LTC1279 LINER [Linear Technology], LTC1279 Datasheet - Page 12

• Manufacturer Part Number: LTC1279
• Description: 12-Bit, 600ksps Sampling A/D Converter with Shutdown
• Manufacturer: LINER [Linear Technology]

0V TO 5V
Figure 9b. LTC1279 Unipolar Offset and Full-Scale Adjust Circuit
LTC1279
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.
12
A
V1
ANALOG
INPUT
PPLICATI
5V
50
R1
ADDITIONAL PINS OMITTED FOR CLARITY
10k
10k
R1
20LSB TRIM RANGE
R2
10k
Figure 9a. Full-Scale Adjust Circuit
R2
10k
R9
20
R3
10k
O
+
–
U
S
A1
+
–
FULL-SCALE
I FOR ATIO
ADJUST
U
100
R4
R4
100k
R3
100k
R6
400
R5
4.3k
FULL-SCALE
ADJUST
W
100k
R7
A
AGND
IN
LTC1279
A
5V
IN
U
LTC1279
R8
10k
OFFSET
ADJUST
1279 F09a
1279 F09b
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 AV
Figure 10. For the bipolar mode, a 0.1 F ceramic provides
ANALOG
Figure 9c. LTC1279 Bipolar Offset and Full-Scale Adjust Circuit
INPUT
10k
R1
R2
10k
+
–
DD
and V
REF
R4
100k
R3
100k
R6
200
R5
4.3k
FULL-SCALE
ADJUST
100k
pins as shown in
R7
–5V
5V
A
R8
20k
OFFSET
ADJUST
IN
LTC1279
1279 F09c