LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 22

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
For a simple approximation, the source impedance R
driving an analog input pin (IN
considered to form, together with R
Figure 12), a first order passive network with a time con-
stant τ = (R
the input signal with better than 1ppm accuracy if the
sampling period is at least 14 times greater than the input
circuit time constant τ. The sampling process on the four
input analog pins is quasi-independent so each time con-
stant should be considered by itself and, under worst-case
circumstances, the errors may add.
When using the internal oscillator, the LTC2485’s front-
end switched-capacitor network is clocked at 123kHz
corresponding to an 8.1µs sampling period. Thus, for
settling errors of less than 1ppm, the driving source
impedance should be chosen such that τ ≤ 8.1µs/14 =
580ns. When an external oscillator of frequency f
used, the sampling period is 2.5/f
error of less than 1ppm, τ ≤ 0.178/f
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 of the sensor is possible.
For many applications, the sensor output impedance com-
bined with external bypass capacitors produces RC time
constants much greater than the 580ns required for 1ppm
accuracy. For example, a 10kΩ bridge driving a 0.1µF
bypass capacitor has a time constant an order of magni-
tude greater than the required maximum. Historically,
settling issues were solved using buffers. These buffers
led to increased noise, reduced DC performance (Offset/
Drift), limited input/output swing (cannot digitize signals
near ground or V
power. The LTC2485 uses a proprietary switching algo-
rithm that forces the average differential input current to
zero independent of external settling errors. This allows
accurate direct digitization of high impedance sensors
without the need of buffers (see Figures 13 to 15). Addi-
tional errors resulting from mismatched leakage currents
must also be taken into account.
LTC2485
22
S
+ R
SW
CC
) • C
U
), added system cost and increased
EQ
. The converter is able to sample
U
+
, IN
–
EOSC
W
, REF
EOSC
SW
and, for a settling
+
.
or REF
and C
U
–
) can be
EQ
EOSC
(see
is
S
The switching algorithm forces the average input current
on the positive input (I
current on the negative input (I
conversion cycle, the average differential input current
(I
zero, the common mode input current (I
proportional to the difference between the common mode
input voltage (V
voltage (V
In applications where the input common mode voltage is
equal to the reference common mode voltage, as in the
case of a balance bridge type application, both the differ-
ential and common mode input current are zero. The
accuracy of the converter is unaffected by settling errors.
Mismatches in source impedances between IN
also do not affect the accuracy.
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
common mode of 2.5V and an input common mode of
1.5V, the common mode input current is approximately
0.74µA (in simultaneous 50Hz/60Hz rejection mode). This
common mode input current has no effect on the accuracy
if the external source impedances tied to IN
matched. Mismatches in these source impedances lead to
a fixed offset error but do not affect the linearity or full-
scale reading. A 1% mismatch in 1kΩ source resistances
leads to a 15ppm shift (74µV) in offset voltage.
In applications where the common mode input voltage
varies as a function of input signal level (single-ended
input, RTDs, half bridges, current sensors, etc.), the
common mode input current varies proportionally with
input voltage. For the case of balanced input impedances,
the common mode input current effects are rejected by the
large CMRR of the LTC2485 leading to little degradation in
accuracy. Mismatches in source impedances lead to gain
errors proportional to the difference between the common
mode input voltage and the common mode reference
voltage. 1% mismatches in 1kΩ source resistances lead
to worst-case gain errors on the order of 15ppm or 1LSB
(for 1V differences in reference and input common mode
IN
+
– I
IN
–
REFCM
) is zero. While the differential input current is
).
INCM
) and the common mode reference
INCM
IN
+
) to be equal to the average input
and V
IN
REFCM
–
). Over the complete
. For a reference
IN
+
+
and IN
+ I
+
IN
and IN
–
)/2 is
–
2485fa
are
–