AD9753AST datasheet(18/28 Pages) AD

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REV. B

–18–

AD9753

This is referred to as the Power Supply Rejection Ratio. For dc

variations of the power supply, the resulting performance of the

DAC directly corresponds to a gain error associated with the

DAC’s full-scale current, IOUTFS. AC noise on the dc supplies is

common in applications where the power distribution is gener-

ated by a switching power supply. Typically, switching power

supply noise will occur over the spectrum from tens of kHz to

several MHz. The PSRR versus the frequency of the AD9753

AVDD supply over this frequency range is shown in Figure 25.

FREQUENCY (MHz)

Figure 25. Power Supply Rejection Ratio

Note that the units in Figure 25 are given in units of (amps out/

volts in). Noise on the analog power supply has the effect of

modulating the internal switches, and therefore the output

current. The voltage noise on AVDD will thus be added in a

nonlinear manner to the desired IOUT. Due to the relative

different size of these switches, PSRR is very code-dependent.

This can produce a mixing effect that can modulate low fre-

quency power supply noise to higher frequencies. Worst-case

PSRR for either one of the differential DAC outputs will occur

when the full-scale current is directed toward that output. As a

result, the PSRR measurement in Figure 25 represents a worst-

case condition in which the digital inputs remain static and the

full-scale output current of 20 mA is directed to the DAC out-

put being measured.

An example serves to illustrate the effect of supply noise on the

analog supply. Suppose a switching regulator with a switching

frequency of 250 kHz produces 10 mV rms of noise and, for

simplicity sake (i.e., ignore harmonics), all of this noise is con-

centrated at 250 kHz. To calculate how much of this undesired

noise will appear as current noise superimposed on the DAC’s

full-scale current, IOUTFS, one must determine the PSRR in dB

using Figure 25 at 250 kHz. To calculate the PSRR for a given

RLOAD, such that the units of PSRR are converted from A/V to

V/V, adjust the curve in Figure 25 by the scaling factor 20

× Log

(RLOAD ). For instance, if RLOAD is 50

Ω, the PSRR is reduced

by 34 dB, i.e., PSRR of the DAC at 250 kHz, which is 85 dB in

Figure 25, becomes 51 dB VOUT/VIN.

Proper grounding and decoupling should be a primary objective

in any high speed, high resolution system. The AD9753 features

separate analog and digital supply and ground pins to optimize

the management of analog and digital ground currents in a system.

In general, AVDD, the analog supply, should be decoupled to

ACOM, the analog common, as close to the chip as physically

possible. Similarly, DVDD, the digital supply, should be

decoupled to DCOM as close to the chip as physically possible.

For those applications that require a single 3.3 V supply for

both the analog and digital supplies, a clean analog supply may

be generated using the circuit shown in Figure 26. The circuit

consists of a differential LC filter with separate power supply

and return lines. Lower noise can be attained by using low ESR

type electrolytic and tantalum capacitors.

AVDD

ACOM

100 F

ELECT.

10 F

TANT.

0.1 F

CER.

TTL/CMOS

LOGIC

CIRCUITS

3.3V

POWER SUPPLY

FERRITE

BEADS

Figure 26. Differential LC Filter for a Single 3.3 V Application

APPLICATIONS

QAM/PSK Synthesis

Quadrature modulation (QAM or PSK) consists of two base-

band PAM (Pulse Amplitude Modulated) data channels. Both

channels are modulated by a common frequency carrier. How-

ever, the carriers for each channel are phase-shifted 90

° from

each other. This orthogonality allows twice the spectral efficiency

(data for a given bandwidth) of digital data transmitted via AM.

Receivers can be designed to selectively choose the “in phase” and

“quadrature” carriers, and then recombine the data. The recombi-

nation of the QAM data can be mapped as points representing

digital words in a two dimensional constellation as shown in

Figure 27. Each point, or symbol, represents the transmission of

multiple bits in one symbol period.

0100

0101

0001

0000

0110

0111

0011

0010

1110

1111

1011

1010

1100

1101

1001

1000

Figure 27. 16 QAM Constellation, Gray Coded (Two

4-Level PAM Signals with Orthogonal Carriers)

Typically, the I and Q data channels are quadrature-modulated

in the digital domain. The high data rate of the AD9753 allows

extremely wideband (>10 MHz) quadrature carriers to be syn-

thesized. Figure 28 shows an example of a 25 MSymbol/S QAM

signal, oversampled by 8 at a data rate of 200 MSPS, modu-

lated onto a 25 MHz carrier and reconstructed using the

AD9753. The power in the reconstructed signal is measured

to be –11.92 dBm. In the first adjacent band, the power is

–76.86 dBm, while in the second adjacent band, the power is

–80.96 dBm.

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