LTC1288CS8 Ver la hoja de datos (PDF) - Linear Technology
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LTC1288CS8 Datasheet PDF : 24 Pages
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LTC1285/LTC1288
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.
VIN +
RSOURCE +
VIN –
RSOURCE –
“+”
INPUT
C1
“–”
INPUT
C2
RON = 500Ω
LTC1285
LTC1288
CIN = 20pF
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 CF (e.g., 1µF), 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 IDC = 20pF × VIN/tCYC and is roughly
proportional to VIN. When running at the minimum cycle
time of 133.3µs, the input current equals 0.375µA at VIN
= 2.5V. In this case, a filter resistor of 160Ω 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.
RFILTER IDC
VIN
“+”
CFILTER
LTC1285
“–”
LTC1285/88 • F09
Figure 9. RC Input Filtering
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 1µA (at 125°C) flowing
through a source resistance of 240Ω will cause a voltage
drop of 240µV 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 50kΩ
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 ROUT (ex. LT1004, LT1019 and LT1021)
or a voltage source with low ROUT.
ROUT
VREF
REF+
1
GND
4
LTC1285
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 VCC 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
17
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