DAC39RFS10 datasheet(49/177 Pages) TI2

49 / 177 page

Figure 7-11. Analog Output Structure

Examples of conversions from digital codes to currents on the IOUTx± outputs are given in Table 7-2. The

currents shown in Table 7-2 include both the current steered portion and the bias currents on each leg.

Table 7-2. Example Digital Code to Analog Current Conversions

DIGITAL CODE

2'S COMPLEMENT

OFFSET BINARY

IDACOUTx+

IDACOUTx–

IDACOUTx+ – IDACOUTx–

32767

0111 1111 1111 1111

1111 1111 1111 1111

0.9999847 × IFS +

IBIAS

0.0000153 × IFS +

IBIAS

0.9999694 × IFS

16384

0100 0000 0000 0000 1100 0000 0000 0000

¾ × IFS + IBIAS

¼ × IFS + IBIAS

½ × IFS

0000 0000 0000 0000 0000 0000 0000 0000

½ × IFS + IBIAS

–16384

1100 0000 0000 0000 0100 0000 0000 0000

¼ × IFS + IBIAS

¾ × IFS + IBIAS

–½ × IFS

–32768

1000 0000 0000 0000 0000 0000 0000 0000

IBIAS

IFS + IBIAS

–IFS

7.3.2.2 Full-Scale Current Adjustment

The total DAC output current is set through the external RBIAS resistor and the COARSE_CUR_A or

COARSE_CUR_B and the FINE_CUR_A or FINE_CUR_B registers. There is a switched fullscale current and a

static fullscale current. The switched current is divided between DACOUTA/B+ and DACOUTA/B- in proportion

to the digital signal value at the DAC. The static current is fixed at the output of each ball DACOUTA/B+ and

DACOUTA/B-.

The equation for the DAC switched output current is

IFSSWITCH=3.6kΩRBIAS× 5mA+1mA*COARSE+0.0156mA*FINE ×2CUR_2X_EN

(2)

where

• Rbias is the external bias resistor

• COARSE is the value of the register COARSE_CUR_A or COARSE_CUR_B (0 to 15)

• FINE is the value of register FINE_CUR_A or FINE_CUR_B (0 to 63)

• CUR_2X_EN is the value of register CUR_2X_EN (0 or 1)

The static current is a fixed fraction of the switched current

IFSSTATIC = 0.235 x IFSSWITCH

(3)

With a 3.6kΩ bias resistor, COARSE_CUR_A or COARSE_CUR_B = 15 and FINE_CUR_A or FINE_CUR_B =

31, IFSSWITCHED is ~ 20.5mA and IFSSTATIC ~ 4.82mA (on each ball + and -). Enabling CUR_2X_EN doubles the

currents.

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DDS39RF12, DDS39RFS12

SBASB21 – SEPTEMBER 2024

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