Datasheet

15
LTC1274/LTC1277
U
S
A
O
PP
L
IC
AT
I
WU
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I FOR ATIO
DIGITAL INTERFACE
The ADCs are designed to interface with microproces-
sors as a memory mapped device. The CS and RD control
inputs are common to all peripheral memory interfacing.
A separate CONVST is used to initiate a conversion.
Figures 10a to 10c are the input/output characteristics of
the ADCs. The code transitions occur midway between
successive integer LSB values (i.e., 0.5LSB, 1.5LSB,
2.5LSB…FS – 1.5LSV). The output code is scaled such
that 1.0LSB = FS/4096 = 4.096V/4096 = 1.0mV.
Unipolar Offset and Full-Scale Error Adjustments
In applications where absolute accuracy is important, then
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 11a
shows the extra components required for full-scale error
adjustment. If both offset and full-scale adjustments are
needed, the circuit in Figure 11b can be used. For zero
offset error, apply 0.50mV (i.e., 0.5LSB) at the input and
adjust the offset trim until the LTC1274/LTC1277 output
code flickers between 0000 0000 0000 and 0000 0000
0001. For zero full-scale error, apply an analog input of
4.0945V (i.e., FS – 1.5LSB or last code transition) at the
input and adjust R5 until the ADC output code flickers
between 1111 1111 1110 and 1111 1111 1111.
Bipolar Offset and Full-Scale Error Adjustments
Bipolar offset and full-scale errors are adjusted in a similar
fashion to the unipolar case. Again, bipolar offset must be
INPUT VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
LTC1274/77 • F10c
111...111
111...110
100...001
100...000
000...000
000...001
011...110
1
LSB
BIPOLAR
ZERO
011...111
FS/2 – 1LSB–FS/2
1LSB = = = 1mV
4.096V
4096
FS
4096
Figure 10c. LTC1277 Bipolar Transfer Characteristics
(Offset Binary)
Figure 10a. LTC1274/LTC1277 Unipolar
Transfer Characteristics
Figure 10b. LTC1274 Bipolar Transfer
Characteristics (2’s Complement)
INPUT VOLTAGE (V)
0V
OUTPUT CODE
FS – 1LSB
LTC1274/77 F10a
111...111
111...110
111...101
111...100
000...000
000...001
000...010
000...011
1
LSB
UNIPOLAR
ZERO
1LSB =
FS
4096
4.096V
4096
= = 1mV
INPUT VOLTAGE (V)
0V
OUTPUT CODE
–1
LSB
LTC1274/77 • F10b
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
1LSB = = = 1mV
4.096V
4096
FS
4096