LTC1275 LINER [Linear Technology], LTC1275 Datasheet - Page 13

• Manufacturer Part Number: LTC1275
• Description: 12-Bit, 300ksps Sampling A/D Converters with Reference
• Manufacturer: LINER [Linear Technology]

A
reference pin. The V
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. In the LTC1273/
LT1276, the input spans are 0V to 5V and 5V respec-
tively with the internal reference. Driving the reference is
not recommended on the LTC1273/LTC1276 since the
input spans will exceed the supplies and codes will be lost
at full scale.
Figure 7 shows a typical reference, the LT1019A-2.5
connected to the LTC1275. This will provide an improved
drift (equal to the maximum 5ppm/ C of the LT1019A-2.5)
and a 2.582V full scale.
UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT
Figure 8 shows the ideal input/output characteristics for
the LTC1273. The code transitions occur midway between
successive integer LSB values (i.e., 1/2LSB, 1 1/2LSBs,
2 1/2LSBs, ... FS – 1 1/2LSBs). The output code is natural
binary with 1LSB = FS/4096 = 5V/4096 = 1.22mV. Figure
9 shows the input/output transfer characteristics for the
LTC1275/LTC1276 in 2’s complement format. As stated in
the figure, 1LSB for LTC1275/LTC1276 are 1.22mV and
2.44mV respectively.
Unipolar Offset and Full Scale Adjustment (LTC1273)
In applications where absolute accuracy is important,
offset and full scale errors can be adjusted to zero. Figure
10a shows the extra components required for full scale
error adjustment. If both offset and full scale adjustments
are needed, the circuit in Figure 10b can be used. Offset
INPUT RANGE
PPLICATI
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1275 with the LT1019A-2.5
±2.58V
LT1019A-2.5
GND
V
5V
IN
O
V
OUT
U
REF
S
pin must be driven to at least
I FOR ATIO
U
3
10 F
W
LTC1273/75/76 • F07
A
V
AGND
LTC1275
IN
REF
U
Figure 9. LTC1275/LTC1276 Bipolar Transfer Characteristic
V
1
Figure 8. LTC1273 Unipolar Transfer Characteristic
011...111
011...110
000...101
000...000
111...111
111...110
100...001
100...000
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
50
R1
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
Figure 10a. Full Scale Adjust Circuit
0V
–FS/2
UNIPOLAR
ZERO
1LSB = FS
R2
10k
LSB
1
4096
R3
10k
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
–
LTC1275/LTC1276
+
= 5V
4096
BIPOLAR
–1
LSB
A1
ZERO
0V
FULL SCALE
FS = 5V (LTC1275)
FS = 10V (LTC1276)
1LSB = FS/4096
ADJUST
100
LSB
R4
1
LTC1273/75/76 • F08
LTC1273/75/76 • F09
FS/2 – 1LSB
FS – 1LSB
LTC1273
LTC1273/75/76 • F10a
A
AGND
LTC1273
LTC1275
LTC1276
IN
13