AD9957SLASHPCBZ datasheet(26/61 Pages) AD

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AD9957

Data Sheet

Rev. F | Page 26 of 61

In Figure 37, it can be seen that the sinc envelope introduces a

frequency dependent attenuation that can be as much as 4 dB at

the Nyquist frequency (half of the DAC sample rate). Without

the inverse sinc filter, the DAC output also suffers from the

frequency dependent droop of the sinc envelope. The inverse

sinc filter effectively flattens the droop to within ±0.05 dB as

shown in Figure 38, which shows the corrected sinc response

with the inverse sinc filter enabled.

–1

–2

–3

–4

0.1

0.2

0.4

0.3

0.5

FREQUENCY RELATIVE TO DAC SAMPLE RATE

INVERSE

SINC

Figure 37. Sinc and Inverse Sinc Responses

–2.8

–2.9

–3.0

–3.1

0.1

0.2

0.4

0.3

0.5

FREQUENCY RELATIVE TO DAC SAMPLE RATE

COMPENSATED RESPONSE

Figure 38. DAC Response with Inverse Sinc Compensation

OUTPUT SCALE FACTOR (OSF)

In QDUC and interpolating DAC modes, the output amplitude

is controlled using an 8-bit digital multiplier. The 8-bit multiplier

value is called the output scale factor (OSF) and is programmed

via the appropriate control registers. It is available for each of

the eight profiles. The LSB weight is 2−7, which yields a multiplier

range of 0 to 1.9921875 (2 − 2−7). The gain extends to nearly a

factor of 2 to provide a means to overcome the intrinsic loss

through the modulator when operating in the quadrature

modulation mode.

In interpolating DAC mode, the OSF should not be programmed

to exceed unity, as clipping can result. Programming the 8-bit

multiplier to unity gain (0x80) bypasses the stage and reduces

power consumption.

14-BIT DAC

The AD9957 incorporates an integrated 14-bit current-output

DAC. The output current is delivered as a balanced signal using

two outputs. The use of balanced outputs reduces the amount of

common-mode noise at the DAC output, increasing signal-to-

noise ratio. An external resistor (RSET) connected between the

DAC_RSET pin and AGND establishes a reference current. The

full-scale output current of the DAC (IOUT) is a scaled version of

the reference current (see the Auxiliary DAC section that follows).

Proper attention should be paid to the load termination to keep

the output voltage within the specified compliance range, as

voltages developed beyond this range cause excessive distortion

and can damage the DAC output circuitry.

Auxiliary DAC

The full-scale output current of the main DAC (IOUT) is con-

trolled by an 8-bit auxiliary DAC. An 8-bit code word stored in

the appropriate register map location sets IOUT according to the

following equation:











 +

CODE

SET

OUT

(6)

where:

RSET is the value of the RSET resistor (in ohms).

CODE is the 8-bit value supplied to the auxiliary DAC (default

is 127).

For example, with RSET = 10,000 and CODE = 127, IOUT = 20.07 mA.

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