Datasheet

LTC2756
21
2756f
applicaTions inForMaTion
Table 6. Partial List of LTC Precision References Recommended
for Use with the LTC2756 with Relevant Specifications
REFERENCE
INITIAL
TOLERANCE
TEMPERATURE
DRIFT
0.1Hz to 10Hz
NOISE
LT1019A-5,
LT1019A-10
±0.05% max 5ppm/°C max 12µV
P-P
LT1236A-5,
LT1236A-10
±0.05% max 5ppm/°C max 3µV
P-P
LT1460A-5,
LT1460A-10
±0.075% max 10ppm/°C max 20µV
P-P
LT1790A-2.5 ±0.05% max 10ppm/°C max 12µV
P-P
LTC6652A-5 ±0.05% max 5ppm/°C max 2.8ppm
P-P
LTC6655A-2.5
LTC6655A-5
±0.025% max 2ppm/°C max 0.25ppm
P-P
As precision DAC applications move to 18-bit perfor-
mance, reference output voltage noise may contribute a
dominant share of the system’s noise floor. This in turn can
degrade system dynamic range and signal-to-noise ratio.
Care should be exercised in selecting a voltage reference
with as low an output noise voltage as practical for the
system resolution desired. Precision voltage references
like the LT1236 or LTC6655 produce low output noise in
the 0.1Hz to 10Hz region, well below the 18-bit LSB level
in 5V or 10V full-scale systems. However, as the circuit
bandwidths increase, filtering the output of the reference
may be required to minimize output noise.
Grounding
As with any high-resolution converter, clean grounding is
important. A low-impedance analog ground plane is nec-
essary, as are star grounding techniques. Keep the board
layer used for star ground continuous to minimize ground
resistances; that is, use the star-ground concept without
using separate star traces. The I
OUT2
pin is of particular
importance; INL will be degraded by the code-dependent
currents carried by I
OUT2
if voltage drops to ground are
allowed to develop. The best strategy here is to tie the pins
to the star ground plane by multiple vias located directly
underneath the part. Alternatively, the pin may be routed
to the star ground point if necessary; route a trace of no
more than 30 squares of 1oz copper.
In the rare case in which neither of these alternatives is
practicable, a force/sense amplifier should be used as a
ground buffer (see Figure 4). Note, however, that the volt-
age offset of the ground buffer amp directly contributes to
the effects on accuracy specified in Table 4 under ‘V
OS1
’.
The combined effects of the offsets can be calculated by
substituting the total offset from I
OUT1
to I
OUT2
for V
OS1
in the equations.