUTA (QOUTA)

FIGURE 4.

HI5628