LTC1400
U U
W U
APPLICATIO S I FOR ATIO
R1
error adjustment. Figure 10b shows offset and full-scale
adjustment. Offset error must be adjusted before full-
scale error. Zero offset is achieved by applying 0.5mV
(i.e., 0.5LSB) at the input and adjusting the offset trim
until the LTC1400 output code flickers between 0000
0000 0000 and 0000 0000 0001. For zero full-scale er-
ror, apply an analog input of 4.0945V (FS – 1.5LSB or
last code transition) at the input and adjust R5 until the
LTC1400 output code flickers between 1111 1111 1110
and 1111 1111 1111.
50Ω
V
+
IN
A1
A
IN
–
R4
R2
10k
100Ω
LTC1400
GND
R3
10k
FULL-SCALE
ADJUST
ADDITIONAL PINS OMITTED FOR CLARITY
±20LSB TRIM RANGE
1400 F10a
Bipolar Offset and Full-Scale Error Adjustments
Figure 10a. LTC1400 Full-Scale Adjust Circuit
Bipolaroffsetandfull-scaleerrorsareadjustedinasimilar
fashion to the unipolar case. Bipolar offset error adjust-
ment is achieved by applying an input voltage of –0.5mV
(–0.5LSB) to the input in Figure 10c and adjusting the
op amp until the ADC output code flickers between 0000
00000000and111111111111.Forfull-scaleadjustment,
an input voltage of 2.0465V (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.
R1
10k
ANALOG
INPUT
+
–
0V TO 4.096V
A
A1
IN
R2
10k
10k
R4
5V
100k
LTC1400
R9
20Ω
R5
4.3k
FULL-SCALE
ADJUST
R3
100k
5V
R7
R8
10k
100k
Board Layout and Bypassing
OFFSET
ADJUST
R6
400Ω
To obtain the best performance from the LTC1400, a
printed circuit board is required. Layout for the printed
circuit board should ensure that digital and analog signal
linesareseparatedasmuchaspossible.Inparticular,care
should be taken not to run any digital track alongside an
analog signal track or underneath the ADC. The analog
input should be screened by GND.
1400 F10b
Figure 10b. LTC1400 Offset and Full-Scale Adjust Circuit
R1
10k
ANALOG
INPUT
±2.048V
+
–
A1
R2
10k
A
IN
High quality tantalum and ceramic bypass capacitors
R4
100k
should be used at the V and V pins as shown in the
CC
REF
LTC1400
R5
4.3k
Typical Application on the first page of this data sheet.
For the bipolar mode, a 0.1μF ceramic provides adequate
FULL-SCALE
ADJUST
bypassing for the V pin. For optimum performance, a
SS
R3
10μF surface mount AVX capacitor with a 0.1μF ceramic
100k
5V
R8
R7
isrecommendedfortheV andV pins.Thecapacitors
100k
CC
REF
20k
OFFSET
mustbelocatedasclosetothepinsaspossible.Thetraces
connecting the pins and the bypass capacitors must be
kept short and should be made as wide as possible. In
ADJUST
R6
–5V
200Ω
1400 F10c
unipolar mode operation, V should be isolated from
SS
any noise source before shorting to the GND pin.
Figure 10c. LTC1400 Bipolar Offset and Full-Scale Adjust Circuit
1400fa
11
Linear [ Linear ]
