Datasheet

I =I +I
OUTFS OUT
OUT
I =I
OUT OUTFS
´
Code
16,384
I
OUT
=I (16,383 )-
OUTFS
´
Code
16,384
I =32 I =32
OUTFS REF
´ ´
V
R
REF
SET
V =I R
OUT OUT LOAD
´
= I
OUT
V
OUT
´ R
LOAD
V =V =-
OUTDIFF OUT
´ ´I R
OUTFS LOAD
(2 Code 16,383)
16,384
-
´
V
OUT
DAC2904
www.ti.com .............................................................................................................................................. SBAS198C –AUGUST 2001–REVISED OCTOBER 2009
APPLICATION INFORMATION
THEORY OF OPERATION DAC TRANSFER FUNCTION
The architecture of the DAC2904 uses the current Each of the DACs in the DAC2904 has a set of
steering technique to enable fast switching and a high complementary current output, I
OUT
and I
OUT
. The
update rate. The core element within the monolithic full-scale output current, I
OUTFS
, is the summation of
DAC is an array of segmented current sources that the two complementary output currents:
are designed to deliver a full-scale output current of
(1)
up to 20mA, as shown in Figure 18. An internal
decoder addresses the differential current switches The individual output currents depend on the DAC
each time the DAC is updated and a corresponding code and can be expressed as:
output current is formed by steering all currents to
either output summing node, I
OUT
or I
OUT
. The
(2)
complementary outputs deliver a differential output
signal, which improves the dynamic performance
through reduction of even-order harmonics,
(3)
common-mode signals (noise), and double the
where Code is the decimal representation of the DAC
peak-to-peak output signal swing by a factor of two,
data input word. Additionally, I
OUTFS
is a function of
compared to single-ended operation.
the reference current I
REF
, which is determined by the
The segmented architecture results in a significant
reference voltage and the external setting resistor,
reduction of the glitch energy, improves the dynamic
R
SET
.
performance (SFDR), and DNL. The current outputs
maintain a very high output impedance of greater
than 200kΩ. (4)
The full-scale output current is determined by the In most cases the complementary outputs will drive
ratio of the internal reference voltage (1.25V) and an resistive loads or a terminated transformer. A signal
external resistor, R
SET
. The resulting I
REF
is internally voltage will develop at each output according to:
multiplied by a factor of 32 to produce an effective
(5)
DAC output current that can range from 2mA to
(6)
20mA, depending on the value of R
SET
.
The value of the load resistance is limited by the
The DAC2904 is split into a digital and an analog
output compliance specification of the DAC2904. To
portion, each of which is powered through its own
maintain specified linearity performance, the voltage
supply pin. The digital section includes edge-triggered
for I
OUT
and I
OUT
should not exceed the maximum
input latches and the decoder logic, while the analog
allowable compliance range.
section comprises the current source array with its
associated switches, and the reference circuitry.
The two single-ended output voltages can be
combined to find the total differential output swing:
(7)
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