13
LTC1416
APPLICATIONS INFORMATION
WUU
U
0000 0000 00 and 1111 1111 1111 11. For full-scale
adjustment, an input voltage of 2.499544V (FS/2 – 1.5LSB)
is applied to A
IN
and R2 is adjusted until the output code
flickers between 0111 1111 1111 10 and 0111 1111
1111 11.
applications, however, do not have a –5V supply readily
available and most ADCs have inadequate PSRR to suffi-
ciently attenuate the noise created by a switching or
charge pump supply. The LTC1416’s excellent PSRR
makes it possible to achieve good performance, even at 14
bits, using a switch based regulator for a –5V supply.
Figure 12a shows a circuit using an LT1373 configured as
a Cuk converter creating –5V from a 5V supply. The circuit
shown in Figure 12b uses an LT1054 regulated charge
pump to provide –5V. This circuit has the advantage of
reduced board space and fewer passive components. (For
further details refer to
Linear Technology
Magazine, June
1997, Page 29.)
BOARD LAYOUT AND BYPASSING
Wire wrap boards are not recommended for high resolu-
tion or high speed A/D converters. To obtain the best
performance from the LTC1416, a printed circuit board
with ground plane is required. Layout for the printed
circuit board should ensure that digital and analog signal
lines are separated as much as possible. In particular, care
should be taken not to run any digital track alongside an
analog signal track or underneath the ADC. The analog
input should be screened by AGND.
An analog ground plane separate from the logic system
ground should be established under and around the ADC
(see Figure 13). Pin 5 (AGND), Pins 14 and 19 (ADC’s
DGND) and all other analog grounds should be connected
to this single analog ground point. The REFCOMP bypass
capacitor and the DV
DD
bypass capacitor should also be
connected to this analog ground plane. No other digital
grounds should be connected to this analog ground plane.
Low impedance analog and digital power supply common
returns are essential to low noise operation of the ADC and
the foil width for these tracks should be as wide as
possible. In applications where the ADC data outputs and
control signals are connected to a continuously active
microprocessor bus, it is possible to get errors in the
conversion results. These errors are due to feedthrough
from the microprocessor to the successive approximation
comparator. The problem can be eliminated by forcing the
microprocessor into a Wait state during conversion or by
using three-state buffers to isolate the ADC data bus. The
1416 F11a
011...111
011...110
000...001
000...000
111...111
111...110
100...001
100...000
FS – 1LSB
–(FS – 1LSB)
INPUT VOLTAGE (A
IN
+
– A
IN
–
)
OUTPUT CODE
Figure 11a. LTC1416 Transfer Characteristics
ANALOG
INPUT
1416 F11b
1
2
3
R4
100Ω
R2
50k
R3
24k
–5V
R6
24k
R1
50k
R5
47k
4
5
22µF
A
IN
+
A
IN
–
V
REF
REFCOMP
AGND
LTC1416
Figure 11b. Offset and Full-Scale Adjust Circuit
Generating a –5V Supply
There are several advantages to using ±5V supplies rather
than a single 5V supply. A larger signal magnitude is
possible which increases the dynamic range and
improves the signal-to-noise ratio. Operating on ±5V
supplies also offers increased headroom which eases the
requirements for signal conditioning circuitry, avoids the
limitations of rail-to-rail operation and widens the selec-
tion of high performance operational amplifiers. Some
