LTC1279 LINER [Linear Technology], LTC1279 Datasheet - Page 11

• Manufacturer Part Number: LTC1279
• Description: 12-Bit, 600ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
op amps are used, more settling time can be provided by
increasing the time between conversions. Suitable de-
vices capable of driving the ADC’s A
LT
Internal Reference
The LTC1279 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.42V. It is internally connected to the DAC and
is available at pin 2 to provide up to 800 A 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 in bipolar mode. The V
pin must be driven to at least 2.45V to prevent conflict with
the internal reference. The reference should be driven to
no more than 4.8V to keep the input span within the 5V
supplies.
Figure 6 shows an LT1006 op amp driving the V
the unipolar mode, the input span is already 0V to 5V with
(= 1.033 V
1.033 V
INPUT RANGE
®
INPUT RANGE
1360, LT1220, LT1223 and LT1224 op amps.
REF
Figure 6. Driving the V
REF(OUT)
2.58V
REF
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1279 with the LT1019A-2.5
pin can be driven with a DAC or other means to
)
LT1019A-2.5
–
GND
+
V
5V
IN
LT1006
U
V
OUT
INFORMATION
U
REF
V
REF(OUT)
with the LT1006 Op Amp
3
2.45V
W
10 F
IN
3
10 F
input include the
A
V
AGND
A
V
AGND
REF
IN
REF
U
IN
REF
–5V
–5V
5V
LTC1279
pin. (In
LTC1279
5V
1279 F06
1279 F07
REF
the internal reference so driving the reference is not
recommended, since the input span will exceed the supply
and codes will be lost at the full scale.) Figure 7 shows a
typical reference, the LT1019A-2.5 connected to the
LTC1279. This will provide an improved drift (equal to the
LT1019A-2.5’s maximum of 5ppm/ C) and a 2.582V full
scale.
UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT
Figure 8a shows the ideal input/output characteristics for
the LTC1279. The code transitions occur midway between
successive integer LSB values (i.e., 0.5LSB, 1.5LSB,
2.5LSB, ... FS – 1.5LSB). The output code is naturally
binary with 1LSB = FS/4096 = 5V/4096 = 1.22mV. Figure
8b shows the input/output transfer characteristics for the
bipolar mode in two’s complement format.
Figure 8a. LTC1279 Unipolar Transfer Characteristics
Figure 8b. LTC1279 Bipolar Transfer Characteristics
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
011...111
011...110
000...001
000...000
111...111
111...110
100...001
100...000
0V
–FS/2
UNIPOLAR
ZERO
1LSB = FS
LSB
1
4096
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
=
4096
BIPOLAR
5V
–1
LSB
ZERO
0V
LSB
FS = 5V
1LSB = FS/4096
1
FS/2 – 1LSB
FS – 1LSB
LTC1279
1279 F08b
1279 F08a
11