AD9785 Analog Devices, AD9785 Datasheet - Page 50

• Manufacturer Part Number: AD9785
• Description: Dual 12-Bit 800 MSPS DAC with Low Power 32-Bit Complex NCO
• Manufacturer: Analog Devices

Specifications of AD9785

• Resolution (bits): 12bit
• Dac Update Rate: 800MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

AD9785/AD9787/AD9788
ANALOG OUTPUTS
Full-scale current on the I DAC and Q DAC can be set from
8.66 mA to 31.66 mA. Initially, the 1.2 V band gap reference is
used to set up a current in an external resistor connected to
I120 (Pin 75). A simplified block diagram of the reference
circuitry is shown in Figure 69.
The recommended value for the external resistor is 10 kΩ,
which sets up an I
turn provides a DAC output full-scale current of 20 mA. Because
the gain error is a linear function of this resistor, a high precision
resistor improves gain matching to the internal matching
specification of the devices. Internal current mirrors provide
a current-gain scaling, where DAC gain is a 10-bit word in the
SPI port register (Register 0x05 and Register 0x07). The default
value for the DAC gain registers gives an I
20 mA, where I
0.1µF
1
.
2
R
35
30
25
20
15
10
VREF
V
5
0
10kΩ
I120
0
Figure 70. DAC Full-Scale Current vs. DAC Gain Code

Figure 69. Full-Scale Current Generation Circuitry



12
27
FS
for either I DAC or Q DAC is equal to

REFERENCE
5kΩ
200
1.2V BAND GAP
REFERENCE



1024
AD9788
6

in the resistor of 120 μA, which in
DAC GAIN CODE
DAC gain
400
Q DAC GAIN
I DAC GAIN
CURRENT
SCALING
600







FS
32
of approximately
Q DAC
I DAC
800
DAC FULL-SCALE
REFERENCE
CURRENT
1000
Rev. A | Page 50 of 64
DIGITAL AMPLITUDE SCALING
Gain scaling of the analog DAC output can be achieved by
changing the values in Register 0x05 and Register 0x07.
However, if this is done, the output common-mode voltage at
the analog output also decreases proportionally. This poses a
problem when the AD9785/AD9787/AD9788 are dc-coupled to
a quadrature modulator. Typical quadrature modulators have
tight restrictions on input common-mode variation.
The AD9785/AD9787/AD9788 use a digital gain scaling block
to get around this problem. Because the gain scaling is done in
the digital processing of the AD9785/AD9787/AD9788, there is
no effect on the output full-scale current. This digital gain
scaling is done in such a way that the midscale value of the
signal is unaffected; the swing of the signal around midscale is
the value that is adjusted with the register settings. Digital gain
scaling is done using the amplitude scale factor (ASF) register
(Register 0x0C).
Auxiliary DAC Operation
Two auxiliary DACs are provided on the AD9785/AD9787/
AD9788. The full-scale output current on these DACs is derived
from the 1.2 V band gap reference and external resistor. The
gain scale from the reference amplifier current, I
auxiliary DAC reference current is 16.67 with the auxiliary DAC
gain set to full scale (10-bit values, Register 0x06, Bits [9:0] and
Register 0x08, Bits [9:0]). This gives a full-scale current of approx-
imately 2 mA for Auxiliary DAC 1 and Auxiliary DAC 2.
The auxiliary DAC outputs are not differential. Only one side of
the auxiliary DAC (P or N) is active at one time. The inactive side
goes into a high impedance state (100 kΩ). In addition, the P or N
output can act as a current source or a current sink. Control of
the P and N sides for both auxiliary DACs is via Register 0x06 and
Register 0x08, Bits [15:14]. 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.
REFERENCE
, to the