LTC2497 Linear Technology, LTC2497 Datasheet - Page 21

• Manufacturer Part Number: LTC2497
• Description: 16-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
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 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 V
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
impedance lead to a fi xed 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
+
REF(CM)
and IN
IN(CM)
).
–
are matched. Mismatches in source
IN(CM)
) and the common mode reference
and V
ANALOG
INPUTS
Figure 10. External Buffers Provide High Impedance Inputs
and Amplifi er Offsets are Automatically Cancelled.
REF(CM)
17
INPUT
MUX
. For a reference
IN
+
+ I
IN
3
2
6
5
–
–
1/2 LTC6078
+
–
1/2 LTC6078
+
)/2 is
LTC2497
1
7
type sensors), 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 LTC2497, 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
1µV typical and a 10µV maximum offset voltage.
Automatic Offset Calibration of External Buffers/
Amplifi ers
In addition to the Easy Drive input current cancellation,
the LTC2497 allows an external amplifi er to be inserted
between the multiplexer output and the ADC input (see
Figure 10). This is useful in applications where balanced
0.1µF
0.1µF
1k
1k
2497 F10
EASY DRIVE
∆Σ ADC
INPUTS
WITH
SDA
SCL
LTC2497
21
2497f