DAC900 Datasheet, PDF (11/16 Pages) Burr-Brown (TI) – 10-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER

APPLICATION INFORMATION
THEORY OF OPERATION
The architecture of the DAC900 uses the current steering
technique to enable fast switching and a high update rate.
The core element within the monolithic D/A converter is an
array of segmented current sources, which are designed to
deliver a full-scale output current of up to 20mA (see
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, which 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 DAC900 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 DAC900 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/1024)
(2)
I_OUT = I_OUTFS • (1023 - Code/1024)
(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)
Full-Scale
Adjust
Resistor
R_SET
2kΩ
+3V to +5V
Analog
0.1µF
+3V to +5V
Digital
Bandwidth
Control
DAC900 +V_A
BW
+V_D
I_OUT
FSA
Ref
Input REF_IN
Ref
Control
Amp
0.1µF
INT/EXT _Ref
400pF
PMOS
Current
Source
Array
LSB
Switches
I_OUT
Segmented
MSB
Switches
50Ω
0.1µF
BYP
20pF 50Ω
Buffer
Latches and Switch
+1.24V Ref
Decoder Logic
PD
Power Down
(internal pull-down)
AGND
CLK
10-Bit Data Input _DGND
Analog
Ground
Clock
Input
D9...D0
Digital
Ground
1:1
V_OUT
20pF
NOTE: Supply bypassing not shown.
FIGURE 1. Functional Block Diagram of the DAC900.
11
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DAC900