LTC1401_15 Datasheet, PDF (10/20 Pages) Linear Technology – Complete SO-8, 12-Bit, 200ksps ADC with Shutdown

LTC1401
APPLICATIONS INFORMATION
is available at Pin 3 to provide up to 1mA 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 is recommended). The V_REF
pin can be driven with a DAC or other means to provide
input span adjustment. The V_REF pin must be driven to at
least 1.25V to prevent conflict with the internal reference.
The reference should not be driven to more than 3V.
Figure 6 shows an LT1360 op amp driving the reference
pin. Figure 7 shows a typical reference (LT1634-1.25)
connected to the LTC1401. This will provide improved
drift (equal to the maximum 25ppm/°C of the LT1634-
1.25) and a 2.1338V full scale.
INPUT RANGE
1.707 • V_REF(OUT)
+
LT1360
–
V_REF(OUT) ≥ 1.25V
3Ω
10µF
3V
A_IN ^VCC
LTC1401
V_REF
GND
LTC1401 • F06
Figure 6. Driving the V_REF with the LT1360 Op Amp
INPUT RANGE 1.707 • V_REF
(= 2.1338V)
10V
V_IN
V_OUT
LT1634-1.25
GND
3Ω
10µF
3V
A_IN ^VCC
LTC1401
V_REF
GND
LTC1401 • F07
Figure 7. Supplying a 2.5V Reference Voltage to
the LTC1401 with the LT1634-1.25
UNIPOLAR OPERATION AND ADJUSTMENT
Figure 8 shows the ideal input/output characteristics for
the LTC1401. 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 = 2.048V/4096 = 0.5mV.
111...111
111...110
111...101
111...100
1LSB = ^FS = ^2.048
4096 4096
UNIPOLAR
ZERO
000...011
000...010
000...001
000...000
0V 1
LSB
INPUT VOLTAGE (V)
FS – 1LSB
LTC1401 • F08
Figure 8. LTC1401 Unipolar Transfer Characteristics
Unipolar Offset and Full-Scale Error Adjustments
In applications where absolute accuracy is important, the
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 9a
shows the extra components required for full scale error
adjustment. If both offset and full-scale adjustments are
needed, the circuit in Figure 9b can be used. For zero offset
error, apply 0.25mV (i.e., 0.5LSB) at the input and adjust
the offset trim until the LTC1401 output code flickers
between 0000 0000 0000 and 0000 0000 0001. For zero
full-scale error, apply an analog input of 2.04725V ( FS –
1.5LSB or last code transition ) at the input and adjust R5
until the LTC1401 output code flickers between 1111 1111
1110 and 1111 1111 1111.
1401fa
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