REV. 0

AD8346

–9–

AC Coupled Interface

An ac coupled interface can also be implemented. This is shown

in Figure 24. This has the advantage that there is almost no

voltage loss due to the biasing network, allowing the AD8346

inputs to be driven by the full 2 V p-p differential signal from the

AD9761 (each of the DAC’s four outputs delivering 1 V p-p).

As in the dc coupled case, the bias levels on the I and Q inputs

should be set to as precise a level as possible, relative to each

other. This prevents the introduction of additional input offset

voltages. In the example shown, the bias level on each input is

set to approximately 1.2 V. The 2.43 k

Ω resistors should have a

ratio tolerance of 0.1% or better.

"I"

DAC

2

LATCH

"I"

IOUTB

IOUTA

"Q"

DAC

2

LATCH

"Q"

QOUTB

QOUTA

MUX

CONTROL

SELECT

WRITE

CLOCK

AD9761

DVDD

DCOM

AVDD

0.1 F

2k

SLEEP

FS ADJ

REFIO

DAC

DATA

INPUTS

100

100

100

100

500

500

500

500

500

500

500

500

0.1 F

634

+5V

PHASE

SPLITTER

VOUT

IBBP

IBBN

QBBP

QBBN

AD8346

LOIP

LOIN

VPS1

VPS2

0.5V p-p EACH PIN

+5V

Figure 23. AD8346 Interface to AD9761 TxDAC

2

LATCH

"I"

IOUTB

IOUTA

2

LATCH

"Q"

QOUTB

QOUTA

MUX

CONTROL

SELECT

WRITE

CLOCK

AD9761

DVDD

DCOM

AVDD

0.1 F

2k

SLEEP

FS ADJ

REFIO

DAC

DATA

INPUTS

100

100

100

100

0.1 F

1k

+5V

PHASE

SPLITTER

IBBP

IBBN

QBBP

QBBN

AD8346

LOIP

LOIN

VPS1

VPS2

1V p-p EACH PIN

+5V

0.01 F

0.01 F

0.01 F

0.01 F

"I"

DAC

2.43k

2.43k

2.43k

2.43k

2.43k

2.43k

2.43k

2.43k

"Q"

DAC

Figure 24. AC-Coupled DAC Interface

The network shown has a high-pass corner frequency of

approximately 14.3 kHz (note that the 12 k

Ω input imped-

ance of the AD8346 has been factored into this calcula-

tion). Increasing the resistors in the network or increasing

the coupling capacitance will reduce the corner frequency

further.

Note that the LO suppression can be manually optimized

by replacing a portion of the four “top” 2.43 k

Ω resistors

with potentiometers. In this case, the “bottom” four resis-

tors in the biasing network would no longer need to be

precision devices.