Datasheet

Data Sheet AD9119/AD9129
Rev. A | Page 51 of 68
VOLTAGE REFERENCE
The AD9119/AD9129 output current is set by a combination of
digital control bits and the I250U reference current, as shown in
Figure 149.
CURRENT
SCALING
FSC[9:0]
AD9129
DAC
I
FULLSCALE
4kΩ
1nF
VREF
I250
V
SSA
I250U
V
BG
1.0V
+
–
11149-153
Figure 149. Voltage Reference Circuit
The reference current is obtained by forcing the band gap
voltage across an external 4.0 kΩ resistor from I250U (Pin A1)
to VSSA. The 1.0 V nominal band gap voltage (VREF) generates a
250 µA reference current in the 4.0 kΩ resistor. Note the
following constraints when configuring the voltage reference
circuit:
• Both the 4.0 kΩ resistor and 1 nF bypass capacitor are
required for proper operation.
• Adjusting the DAC output full-scale current, I
OUTFS
, from
its default setting of 20 mA should be performed digitally.
• The AD9119/AD9129 are not multiplying DACs. Modula-
tion of the reference current, I250U, with an ac signal is not
supported.
• The band gap voltage appearing at the VREF pin must be
buffered for use with an external circuitry because its output
impedance is approximately 7.5 kΩ.
• An external reference can be used to overdrive the internal
reference by connecting it to the VREF pin.
As mentioned, the I
OUTFS
can be adjusted digitally over a 9.4 mA
to 34.2 mA range by the FSC_x[9:0] bits (Register 0x20, Bits[7:0]
and Register 0x21, Bits[1:0]). The following equation relates
I
OUTFS
to the FSC_x[9:0] bits, which can be set from 0 to 1023.
I
OUTFS
= 24.21875 mA × FSC_x[9:0]/1000 + 9.4 mA (1)
Note that the default value of 0x200 generates 21.937 mA full scale,
but most of the characterization presented in this datasheet uses
33 mA, unless noted otherwise.
ANALOG OUTPUTS
Equivalent DAC Output and Transfer Function
The AD9119/AD9129 provide complementary current outputs,
IOUTP and IOUTN, that sink current from an external load
that is referenced to the 1.8 V VDDA supply. Figure 150 shows
an equivalent output circuit for the DAC. Compared to most
current output DACs of this type, the outputs of the AD9119/
AD9129 exhibit a slight offset current (that is, I
OUTFS
/17), and the
peak differential ac current is slightly below I
OUTFS
/2 (that is,
8/17 × I
OUTFS
).
(9/17) × I
OUTFS
I
PEAK
=
(8/17) × I
OUTFS
I
OUTFS
= 9.5mA – 34mA
(9/17) × I
OUTFS
AC
11149-154
Figure 150. Equivalent DAC Output Circuit
The example shown in Figure 150 can be modeled as a pair of
dc current sources that source a current of 9/17 × I
OUTFS
to each
output. A differential ac current source, I
PEAK
, is used to model
the signal (that is, a digital code) dependent nature of the DAC
output. The polarity and signal dependency of this ac current
source is related to the digital code (F) by the following equation:
F (code) = (DACCODE − 8192)/8192 (2)
−1 < F (code) < +1 (3)
where DACCODE = 0 to 16,383 (decimal).
Because I
PEAK
can swing ±(8/17) × I
OUTFS
, the output currents
that are measured at IOUTP and IOUTN can span from
I
OUTFS
/17 to I
OUTFS
. However, because the ac signal-dependent
current component is complementary, the sum of the two
outputs is always constant (that is, IOUTP + IOUTN =
(18/17) × I
OUTFS
).
The code-dependent current that is measured at the IOUTP
(and IOUTN) output is as follows:
IOUTP = (9/17) × I
OUTFS
(mA) + (8/17) × I
OUTFS
(mA)
× F (code) (4)
IOUTN = (9/17) × I
OUTFS
(mA) − (8/17) × I
OUTFS
(mA)
× F (code)
Figure 151 shows the IOUTP vs. DACCODE transfer function
when I
OUTFS
is set to 19.65 mA.
20
18
10
12
14
16
OUTPUT CURRENT (mA)
8
6
4
2
0
0 4096 8192 12,288
DAC CODE
16,384
11149-155
Figure 151. Gain Curve for FSC_x[9:0] = 512, DAC Offset = 1.228 mA