LTC2486
27
2486fe
For more information www.linear.com/LTC2486
APPLICATIONS INFORMATION
When using the LTC2486’s internal oscillator, the input
capacitor array is switched at 123kHz. The effect of the
charge transfer depends on the circuitry driving the input/
reference pins. If the total external RC time constant is less
than 580ns the errors introduced by the sampling process
are negligible since complete settling occurs.
Typically, the reference inputs are driven from a low imped
-
ance source. In this case, complete settling occurs even
with
large external bypass capacitors. The inputs (CH0 to
CH3, COM), on the other hand, are typically driven from
larger source resistances. Source resistances up to 10k
may interface directly to the LTC2486 and settle completely;
however, the addition of external capacitors at the input
terminals in order to filter unwanted noise (anti-aliasing)
results in incomplete settling.
Automatic Differential Input Current Cancellation
In applications where the sensor output impedance is
low (up to 10kΩ with no external bypass capacitor or up
to 500Ω with 0.001µF bypass), complete settling of the
input occurs. In this case, no errors are introduced and
direct digitization is possible.
For many applications, the sensor output impedance
combined with external input bypass capacitors produces
RC time constants much greater than
the 580ns required
for
1ppm accuracy. For example, a 10kΩ bridge driving a
0.1µF capacitor has a time constant an order of magnitude
greater than the required maximum.
The LTC2486 uses a proprietary switching algorithm that
forces the average differential input current to zero indepen
-
dent of external settling errors. This allows direct digitization
of high impedance sensors without the need of buffers.
The switching algorithm forces the average input current
on the positive input (I
IN
+
) to be equal to the average input
current on the negative input (I
IN
–
). Over the complete
conversion cycle, the average differential input current
(I
IN
+
– I
IN
–
) is zero. While the differential input current is
zero, the common mode input current (I
IN
+
+ I
IN
–
)/2 is
proportional to the difference between the common mode
input voltage (V
IN(CM)
) and the common mode reference
voltage (V
REF(CM)
).
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balanced bridge, both the differential and com
-
mon mode input currents are zero. The accuracy of the
converter is not compromised by settling errors.
In applications where the input common mode
voltage is
constant
but different from the reference common mode
voltage, the differential input current remains zero while
the common mode input current is proportional to the
difference between V
IN(CM)
and V
REF(CM)
. For a reference
common mode voltage of 2.5V and an input common
mode of 1.5V, the common mode input current is ap
-
proximately 0.74µA. This common mode input current
does not degrade the accuracy if the source impedances
tied to IN
+
and IN
–
are matched. Mismatches in source
impedance lead to a fixed offset error but do not effect
the linearity or full scale reading. A 1% mismatch in a 1k
source resistance leads to a 74µV shift in offset voltage.
In applications where the common mode input voltage
varies as a function of the input signal level (single ended
type sensors), the common mode input current varies pro
-
portionally with input
voltage. For the case of balanced input
impedances, the common mode input current effects are
rejected by the large CMRR of the LTC2486, leading to little
degradation in accuracy. Mismatches in source impedances
lead to gain errors proportional to the difference between
the common mode input and common mode reference. 1%
mismatches
in 1k source resistances lead to gain errors on
the order of 15ppm. Based on the stability of the internal
sampling capacitors and the accuracy of the internal oscil
-
lator, a one-time calibration will remove this error.
In addition to the input sampling current, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA Max), results
in a small offset shift. A 1k source resistance will create a
1µV typical and a 10µV maximum offset voltage.
Reference Current
Similar to the analog inputs, the LTC2486 samples the
differential reference pins (REF
+
and REF
–
) transferring
small amounts of charge to and from these pins, thus
producing a dynamic reference current. If incomplete set
-
tling occurs (as a function the reference source resistance
