LTC2486IDE datasheet(28/36 Pages) LINER

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LTC2486

2486f

APPLICATIONS INFORMATION

When using the LTC2486’s internal oscillator, the input

capacitor array is switched at 123kHz. The effect of the

charge transfer depends on the circuitry driving the in-

put/reference pins. If the total external RC time constant

is less than 580ns the errors introduced by the sampling

process are negligible since complete settling occurs.

Typically, the reference inputs are driven from a low imped-

ance source. In this case, complete settling occurs even

with large external bypass capacitors. The inputs (CH0 to

CH3, COM), on the other hand, are typically driven from

larger source resistances. Source resistances up to 10k

may interface directly to the LTC2486 and settle completely;

however, the addition of external capacitors at the input

terminals in order to ﬁlter unwanted noise (anti-aliasing)

results in incomplete settling.

Automatic Differential Input Current Cancellation

In applications where the sensor output impedance is

low (up to 10k

Ω with no external bypass capacitor or up

to 500

Ω with 0.001µF bypass), complete settling of the

input occurs. In this case, no errors are introduced and

direct digitization is possible.

For many applications, the sensor output impedance

combined with external input bypass capacitors produces

RC time constants much greater than the 580ns required

for 1ppm accuracy. For example, a 10k

Ω bridge driving a

0.1µF capacitor has a time constant an order of magnitude

greater than the required maximum.

The LTC2486 uses a proprietary switching algorithm that

forces the average differential input current to zero indepen-

dent 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 on the negative input (IIN

–). Over the complete

conversion cycle, the average differential 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 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 currents 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 approximately

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 pro-

portionally with input voltage. For the case of balanced input

impedances, the common mode input current effects are

rejected by the large CMRR of the LTC2486, leading to little

degradation in accuracy. Mismatches in source impedances

lead to gain errors proportional to the difference 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 accuracy of the internal oscil-

lator, 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.

Reference Current

Similar to the analog inputs, the LTC2486 samples the

differential reference pins (REF+ and REF–) transferring

small amounts of charge to and from these pins, thus

producing a dynamic reference current. If incomplete set-

tling occurs (as a function the reference source resistance

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