AD9739 Analog Devices, AD9739 Datasheet - Page 40

• Manufacturer Part Number: AD9739
• Description: 14-Bit, 2500 MSPS, RF Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9739

• Resolution (bits): 14bit
• Dac Update Rate: 2.5GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9739
VOLTAGE REFERENCE
The
control bits and the I120 reference current, as shown in Figure 62.
The reference current is obtained by forcing the band gap voltage
across an external 10 kΩ resistor from I120 (Pin B14) to ground.
The 1.2 V nominal band gap voltage (VREF) generates a 120 μA
reference current in the 10 kΩ resistor. Note the following
constraints when configuring the voltage reference circuit:
•
•
•
•
•
I
FSC[9:0] (Register 0x06 and Register 0x07).
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AD9739
Both the 10 kΩ resistor and 1 nF bypass capacitor are required
for proper operation.
Adjusting the output full-scale current, I
from its default setting of 20 mA should be performed
digitally.
The
reference current, I120, with an ac signal is not supported.
The band gap voltage appearing at VREF (Pin C14) must
be buffered for use with external circuitry because its
output impedance is approximately 5 kΩ.
An external reference can be used to overdrive the internal
reference by connecting it to VREF (Pin C14).
can be adjusted digitally over 8.7 mA to 31.7 mA by using
1.10
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
1nF
AD9739
–15 –13 –11 –9 –7 –5 –3 –1
AGND
Figure 61. Common-Mode Voltage with Respect to
CLKP_OFFSET/CLKN_OFFSET and DIR_P/DIR_N
10kΩ
output current is set by a combination of digital
VREF
I120
Figure 62. Voltage Reference Circuit
is not a multiplying DAC. Modulating the
I120
V
1.2V
BG
AD9739
–
+
OFFSET CODE
CURRENT
SCALING
FSC[9:0]
1
3
5
IFULL-SCALE
7
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DAC
9
, of the DAC
11 13 15
CLKP
CLKN
Rev. B | Page 40 of 48
The following equation relates I
which can be set from 0 to 1023:
Note that a default value of 0x200 generates 20 mA full scale,
which is used for most of the characterization presented in this
data sheet (unless noted otherwise).
ANALOG OUTPUTS
Equivalent DAC Output and Transfer Function
The
and IOUTN, that source current into an external ground reference
load. Figure 63 shows an equivalent output circuit for the DAC.
Note that, compared to most current output DACs of this type,
the
I
I
As shown in Figure 63, the DAC output can be modeled as a pair of
dc current sources that source a current of 17/32 × I
output. A differential ac current source, I
the signal-dependent nature of the DAC output. The polarity
and signal dependency of this ac current source are related to
the digital code by the following equations:
where DACCODE = 0 to 16,383 (decimal).
Because I
measured at IOUTP and IOUTN can span from I
I
component is complementary, the sum of the two outputs is
always constant (that is, IOUTP + IOUTN = (34/32) × I
The code-dependent current measured at the IOUTP (and
IOUTN) output is as follows:
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AD9739
AD9739
I
F(Code) = (DACCODE − 8192)/8192
−1 ≤ F(Code) < 1
IOUTP = 17/32 × I
IOUTN = 17/32 × I
/16), and the peak differential ac current is slightly below
/2 (that is, 15/32 × I
. However, because the ac signal-dependent current
OUTFS
PEAK
= 22.6 × FSC[9:0]/1000 + 8.7
outputs exhibit a slight offset current (that is,
provides complementary current outputs, IOUTP
can swing ±(15/32) × I
Figure 63. Equivalent DAC Output Circuit
15/32 × I
I
PEAK
I
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OUTFS
=
17/32 × I
17/32 × I
OUTFS
OUTFS
OUTFS
= 8.6 – 31.2mA
+ 15/32 × I
− 15/32 × I
).
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OUTFS
OUTFS
AC
OUTFS
to the FSC[9:0] register,
70Ω
OUTFS
OUTFS
PEAK,
, the output currents
2.2pF
× F(Code)
× F(Code)
is used to model
Data Sheet
OUTFS
OUTFS
/16 to
OUTFS
to each
).
(1)
(4)
(5)
(2)
(3)