LTC1274 Linear Technology, LTC1274 Datasheet - Page 13

• Manufacturer Part Number: LTC1274
• Description: 12-Bit/ 10mW/ 100ksps ADCs with 1uA Shutdown
• Manufacturer: Linear Technology

0 TO 1.69V
A
LTC1277 A
The input capacitor for the LTC1277 is switched onto the
A
A
period. At the end of the sample phase the input capacitor
switches to the A
During the conversion the A
tively “held” by the sample-and-hold and will not affect
the conversion result. It is critical that the A
voltage be free of noise and settles completely during the
conversion.
Internal Reference
The ADCs have an on-chip, temperature compensated,
curvature corrected bandgap reference which is factory
trimmed to 2.42V. It is internally connected to the DAC and
is available at Pin 2 (LTC1274) or Pin 3 (LTC1277) to
provide up to 1mA current to an external load.
For minimum code transition noise the reference output
should be decoupled with a capacitor to filter wideband
noise from the reference (10 F tantalum in parallel with a
0.1 F ceramic).
The V
provide input span adjustment. The V
driven to at least 2.45V to prevent conflict with the internal
reference. The reference should be driven to no more than
IN BIPOLAR MODE
IN
IN
UNIPOLAR MODE
PPLICATI
+
+
0.846V
INPUT RANGE:
input during the sample phase. The voltage on the
input must settle completely within the sample
REF
Figure 6. Driving the V
REF(OUT)
REF(OUT)
pin can be driven with a DAC or other means to
IN
IN
+
/A
IN
O
–
–
+
IN
LT1006
Input Settling
U
–
S
input and the conversion starts.
I FOR ATIO
REF
U
V
with the LT1006 Op Amp
IN
REF(OUT)
+
input voltage is effec-
W
2.45V
3
REF
10 F
pin must be
A
V
AGND
IN
LTC1274
U
LTC1274/77 • F06
IN
REF
–
5V
input
0V TO 4.231V (1.69V
3V to keep the input span within the 5V supply in unipolar
mode. In bipolar mode the reference should be driven to
no more than 5V, the positive supply voltage of the chip.
Figure 6 shows an LT1006 op amp driving the Reference
pin. In unipolar mode, the reference can be driven up to
2.95V at which point it will provide a 0V to 5V input span.
For the bipolar mode, the reference can be driven up to 5V
at which point it will provide a 4.23V input span. Figure
7 shows a typical reference, the LT1019A-2.5 connected
to the LTC1274. This will provide an improved drift (equal
to the maximum 5ppm/ C of the LT1019A-2.5) and a
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 LTC1274/LTC1277, a printed cir-
cuit board 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.
High quality tantalum and ceramic bypass capacitors
should be used at the V
2.115V (bipolar) or 4.231V (unipolar) full scale.
2.115V ( 0.846
IN UNIPOLAR MODE
IN BIPOLAR AND
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1274 with the LT1019A-2.5
INPUT RANGE:
REF(OUT)
V
REF
)
)
LT1019A-2.5
GND
LTC1274/LTC1277
V
5V
IN
DD
V
OUT
and V
REF
3
pins as shown in
10 F
A
V
AGND
13
LTC1274
LTC1274/77 • F07
IN
REF
5V