LTC2497 datasheet(21/28 Pages) LINER

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LTC2497

2497f

zero, the common mode input current (IIN

+ + IIN–)/2 is

proportional to the difference between the common mode

input voltage (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

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

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/

Ampliﬁers

In addition to the Easy Drive input current cancellation,

the LTC2497 allows an external ampliﬁer to be inserted

between the multiplexer output and the ADC input (see

Figure 10). This is useful in applications where balanced

Figure 10. External Buffers Provide High Impedance Inputs

and Ampliﬁer Offsets are Automatically Cancelled.

–

1/2 LTC6078

∆Σ ADC

WITH

EASY DRIVE

INPUTS

INPUT

MUX

2497 F10

LTC2497

ANALOG

INPUTS

SCL

SDA

0.1

µF

0.1

µF

APPLICATIONS INFORMATION

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