LTC1668IG#PBF Linear Technology, LTC1668IG#PBF Datasheet - Page 11

• Manufacturer Part Number: LTC1668IG#PBF
• Description: IC D/A CONV 16BIT 50MSPS 28-SSOP
• Manufacturer: Linear Technology

Specifications of LTC1668IG#PBF

• Settling Time: 20ns
• Number Of Bits: 16
• Data Interface: Parallel
• Number Of Converters: 1
• Voltage Supply Source: Dual ±
• Power Dissipation (max): 180mW
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Resolution (bits): 16bit
• Sampling Rate: 50MSPS
• Input Channel Type: Parallel
• Supply Voltage Range - Analog: ± 4.75V To ± 5.25V
• Supply Current: 33mA
• Digital Ic Case Style: SSOP
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIO S I FOR ATIO
Theory of Operation
The LTC1666/LTC1667/LTC1668 are high speed current
steering 12-/14-/16-bit DACs 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 devices are complete with a 2.5V internal bandgap
reference and edge triggered latches, and set a new
standard for DAC applications requiring very high dy-
namic range at output frequencies up to several mega-
hertz.
Referring to the Block Diagrams, the DACs contain an
array of current sources that are steered to I
with NMOS differential current switches. The four most
significant bits are made up of 15 current segments of
equal weight. The remaining lower bits 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.
INT
OUTFS
I
OUTFS
is a scaled replica of the DAC current sources and
OUTFS
REFIN
= 8 • (I
INT
= 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
), 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
)
REF
= 8 • (1.25mA)
, is nominally
OUTA
= REFOUT =
U
REFIN
or I
REFIN
OUTB
pin
(1)
.
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").
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
Reference Operation
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.
External Reference Operation
Figure 1, shows how to use an external reference to control
the LTC1666/LTC1667/LTC1668 full-scale current.
COMP1
LTC1666/LTC1667/LTC1668
Figure 1. Using the LTC1666/LTC1667/LTC1668
with an External Reference
5V
COMP1
= 0.1 F.
REFERENCE
EXTERNAL
should be connected to V
OUTFS
0.1 F
= 10mA). REFOUT has nominal
R
REFOUT
SET
REFIN
I
REFIN
REFERENCE
+
–
, corresponding to a
2.5V
COMP1
SS
LTC1666/
LTC1667/
LTC1668
COMP1
and be placed
1666/7/8
• 80 or 20kHz
F02
11
should