Datasheet

17
LTC1286/LTC1298
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
.
Reduced Reference Operation
The minimum reference voltage of the LTC1298 is limited
to 4.5V because the V
CC
supply and reference are inter-
nally tied together. However, the LTC1286 can operate
with reference voltages below 1V.
The effective resolution of the LTC1286 can be increased
by reducing the input span of the converter. The LTC1286
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
Voltage). However, care must be taken when operating at
low values of V
REF
because of the reduced LSB step size
and the resulting higher accuracy requirement placed on
the converter. The following factors must be considered
when operating at low V
REF
values:
1. Offset
2. Noise
3. Conversion speed (CLK frequency)
Offset with Reduced V
REF
The offset of the LTC1286 has a larger effect on the output
code. When the ADC is operated with reduced reference
voltage. The offset (which is typically a fixed voltage)
becomes a larger fraction of an LSB as the size of the LSB
is reduced. The typical curve of Change in Offset vs
Reference Voltage shows how offset in LSBs is related to
reference voltage for a typical value of V
OS
. For example,
a V
OS
of 122µV which is 0.1LSB with a 5V reference
becomes 0.5LSB with a 1V reference and 2.5LSBs with a
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
F
(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 I
DC
= 20pF × V
IN
/t
CYC
and is roughly
proportional to V
IN
. When running at the minimum cycle
time of 64µs, the input current equals 1.56µA at V
IN
= 5V.
In this case, a filter resistor of 75Ω 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
LTC1286/98 • F09
LTC1286
“+”
“–”
I
DC
Figure 9. RC Input Filtering
LTC1286
REF
+
R
OUT
V
REF
1
4
GND
LTC1286/98 • F10
Figure 10. Reference Input Equivalent Circuit
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 0.2LSB. This error will be much reduced
at lower temperatures because leakage drops rapidly (see
typical curve of Input Channel Leakage Current vs Tem-
perature).
REFERENCE INPUTS
The reference input of the LTC1286 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
APPLICATION INFORMATION
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