參數(shù)資料
型號(hào)： LTC1285CS8
廠商： LINEAR TECHNOLOGY CORP
元件分類： ADC
英文描述： 3V Micropower Sampling 12-Bit A/D Converters in SO-8 Packages
中文描述： 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, SERIAL ACCESS, PDSO8
封裝： 0.150 INCH, PLASTIC, SO-8
文件頁(yè)數(shù)： 17/24頁(yè)
文件大?。?/td> 412K
代理商： LTC1285CS8
17
LTC1285/LTC1288
APPLICATIO
I
FOR
ATIO
U
selected “+” and “–” inputs once during each conversion
cycle. Large external source resistors and capacitances
will slow the settling of the inputs. It is important that the
overall RC time constants be short enough to allow the
analog inputs to completely settle within the allowed time.
W
U
U
Input Leakage Current
Input leakage currents can also create errors if the source
resistance gets too large. For instance, the maximum
input leakage specification of 1
μ
A (at 125
°
C) flowing
through a source resistance of 240
will cause a voltage
drop of 240
μ
V or 0.4LSB. This error will be much
reduced at lower temperatures because leakage drops
rapidly (see typical curve of Input Channel Leakage
Current vs Temperature).
REFERENCE INPUTS
The reference input of the LTC1285 is effectively a 50k
resistor from the time CS goes low to the end of the
conversion. The reference input becomes a high impedence
node at any other time (see Figure 10). Since the voltage
on the reference input defines the voltage span of the A/D
converter, the reference input should be driven by a
reference with low R
OUT
(ex. LT1004, LT1019 and LT1021)
or a voltage source with low R
OUT
.
LTC1285
REF
+
1
R
OUT
V
REF
4
GND
LTC1285/88 F10
Figure 10. Reference Input Equivalent Circuit
Reduced Reference Operation
The minimum reference voltage of the LTC1288 is limited
to 2.7V because the V
CC
supply and reference are inter-
nally tied together. However, the LTC1285 can operate
with reference voltages below 1.5V.
The effective resolution of the LTC1285 can be increased
by reducing the input span of the converter. The LTC1285
exhibits good linearity and gain over a wide range of
reference voltages (see typical curves of Change in Linear-
ity vs Reference Voltage and Change in Gain vs Reference
R
ON
= 500
C
IN
= 20pF
LTC1285
LTC1288
“+”
INPUT
R
SOURCE
+
V
IN
+
C1
“–”
INPUT
R
SOURCE
–
V
IN
–
C2
LTC1285/88 F08
Figure 8. Analog Input Equivalent Circuit
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 9. For large values of C
F
(e.g., 1
μ
F), the
capacitive input switching currents are averaged into a
net DC current. Therefore, a filter should be chosen with
a small resistor and large capacitor to prevent DC drops
across the resistor. The magnitude of the DC current is
approximately I
DC
= 20pF
×
V
IN
/t
CYC
and is roughly
proportional to V
IN
. When running at the minimum cycle
time of 133.3
μ
s, the input current equals 0.375
μ
A at V
IN
= 2.5V. In this case, a filter resistor of 160
will cause
0.1LSB of full-scale error. If a larger filter resistor must
be used, errors can be eliminated by increasing the cycle
time.
R
FILTER
V
IN
C
FILTER
LTC1285/88 F09
LTC1285
“
+
”
“
–
”
I
DC
Figure 9. RC Input Filtering
相關(guān)PDF資料
PDF描述
LTC1288 Single Channel Codec-Bandwidth Independent of Sampling Rate 28-PLCC 0 to 70
LTC1288CS8 3V Micropower Sampling 12-Bit A/D Converters in SO-8 Packages
LTC1289BC 3 Volt Single Chip 12-Bit Data Acquisition System
LTC1289CCJ 3 Volt Single Chip 12-Bit Data Acquisition System
LTC1289CCN 3 Volt Single Chip 12-Bit Data Acquisition System
相關(guān)代理商/技術(shù)參數(shù)
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