LTC1282 Datasheet, PDF (13/24 Pages) Linear Technology – 3V 140ksps 12-Bit Sampling A/D Converter with Reference

LTC1282
APPLICATI S I FOR ATIO
Overdriving the Internal Reference
The V_REF pin can be driven above its normal value with a
DAC or other means to provide input span adjustment.
Figure 6 shows an LT1006 op amp driving the reference
pin. The V_REF pin must be driven to at least 1.25V to
prevent conflict with the internal reference. The reference
should be driven to no more than 1.44V in unipolar mode
or 2.88V for bipolar mode to keep the input span within the
single 3V or ±3V supplies.
INPUT RANGE
±1.033V_REF(OUT)
+
LT1006
–
V_REF(OUT) ≥ 1.25V
3Ω
10µF
A_IN V_DD 3V
LTC1282
V_REF
AGND V_SS –3V
LTC1282 • F06
Figure 6. Driving the V_REF with the LT1006 Op Amp
Figure 7 shows a typical reference, the LT1019A-2.5
connected to the LTC1282 operating in bipolar mode. This
will provide an improved drift (due to the 5ppm/°C of the
LT1019A-2.5) and a ±2.604V full scale.
INPUT RANGE
± 2.60V
5V
V_IN
V_OUT
LT1019A-2.5
GND
3Ω
+
10µF
A_IN V_DD 3V
LTC1282
V_REF
AGND V_SS
LTC1282 • F07
– 3V
Figure 7. Supplying a 2.5V Reference Voltage
to the LTC1282 with the LT1019A-2.5
UNIPOLAR/BIPOLAR OPERATION AND ADJUSTMENT
Figure 8 shows the ideal input/output characteristics for
the LTC1282. The code transitions occur midway be-
tween successive integer LSB values (i.e., 0.5LSB,
1.5LSBs, 2.5LSBs, FS – 1.5LSBs). The output code is
natural binary with 1LSB = FS/4096 = 2.5V/4096 = 0.61mV.
Figure 9 shows the input/output transfer characteristics
for the LTC1282 in bipolar operation. The full scale for
LTC1282 in bipolar mode is still 2.5V and 1LSB = 0.61mV.
111...111
111...110
111...101
111...100
FS = 2.5V
1LSB = FS/4096
UNIPOLAR
ZERO
000...011
000...010
000...001
000...000
0V 1
LSB
INPUT VOLTAGE (V)
FS – 1LSB
LTC1282 • F8
Figure 8. LTC1282 Unipolar Transfer Characteristic
011...111
011...110
BIPOLAR
ZERO
000...001
000...000
111...111
111...110
100...010
100...001
100...000
FS = 2.5V
1LSB = FS/4096
– FS/2
–1 0V 1
LSB LSB
INPUT VOLTAGE (V)
FS/2 – 1LSB
LTC1282 • F09
Figure 9. LTC1282 Bipolar Transfer Characteristic
Unipolar Offset and Full-Scale Adjustment
In applications where absolute accuracy is important,
offset and full-scale errors can be adjusted to zero. Figure
10 shows the extra components required for full-scale
error adjustment. If both offset and full-scale adjust-
ments are needed, the circuit in Figure 11 can be used.
Offset should be adjusted before full scale. To adjust
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