AD9174 datasheet(71/164 Pages) AD

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Data Sheet

AD9174

Rev. A | Page 71 of 164

As previously mentioned, MSB shuffling is a form of error

averaging. A particular AD9174 device, with its own auto-

calibration factors and unique production process variations,

may show improved spurious performance at some signal levels

with MSB shuffle disabled. However, when a statically meaningful

set of devices is considered, the overall spurious performance is

shown to improve, on average.

MSB shuffle can be enabled via the MSB_SHUFFLE_EN bit (Bit 4

of Register 0x151).

DC-Coupled Operation

In certain applications, it is desirable to dc couple the analog

outputs to an external device, such as a modulator or a

differential amplifier. The AD9174 analog outputs can be

dc-coupled without performance degradation, as long as the

common-mode voltage (the dc voltage common to both the

positive and negative branches of a particular analog output) is

kept below 100 mV. For ac-coupled operation, the outputs are

typically dc shorted to GND or 0 V through RF chokes or

particular balun configurations.

Elevating the common-mode voltage to between 100 mV and

300 mV leads to performance degradation. Increasing the

common-mode voltage beyond 300 mV may lead to long-

term, irreversible damage of the analog outputs.

Ideally, the common-mode voltage at the analog outputs is kept

near 0 V or GND, while the load impedance seen by the analog

outputs is matched to their internal impedance. Replacing the

RF chokes used in ac-coupled operation with 50 Ω resistors to

GND is not recommended because this results in an excessive

common-mode voltage, near 250 mV. Instead, tie the 50 Ω resistors

to a −0.6 V reference supply, thus maintaining proper dc bias at the

analog output devices internal to each DAC output.

It is possible to resistively match the 0 V common-mode output

voltage of the AD9174 to a nonzero common-mode input voltage

of a downstream device, such as the 0.5 V input common mode

typical to some modulators. This matching inevitably leads to a

loss in the maximum power that can be delivered to the down-

stream device, because some of the power is dissipated in the

resistive matching network.

2:1

1:1

50Ω

LOAD

50Ω

DC IOUTFS/2

AC IOUTFS/2

DAC

BALUN

CHOKE

DACx+

DACx–

IAC

IOUTFS = 15.625mA TO 25.977mA

Figure 91. Equivalent DAC Output Circuit and Recommended DAC Output Network

2:1

1:1

50Ω

LOAD

50Ω

DC IOUTFS/2

AC IOUTFS/2

DAC

BALUN

CHOKE

DACx+

DC PATH

DACx–

IAC

IOUTFS = 15.625mA TO 25.977mA

Figure 92. DACx Output, DC Path (AC-Coupled Operation)

50Ω

LOAD

2:1

1:1

50Ω

DC IOUTFS/2

AC IOUTFS/2

DAC

BALUN

CHOKE

DACx+

AC ILOAD

DACx–

AC PATH

IOUTFS = 15.625mA TO 25.977mA

Figure 93. DACx Output, AC Path (AC-Coupled Operation)

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