Datasheet
LTC1400
10
1400fa
trimmed to 2.42V. It is internally connected to the DAC
and is available at Pin 3 to provide up to 1mA of 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 (10μF tantalum in
parallel with a 0.1μF ceramic). The V
REF
pin can be driven
with a DAC or other means to provide input span adjust-
ment in bipolar mode. The V
REF
pin must be driven to at
least 2.45V to prevent conflict with the internal reference.
The reference should not be driven to more than 5V.
Figure 6 shows an LT1360 op amp driving the reference
pin. Figure 7 shows a typical reference, the LT1019A-5
connected to the LTC1400. This will provide an improved
drift (equal to the maximum 5ppm/°C of the LT1019A-
5) and a ±4.231V full scale. If V
REF
is forced lower than
2.42V, the REFRDY bit in the serial data output will be
forced to low.
APPLICATIO S I FOR ATIO
W UU U
1400 F06
–
+
V
REF(OUT)
≥ 2.45V
A
IN
V
REF
GND
10µF
3Ω
INPUT RANGE
±0.846 • V
REF(OUT)
5V
–5
V
LTC1400
LT1360
V
CC
V
SS
Figure 9. LTC1400 Bipolar Transfer Characteristics
Figure 6. Driving the V
REF
with the LT1360 Op Amp
1400 F07
10µF
3Ω
INPUT RANGE ±4.231V
(= ±0.846 • V
REF
)
–5
V
LT1019A-5
10
V
V
IN
V
OUT
GND
5V
A
IN
V
REF
GND
LTC1400
V
CC
V
SS
Figure 7. Supplying a 5V Reference Voltage
to the LTC1400 with the LT1019A-5
Unipolar/Bipolar Operation and Adjustment
Figure 8 shows the ideal input/output characteristics for
the LTC1400. 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 straight
binary with 1LSB = 4.096V/4096 = 1mV. Figure 9 shows
the input/output transfer characteristics for the bipolar
mode in two’s complement format.
INPUT VOLTAGE (V)
0V
OUTPUT CODE
FS – 1LSB
1400 F08
111...111
111...110
111...101
111...100
000...000
000...001
000...010
000...011
1
LSB
UNIPOLAR
ZERO
1LSB =
FS
4096
Figure 8. LTC1400 Unipolar Transfer Characteristics
INPUT VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
11400 F09
011...111
011...110
000...001
000...000
100...000
100...001
111...110
1
LSB
BIPOLAR
ZERO
111...111
FS/2 – 1LSB–FS/2
Unipolar Offset and Full-Scale Error Adjustments
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