AD9744ACP-PCB Analog Devices Inc, AD9744ACP-PCB Datasheet - Page 13

• Manufacturer Part Number: AD9744ACP-PCB
• Description: BOARD EVAL FOR AD9744ACP
• Manufacturer: Analog Devices Inc
• Series: TxDAC®r

Specifications of AD9744ACP-PCB

• Rohs Status: RoHS non-compliant
• Number Of Dac's: 1
• Number Of Bits: 14
• Outputs And Type: 1, Differential
• Sampling Rate (per Second): 210M
• Data Interface: Parallel
• Settling Time: 11ns
• Dac Type: Current
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9744

FUNCTIONAL DESCRIPTION
Figure 24 shows a simplified block diagram of the AD9744. The
AD9744 consists of a DAC, digital control logic, and full-scale
output current control. The DAC contains a PMOS current
source array capable of providing up to 20 mA of full-scale cur-
rent (I
make up the five most significant bits (MSBs). The next four
bits, or middle bits, consist of 15 equal current sources whose
value is 1/16th of an MSB current source. The remaining LSBs
are binary weighted fractions of the middle bits current sources.
Implementing the middle and lower bits with current sources,
instead of an R-2R ladder, enhances its dynamic performance
for multitone or low amplitude signals and helps maintain the
DAC’s high output impedance (that is, >100 kΩ).
All of these current sources are switched to one or the other of
the two output nodes, that is, IOUTA or IOUTB, via PMOS
differential current switches. The switches are based on the
architecture that was pioneered in the AD9764 family, with
further refinements to reduce distortion contributed by the
switching transient. This switch architecture also reduces vari-
ous timing errors and provides matching complementary drive
signals to the inputs of the differential current switches.
The analog and digital sections of the AD9744 have separate
power supply inputs, that is, AVDD and DVDD, that can
operate independently over a 2.7 V to 3.6 V range. The
digital section, which is capable of operating at a rate of up
to 210 MSPS, consists of edge-triggered latches and segment
decoding logic circuitry. The analog section includes the PMOS
current sources, the associated differential switches, a 1.2 V
band gap voltage reference, and a reference control amplifier.
The DAC full-scale output current is regulated by the reference
control amplifier and can be set from 2 mA to 20 mA via an
external resistor, R
(FS ADJ) pin. The external resistor, in combination with both
the reference control amplifier and voltage reference V
the reference current I
current sources with the proper scaling factor. The full-scale
current, I
REFERENCE OPERATION
The AD9744 contains an internal 1.2 V band gap reference. The
internal reference cannot be disabled, but can be easily overrid-
den by an external reference with no effect on performance.
Figure 25 shows an equivalent circuit of the band gap reference.
REFIO serves as either an output or an input depending on
whether the internal or an external reference is used. To use the
internal reference, simply decouple the REFIO pin to ACOM
with a 0.1 µF capacitor and connect REFLO to ACOM via a
resistance less than 5 Ω. The internal reference voltage will be
present at REFIO. If the voltage at REFIO is to be used any-
where else in the circuit, an external buffer amplifier with an
OUTFS
OUTFS
). The array is divided into 31 equal currents that
, is 32 times I
SET
, connected to the full-scale adjust
REF
, which is replicated to the segmented
REF
.
REFIO
, sets
Rev. B | Page 13 of 32
input bias current of less than 100 nA should be used. An ex-
ample of the use of the internal reference is shown in Figure 26.
ADDITIONAL
An external reference can be applied to REFIO, as shown in
Figure 27. The external reference may provide either a fixed
reference voltage to enhance accuracy and drift performance or
a varying reference voltage for gain control. Note that the 0.1 µF
compensation capacitor is not required since the internal refer-
ence is overridden, and the relatively high input impedance of
REFIO minimizes any loading of the external reference.
REFERENCE CONTROL AMPLIFIER
The AD9744 contains a control amplifier that is used to regulate
the full-scale output current, I
configured as a V-I converter, as shown in Figure 26, so that its
current output, I
an external resistor, R
to the segmented current sources with the proper scale factor to
set I
EXTERNAL
AVDD
REF
OUTFS
LOAD
, as stated in Equation 3.
R
REF BUFFER
EXTERNAL
OPTIONAL
SET
Figure 25. Equivalent Circuit of Internal Reference
Figure 27. External Reference Configuration
Figure 26. Internal Reference Configuration
V
REFIO
REF
I
V
REF
REFIO
, is determined by the ratio of the V
0.1µF
=
2kΩ
84µA
SET
/R
SET
, as stated in Equation 4. I
REFLO
AVDD
REFIO
FS ADJ
REFIO
FS ADJ
+1.2V REF
AD9744
OUTFS
AD9744
7kΩ
+1.2V REF
REFLO
. The control amplifier is
REFIO
REFLO
150pF
150pF
REFERENCE
CONTROL
AMPLIFIER
REF
CURRENT
CURRENT
SOURCE
SOURCE
ARRAY
ARRAY
AD9744
3.3V
3.3V
is copied
AVDD
AVDD
REFIO
and