LTC2482
22
2482fc
APPLICATIONS INFORMATION
When using the internal oscillator, the LTC2482’s front-end
switched-capacitor network is clocked at 123kHz corre-
sponding 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
EOSC
is used, the sampling
period is 2.5/f
EOSC
and, for a settling error of less than
1ppm, τ ≤ 0.178/f
EOSC
.
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
combined 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
magnitude 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
CC
), added system cost and increased power.
The LTC2482 uses a proprietary switching algorithm that
forces the average differential input current to zero indepen-
dent of external settling errors. This allows accurate direct
digitization of high impedance sensors without the need
for buffers. Additional errors resulting from mismatched
leakage currents must also be taken into account.
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
INCM
) and the common mode reference
voltage (V
REFCM
).
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
differential and common mode input current are zero.
The accuracy of the converter is unaffected by settling
errors. Mismatches in source impedances between IN
+
and 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 differ-
ence between V
INCM
and V
REFCM
. For a reference common
mode of 2.5V and an input common mode of 1.5V, the
common mode input current is approximately 0.74μA. This
common mode input current has no effect on the accuracy
if the external source impedances tied to IN
+
and IN
–
are
matched. Mismatches in these source impedances lead to a
fi xed offset error but do not affect the linearity or full-scale
reading. A 1% mismatch in 1k source resistances leads to
a 1LSB 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 com-
mon 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 LTC2482 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 ref-
erence voltage. 1% mismatches in 1k source resistances
lead to gain worst-case gain errors on the order of 1LSB
(for 1V differences in reference and input common mode
voltage). Table 5 summarizes the effects of mismatched
source impedance and differences in reference/input
common mode voltages.
Table 5. Suggested Input Confi guration for LTC2482
BALANCED INPUT
RESISTANCES
UNBALANCED INPUT
RESISTANCES
Constant
V
IN(CM)
– V
REF(CM)
C
IN
> 1nF at Both IN
+
and IN
–
. Can Take Large
Source Resistance with
Negligible Error
C
IN
> 1nF at Both IN
+
and
IN
–
. Can Take Large Source
Resistance. Unbalanced
Resistance Results in
an Offset Which Can be
Calibrated
Varying
V
IN(CM)
– V
REF(CM)
C
IN
> 1nF at Both IN
+
and IN
–
. Can Take Large
Source Resistance with
Negligible Error
Minimize IN
+
and IN
–
Capacitors and Avoid
Large Source Impedance
(<5k Recommended)
