Datasheet

DAC908
10
SBAS109B
APPLICATION INFORMATION
THEORY OF OPERATION
The architecture of the DAC908 uses the current steering
technique to enable fast switching and a high update rate. The
core element within the monolithic DAC is an array of
segmented current sources that are designed to deliver a full-
scale output current of up to 20mA, as shown in Figure 1. An
internal decoder addresses the differential current switches
each time the DAC is updated and a corresponding output
current is formed by steering all currents to either output
summing node, I
OUT
or I
OUT
. The complementary outputs
deliver a differential output signal that improves the dynamic
performance through reduction of even-order harmonics,
common-mode signals (noise), and double the peak-to-peak
output signal swing by a factor of two, compared to single-
ended operation.
The segmented architecture results in a significant reduc-
tion of the glitch energy, and improves the dynamic perfor-
mance (SFDR) and DNL. The current outputs maintain a
very high output impedance of greater than 200kΩ.
The full-scale output current is determined by the ratio of
the internal reference voltage (1.24V) and an external
resistor, R
SET
. The resulting I
REF
is internally multiplied by
a factor of 32 to produce an effective DAC output current
that can range from 2mA to 20mA, depending on the value
of R
SET
.
The DAC908 is split into a digital and an analog portion,
each of which is powered through its own supply pin. The
digital section includes edge-triggered input latches and the
decoder logic, while the analog section comprises the cur-
rent source array with its associated switches and the
reference circuitry.
DAC TRANSFER FUNCTION
The total output current, I
OUTFS
, of the DAC908 is the
summation of the two complementary output currents:
I
OUTFS
= I
OUT
+ I
OUT
(1)
The individual output currents depend on the DAC code and
can be expressed as:
I
OUT
= I
OUTFS
• (Code/256) (2)
I
OUT
= I
OUTFS
• (255 – Code/256) (3)
where ‘Code’ is the decimal representation of the DAC data
input word. Additionally, I
OUTFS
is a function of the refer-
ence current I
REF
, which is determined by the reference
voltage and the external setting resistor, R
SET
.
I
OUTFS
= 32 • I
REF
= 32 • V
REF
/R
SET
(4)
In most cases the complementary outputs will drive resistive
loads or a terminated transformer. A signal voltage will
develop at each output according to:
V
OUT
= I
OUT
• R
LOAD
(5)
V
OUT
= I
OUT
• R
LOAD
(6)
FIGURE 1. Functional Block Diagram of the DAC908.
PMOS
Current
Source
Array
LSB
Switches
Segmented
MSB
Switches
+1.24V Ref
Latches and Switch
Decoder Logic
8-Bit Data Input
D7...D0
DAC908
Full-Scale
Adjust
Resistor
Ref
Control
Amp
Ref
Buffer
BW
+V
D
+V
A
R
SET
2kΩ
CLK
DGND
Ref
Input
0.1µF
INT/EXT
I
OUT
I
OUT
BYP
PD
20pF
50Ω
50Ω
20pF
1:1
V
OUT
0.1µF
400pF
0.1µF
+3V to +5V
Analog
Bandwidth
Control
+3V to +5V
Digital
FSA
REF
IN
AGND
Analog
Ground
Digital
Ground
Power Down
(internal pull-down)
Clock
Input
NOTE: Supply bypassing not shown.