ad80164absvz Analog Devices, Inc., ad80164absvz Datasheet - Page 39

• Manufacturer Part Number: ad80164absvz
• Description: Dual, 12-/14-/16-bit,1 Gsps Digital-to-analog Converters
• Manufacturer: Analog Devices, Inc.

0.1µF
Application of Auxiliary DACs in Single Sideband
Transmitter
Two auxiliary DACs are provided on the AD9776A/AD9778A/
AD9779A. The full-scale output current on these DACs is derived
from the 1.2 V band gap reference and external resistor between
the I120 pin and ground. The gain scale from the reference
amplifier current I
current is 16.67 with the auxiliary DAC gain set to full scale
(10-bit values, SPI Register 0x0D, and SPI Register 0x11), this
gives a full-scale current of approximately 2 mA for auxiliary
DAC1 and auxiliary DAC2.
The AUX DAC structure is shown in Figure 78. Only one of
the two output pins of the AUX DAC is active at a time. The
inactive side goes to a high impedance state (>100 kΩ). The
active output pin is chosen by writing to Register 0x0E and
Register 0x10, Bit 7.
The active output can act as either a current source or a current
sink. When sourcing current, the output compliance voltage is
0 V to 1.6 V. When sinking current, the output compliance
voltage is 0.8 V to 1.6 V. The output pin is chosen to be a
current source or current sink by writing to Register 0x0E
and Register 0x10, Bit 6.
35
30
25
20
15
10
VREF
5
0
10kΩ
I120
0
AD9779A
200
REFERENCE
1.2V BAND GAP
Figure 77. I
Figure 76. Reference Circuitry
to the auxiliary DAC reference
DAC GAIN CODE
400
FS
vs. DAC Gain Code
Q DAC GAIN
I DAC GAIN
CURRENT
SCALING
600
Q DAC
I DAC
800
DAC FULL-SCALE
REFERENCE
CURRENT
1000
Rev. 0 Page 39 of 68
The magnitude of the AUX DAC 1 current is controlled by the
AUX DAC 1 Control Register 0x06, and the magnitude of the
AUX DAC 2 current is controlled by the AUX DAC 2 Control
Register 0x08. These AUX DACs have the ability to source or
sink current. This is programmable via Bit 14 in either AUX
DAC control register. The choice of sinking or sourcing should
be made at circuit design time. There is no advantage to switch-
ing between source or sinking current once the circuit is in place.
The auxiliary DACs can be used for local oscillator (LO) cancella-
tion when the DAC output is followed by a quadrature modulator.
This LO feedthrough is caused by the input referred dc offset
voltage of the quadrature modulator (and the DAC output offset
voltage mismatch) and can degrade system performance. Typical
DAC-to-quadrature modulator interfaces are shown in Figure 79
and Figure 80. Often, the input common-mode voltage for the
modulator is much higher than the output compliance range of
the DAC, so that ac coupling or a dc level shift is necessary. If the
required common-mode input voltage on the quadrature modu-
lator matches that of the DAC, then the dc blocking capacitors in
Figure 79 can be removed. A low-pass or band-pass passive filter
is recommended when spurious signals from the DAC (distortion
and DAC images) at the quadrature modulator inputs can affect
the system performance. Placing the filter at the location shown
in Figure 79 and Figure 80 allows easy design of the filter, as the
source and load impedances can easily be designed close to 50 Ω.
AD9779A
25Ω TO 50Ω
I DAC
Figure 78. Auxiliary DAC Structure on AD9776A/AD9778A/AD97779A
Figure 79. Typical Use of Auxiliary DACs AC Coupling to
(SOURCE)
0.1µF
0.1µF
0 TO 2mA
0 TO 2mA
AD9779A
25Ω TO 50Ω
Q DAC
(SINK)
AD9776A/AD9778A/AD9779A
V
BIAS
FILTERING
OPTIONAL
Quadrature Modulator
PASSIVE
SOURCE/
MODULATOR V+
SINC
QUADRATURE
0.1µF
0.1µF
P/N
OPTIONAL
FILTERING
AD9779A
PASSIVE
QUAD MOD
I INPUTS
DAC1
AUX
AUXP
AUXN
MODULATOR V+
QUADRATURE
AD9779A
QUAD MOD
Q INPUTS
DAC2
AUX