LTC1273_1 Datasheet, PDF (13/24 Pages) Linear Technology – 12-Bit, 300ksps Sampling A/D Converters with Reference

APPLICATI S 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.
INPUT RANGE
± 2.58V
5V
V_IN
V_OUT
LT1019A-2.5
GND
3Ω
10µF
LTC1275
A_IN
V_REF
AGND
LTC1273/75/76 • F07
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1275 with the LT1019A-2.5
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
LTC1273
LTC1275/LTC1276
111...111
111...110
111...101
111...100
1LSB
=
FS
4096
=
5V
4096
UNIPOLAR
ZERO
000...011
000...010
000...001
000...000
0V 1
LSB
FS – 1LSB
INPUT VOLTAGE (V)
LTC1273/75/76 • F08
Figure 8. LTC1273 Unipolar Transfer Characteristic
011...111
011...110
BIPOLAR
ZERO
000...001
000...000
111...111
111...110
111...110
100...001
100...000
FS = 5V (LTC1275)
FS = 10V (LTC1276)
1LSB = FS/4096
– FS/2
–1 0V 1 FS/2 – 1LSB
LSB LSB
INPUT VOLTAGE (V)
LTC1273/75/76 • F09
Figure 9. LTC1275/LTC1276 Bipolar Transfer Characteristic
R1
50Ω
V₁
+
A1
–
R4
R2
100Ω
10k
R3
10k
FULL SCALE
ADJUST
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
A_IN
LTC1273
LTC1275
LTC1276
AGND
LTC1273/75/76 • F10a
Figure 10a. Full Scale Adjust Circuit
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