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LTC2498
29
2498ff
applications inForMation
independent 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 (IIN+) to be equal to the average input
current in the negative input (IIN–). Over the complete
conversion cycle, the average input current (IIN+ – IIN–)
is zero. While the differential input current is zero, the
common mode input current (IIN+ + IIN–)/2 is proportional
to the difference between the common mode input volt-
age (VIN(CM)) and the common mode reference voltage
(VREF(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 current 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 VIN(CM) and VREF(CM). For a reference
commonmodevoltageof2.5Vandaninputcommonmode
of 1.5V, the common mode input current is approximately
0.74A (in simultaneous 50Hz/60Hz rejection mode). This
commonmodeinputcurrentdoesnotdegradetheaccuracy
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 74V
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
proportionally with input voltage. For the case of balanced
inputimpedances,thecommonmodeinputcurrenteffects
are rejected by the large CMRR of the LTC2498, leading
to little degradation in accuracy. Mismatches in source
impedances lead to gain errors proportional to the dif-
ference 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 ac-
curacy of the internal oscillator, 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
1V typical and a 10V maximum offset voltage.
Automatic Offset Calibration of External Buffers/
Amplifiers
In addition to the Easy Drive input current cancellation,
the LTC2498 enables an external amplifier to be inserted
between the multiplexer output and the ADC input, see
Figure 13. This is useful in applications where balanced
source impedances are not possible. One pair of external
buffers/amplifiers can be shared between all 17 analog
inputs.TheLTC2498performsaninternaloffsetcalibration
every conversion cycle in order to remove the offset and
drift of the ADC. This calibration is performed through a
combination of front end switching and digital process-
ing. Since the external amplifier is placed between the
multiplexer and the ADC, it is inside this correction loop.
This results in automatic offset correction and offset drift
removal of the external amplifier.
Figure 13. External Buffers Provide High Impedance Inputs and
Amplifier Offsets are Automatically Cancelled.
–
+
–
+
1/2 LTC6078
1
2
3
5
6
7
ΔΣ ADC
WITH
EASY DRIVE
INPUTS
INPUT
MUX
MUXOUTP
MUXOUTN
17
2498 F13
LTC2498
ANALOG
INPUTS
SDI
SCK
SDO
CS
1k
0.1F
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