DAC2902

13

SBAS167C

www.ti.com

Figure 8 shows an example of a DC-coupled interface with

DC level-shifting, using a precision resistor network. An ac-

coupled interface, as shown in Figure 9, has the advantage

that the common-mode levels at the input of the modulator

can be set independently of those at the output of the DAC.

Furthermore, no voltage loss is obtained in this setup.

INTERNAL REFERENCE OPERATION

The DAC2902 has an on-chip reference circuit that consists

of a 1.25V bandgap reference and two control amplifiers,

one for each DAC. The full-scale output current, IOUTFS, of

the DAC2902 is determined by the reference voltage, VREF,

and the value of resistor RSET. IOUTFS can be calculated by:

IOUTFS = 32 × IREF = 32 × VREF / RSET

(10)

The external resistor RSET connects to the FSA pin (Full-

Scale Adjust), see Figure 10. The reference control amplifier

operates as a V-to-I converter producing a reference current,

IREF, which is determined by the ratio of VREF and RSET (as

shown in Equation 10). The full-scale output current, IOUTFS,

results from multiplying IREF by a fixed factor of 32.

I

OUT1

I

OUT1

DAC2902

I

OUT1

I

OUT1

V

OUT1

V

DC

R

3

R

4

R

5

V

OUT1

FIGURE 8. DC-Coupled Interface to Quadrature Modulator

Applying Level Shifting.

FIGURE 9. AC-Coupled Interface to Quadrature Modulator Applying Level Shifting.

I

OUT1

I

OUT1

DAC2902

I

OUT1

I

OUT1

V

OUT1

0.01

µF

0.01

µF

V

DC

R

1

R

2

V

OUT1

R

LOAD

50

Ω

50

Ω