AD9780 Analog Devices, AD9780 Datasheet - Page 27

• Manufacturer Part Number: AD9780
• Description: Dual 12-Bit, LVDS Interface 500 MSPS DAC
• Manufacturer: Analog Devices

Specifications of AD9780

• Resolution (bits): 12bit
• Dac Update Rate: 500MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

DAC TRANSFER FUNCTION
Each DAC output of the AD9780/AD9781/AD9783 drives two
complementary current outputs, I
a near I
where N = 12/14/16 bits for AD9780/AD9781/AD9783
(respectively), while I
The current output appearing at I
both the input code, and I
where DAC DATA = 0 to 2
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, I
should be connected to matching resistive loads (R
tied to analog common (AVSS). The single-ended voltage
output appearing at the I
Note that to achieve the maximum output compliance of 1 V at
the nominal 20 mA output current, R
Also note that the full-scale value of V
not exceed the specified output compliance range to maintain
specified distortion and linearity performance.
There are two distinct advantages to operating the AD9780/
AD9781/AD9783 differentially. First, differential operation
helps cancel common-mode error sources associated with I
and I
differential code-dependent current and subsequent output
voltage (V
output (V
DAC CODE = 2
I
I
V
V
V
OUTN
OUTP
OUTN
35
30
25
20
15
10
OUTP
OUTN
DIFF
5
FS
0
, such as noise, distortion, and dc offsets. Second, the
when all bits are high. For example,
OUTP
= (DAC DATA/2
= ((2
= (I
DIFF
= I
= I
) is twice the value of the single-ended voltage
OUTP
OUTN
OUTP
or V
N
− 1) − DAC DATA)/2
× R
– I
× R
OUTN
Figure 63. I
256
OUTN
N
OUTN
LOAD
LOAD
− 1
), providing 2× signal power to the load.
FS
provides no current.
OUTP
) × R
N
DAC GAIN CODE
and can be expressed as
N
− 1 (decimal representation).
FS
) × I
and I
vs. DAC Gain Code
LOAD
512
OUTP
FS
OUTP
OUTN
and I
LOAD
and I
OUTP
N
pins is
× I
OUTN
must be set to 50 Ω.
and V
FS
768
OUTN
is a function of
. I
OUTP
LOAD
OUTN
OUTP
) that are
and I
should
provides
1024
OUTN
OUTP
Rev. A | Page 27 of 36
(1)
(2)
(3)
(4)
(5)
ANALOG MODES OF OPERATION
The AD9780/AD9781/AD9783 use a proprietary quad-switch
architecture that lowers the distortion of the DAC by eliminating a
code-dependent glitch that occurs with conventional dual-switch
architectures. This architecture eliminates the code-dependent
glitches, but creates a constant glitch at a rate of 2 × f
communications systems and other applications requiring good
frequency domain performance from the DAC, this is seldom
problematic.
The quad-switch architecture also supports two additional
modes of operation: mix mode and return-to-zero mode. The
waveforms of these two modes are shown in Figure 64. In mix
mode, the output is inverted every other half clock cycle. This
effectively chops the DAC output at the sample rate. This chop-
ping has the effect of frequency shifting the sinc roll-off from dc
to f
spectrum. The shifted spectrum is also shaped by a second sinc
function with a first null at 2 × f
is that the data is not continuously varying at twice the clock
rate, but is simply repeated.
In return-to-zero mode, the output is set to midscale every
other half clock cycle. The output is similar to the DAC output
in normal mode except that the output pulses are half the width
and half the area. Because the output pulses have half the width,
the sinc function is scaled in frequency by two and has a first
null at 2 × f
pulses in normal mode, the output power is half the normal
mode output power.
The functions that shape the output spectrums for the three
modes of operation, normal mode, mix mode, and return-to-
zero mode, are shown in Figure 65. Switching between the
analog modes reshapes the sinc roll-off inherent at the DAC
output. This ability to change modes in the AD9780/AD9781/
AD9783 makes the parts suitable for direct IF applications. The
user can place a carrier anywhere in the first three Nyquist
zones depending on the operating mode selected. The perfor-
mance and maximum amplitude in all three Nyquist zones is
QUAD-SWITCH
QUAD-SWITCH
(
f
DAC OUTPUT
DAC OUTPUT
(RETURN-TO-
ZERO MODE)
S
MIX MODE)
DAC
Figure 64. Mix Mode and Return-to-Zero Mode DAC Waveforms
. Additionally, there is a second subtle effect on the output
INPUT DATA
DAC CLK
DAC
. Because the area of the pulses is half that of the
D
1
D
2
AD9780/AD9781/AD9783
D
3
D
4
DAC
D
5
. The reason for this shaping
D
6
D
7
D
8
D
9
D
10
DAC
. For
t
t