Datasheet

13
LTC1273
LTC1275/LTC1276
127356fa
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
reference pin. The V
REF
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. 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.
Figure 8. LTC1273 Unipolar Transfer Characteristic
INPUT VOLTAGE (V)
0V
OUTPUT CODE
FS – 1LSB
LTC1273/75/76 • F08
111...111
111...110
111...101
111...100
000...000
000...001
000...010
000...011
1
LSB
1LSB =
FS
4096
=
5V
4096
UNIPOLAR
ZERO
Figure 9. LTC1275/LTC1276 Bipolar Transfer Characteristic
LTC1273
LTC1275
LTC1276
A
IN
AGND
LTC1273/75/76 • F10a
R4
100Ω
FULL SCALE
ADJUST
R3
10k
R2
10k
R1
50Ω
V
1
–
+
A1
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
Figure 10a. Full Scale Adjust Circuit
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1275 with the LT1019A-2.5
3Ω
INPUT RANGE
±2.58V
LTC1275
A
IN
AGND
V
REF
10µF
LTC1273/75/76 • F07
LT1019A-2.5
V
IN
GND
V
OUT
5V
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 VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
LTC1273/75/76 • F09
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
111...110
FS/2 – 1LSB–FS/2
FS = 5V (LTC1275)
FS = 10V (LTC1276)
1LSB = FS/4096