DAC1282IPW datasheet(19/43 Pages) TI1

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5 V

Reference

0.1 µF

(1)

VREF

AVSS (14)

-2.5 V

220 k

Power-down

Switch

Internal

Circuitry

AVSS (23)

DAC1282

www.ti.com

SBAS490

– DECEMBER 2011

The DAC1282 filters the digital data (bitstream) input providing a voltage output proportional to the bitstream

ones-density. The GAIN[2:0] register sets the analog gain in 6-dB steps, from 0 dB to

–36 dB (1/1 to 1/64); see

the SYNC section for the external timing requirements. Table 7 lists several values of the external bitstream

input.

Table 7. External Bitstream Input

ONES-DENSITY BITSTREAM (%)

VOUTP

– VOUTN (V)(1)

–2.5 × Gain

37.5

–1.25 × Gain

62.5

+1.25

× Gain

+2.5

× Gain

(1)

VREF = 5 V. Gain is the analog gain, programmable from 1/1 to 1/64 (0 dB to –36 dB).

REFERENCE VOLTAGE (VREF)

The DAC1282 requires an external reference for operation. Although reference voltage as low as 2.5 V can be

used, best SNR is achieved with a 5-V reference. The reference input is defined as the voltage difference

between VREF and AVSS (that is, VREF = VREF – AVSS). The DAC1282 output scales with VREF; consequently,

reference noise or drift appears on the DAC output. Excessive reference noise may lead to degraded SNR. A

low-drift and low-noise reference is recommended.

Connect the external reference ground pin directly to the AVSS pins using a star connector to AVSS pin 14. Star

connection minimizes the possibility of power-supply crosstalk. Also, connect a 0.1-

µF capacitor close to the

VREF and AVSS terminals to reduce noise susceptibility. Figure 35 shows the reference connection. The

reference input impedance is 220 k

Ω. In power-down the switch is off, resulting in very high input impedance. For

single-supply applications, connect AVSS to a clean analog ground point.

(1) Recommended bypass capacitor.

Figure 35. Reference Input Connection

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