LTC1668 LINER [Linear Technology], LTC1668 Datasheet - Page 6

• Manufacturer Part Number: LTC1668
• Description: 16-Bit, 50Msps DAC
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1668
Theory of Operation
The LTC1668 is a high speed current steering 16-Bit DAC
made on an advanced BiCMOS process. Precision thin
film resistors and well matched bipolar transistors result
in excellent DC linearity and stability. A low glitch current
switching design gives excellent AC performance at sample
rates up to 50Msps. The device is complete with a 2.5V
internal bandgap reference and edge triggered latches,
and sets a new standard for DAC applications requiring
very high dynamic range at output frequencies up to
several megahertz.
Referring to the Block Diagram, the DAC contains an array
of current sources that are steered to I
NMOS differential current switches. The four most signifi-
cant bits, DB15 to DB12 are made up of 15 current
segments of equal weight. The lower bits, DB11 to DB0 are
binary weighted, using a combination of current scaling
and a differential resistive attenuator ladder. All bits and
segments are precisely matched, both in current weight
for DC linearity, and in switch timing for low glitch impulse
and low spurious tone AC performance.
Setting the Full-Scale Current, I
The full-scale DAC output current, I
10mA, and can be adjusted down to 1mA. Placing a
resistor, R
sets I
The internal reference control loop amplifier maintains a
virtual ground at I
source, I
I
I
For example, if R
2.5V, I
= 10mA.
The reference control loop requires a capacitor on the
COMP1 pin for compensation. For optimal AC perfor-
mance, C
very close to the package (less than 0.1").
6
INT
OUTFS
I
OUTFS
is a scaled replica of the DAC current sources and
OUTFS
REFIN
= 8 • (I
INT
COMP1
= 8 • (I
SET
as follows.
, to sink the exact current flowing into I
= 2.5/2k = 1.25mA and I
, between the REFOUT pin, and the I
INT
should be connected to V
), therefore:
REFIN
SET
REFIN
U
= 2k and is tied to V
) = 8 • (V
by servoing the internal current
U
REF
OUTFS
/R
W
OUTFS
SET
OUTFS
OUTA
)
SS
REF
= 8 • (1.25mA)
, is nominally
and be placed
or I
= REFOUT =
U
OUTB
REFIN
REFIN
with
pin
(1)
.
For fixed reference voltage applications, C
be 0.1 F or more. The reference control loop small-signal
bandwidth is approximately 1/(2 ) • C
for C
Internal Reference Output—REFOUT
The onboard 2.5V bandgap voltage reference drives the
REFOUT pin. It is trimmed and specified to drive a 2k
resistor tied from REFOUT to I
1.25mA load (I
output impedance of 6 , or 0.24% per mA, so it must be
buffered to drive any additional external load. A 0.1 F
capacitor is required on the REFOUT pin for compensa-
tion. Note that this capacitor is required for stability, even
if the internal reference is not being used.
DAC Transfer Function
The LTC1668 uses straight binary digital coding. The
complementary current outputs, I
current from 0 to I
I
0) to 10mA when all bits are high (i.e., Code = 65535) (deci-
mal representation). I
I
In typical applications, the LTC1668 differential output
currents either drive a resistive load directly or drive an
equivalent resistive load through a transformer, or as the
feedback resistor of an I-to-V converter. The voltage
outputs generated by the I
are then:
The differential voltage is:
OUT A
OUT A
I
I
V
V
V
OUT A
OUT B
OUT A
OUT B
DIFF
COMP1
swings from 0mA when all bits are low (i.e., Code =
and I
= V
= (I
= I
= I
= I
= I
OUT B
= 0.1 F.
OUT A
OUTFS
OUTFS
OUT A
OUT A
OUT B
OUTFS
are given by the following formulas:
– V
– I
• (DAC Code/65536)
• (65535-DAC Code)/65536
• R
• R
OUTFS
OUT B
OUT B
LOAD
LOAD
OUT B
= 10mA). REFOUT has nominal
. For I
) • (R
OUT A
is complementary to I
OUTFS
LOAD
REFIN
and I
OUT A
)
OUT B
, corresponding to a
COMP1
= 10mA (nominal),
and I
output currents
COMP1
• 80 or 20kHz
OUT B
should
OUT A
, sink
(2)
(3)
(4)
(5)
(6)
.