LTC1288IS8 Linear Technology, LTC1288IS8 Datasheet - Page 17

• Manufacturer Part Number: LTC1288IS8
• Description: IC A/D CONV SAMPLING 12BIT 8SOIC
• Manufacturer: Linear Technology

Specifications of LTC1288IS8

• Number Of Bits: 12
• Sampling Rate (per Second): 6.6k
• Data Interface: MICROWIRE™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 630µW
• Voltage Supply Source: Single Supply
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATION INFORMATION
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.
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 9. For large values of C
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
proportional to V
time of 133.3 s, the input current equals 0.375 A at V
= 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.
V
V
IN
IN
+
–
Figure 8. Analog Input Equivalent Circuit
R
R
SOURCE
SOURCE
V
IN
R
DC
Figure 9. RC Input Filtering
+
–
FILTER
IN
. When running at the minimum cycle
= 20pF
U
C1
INPUT
C2
INPUT
I
“+”
“–”
DC
U
C
FILTER
V
IN
“
“
+
–
LTC1285
R
/t
”
”
W
ON
LTC1285/88 • F09
CYC
= 500
F
and is roughly
(e.g., 1 F), the
C
LTC1285
LTC1288
IN
U
LTC1285/88 • F08
= 20pF
IN
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 A (at 125 C) flowing
through a source resistance of 240 will cause a voltage
drop of 240 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
or a voltage source with low R
Reduced Reference Operation
The minimum reference voltage of the LTC1288 is limited
to 2.7V because the V
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
Figure 10. Reference Input Equivalent Circuit
R
V
OUT
REF
REF
GND
1
4
+
OUT
CC
(ex. LT1004, LT1019 and LT1021)
LTC1285/LTC1288
supply and reference are inter-
OUT
.
LTC1285
LTC1285/88 • F10
17