LTC1279ISW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1279ISW' PDF : 16 Pages View PDF
LTC1279
APPLICATI S I FOR ATIO
Unipolar Offset and Full-Scale Error Adjustments
In applications where absolute accuracy is important,
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 0.61mV (i.e., 0.5LSB) at the input and adjust
the offset trim until the LTC1279 output code flickers
between 0000 0000 0000 and 0000 0000 0001. For zero
full-scale error apply an analog input of 4.99817V (i.e.,
FS – 1.5LSB or last code transition) at the input and adjust
R5 until the LTC1279 output code flickers between 1111
1111 1110 and 1111 1111 1111.
R1
50Ω
V1
+
A1
–
R4
R2
100Ω
10k
R3
10k
FULL-SCALE
ADJUST
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
AIN
LTC1279
AGND
1279 F09a
Figure 9a. Full-Scale Adjust Circuit
R1
ANALOG 10k
INPUT
+
0V TO 5V
R2
10k 10k
5V
–
R9
20Ω
R4
100k
R5
4.3k
FULL-SCALE
ADJUST
R3
100k
R7
100k
R6
400Ω
AIN
LTC1279
5V
R8
10k
OFFSET
ADJUST
1279 F09b
Figure 9b. LTC1279 Unipolar Offset and Full-Scale Adjust Circuit
R1
ANALOG 10k
INPUT
+
R2
10k
–
AIN
R4
100k
LTC1279
R5
4.3k
FULL-SCALE
ADJUST
5V
R3
100k
R7
100k
R6
200Ω
R8
20k
OFFSET
ADJUST
– 5V
1279 F09c
Figure 9c. LTC1279 Bipolar Offset and Full-Scale Adjust Circuit
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
adjusted before full-scale error. Bipolar offset error ad-
justment is achieved by trimming the offset of the op amp
driving the analog input of the LTC1279 while the input
voltage is 0.5LSB below ground. This is done by applying
an input voltage of – 0.61mV (– 0.5LSB) to the input in
Figure 9c and adjusting the R8 until the ADC output code
flickers between 0000 0000 0000 and 1111 1111 1111.
For full scale adjustment, an input voltage of 2.49817V
(FS – 1.5LSBs) is applied to the input and R5 is adjusted
until the output code flickers between 0111 1111 1110
and 0111 1111 1111.
BOARD LAYOUT AND BYPASSING
Wire wrap boards are not recommended for high resolu-
tion or high speed A/D converters. To obtain the best
performance from the LTC1279, a printed circuit board is
required. The printed circuit board’s layout should ensure
that digital and analog signal lines are separated as much
as possible. In particular, care should be taken not to run
any digital trace alongside an analog signal trace or
underneath the ADC. The analog input should be screened
by AGND.
High quality tantalum and ceramic bypass capacitors
should be used at the AVDD and VREF pins as shown in
Figure 10. For the bipolar mode, a 0.1µF ceramic provides
12
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