LTC2489CDEPBF datasheet(17/24 Pages) LINER

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LTC2489

2489f

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 LTC2489, 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.

Reference Current

Similar to the analog inputs, the LTC2489 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

and reference bypass capacitance) linearity and gain errors

are introduced.

For relatively small values of external reference capacitance

(CREF< 1nF), the voltage on the sampling capacitor settles

for reference impedances of many kΩ (if CREF = 100pF up

to 10kΩ will not degrade the performance) (see Figures

10 and 11).

APPLICATIONS INFORMATION

Figure 10. +FS Error vs RSOURCE at VREF (Small CREF)

Figure 11. –FS Error vs RSOURCE at VREF (Small CREF)

RSOURCE (Ω)

10k

2489 F10

–10

100

100k

VCC = 5V

VREF = 5V

VIN

+ = 3.75V

VIN

– = 1.25V

FO = GND

TA = 25°C

CREF = 0.01μF

CREF = 0.001μF

CREF = 100pF

CREF = 0pF

RSOURCE (Ω)

–30

–10

10k

2489 F11

–50

–70

–40

–20

–60

–80

–90

100

100k

VCC = 5V

VREF = 5V

VIN

+ = 1.25V

VIN

– = 3.75V

FO = GND

TA = 25°C

CREF = 0.01μF

CREF = 0.001μF

CREF = 100pF

CREF = 0pF

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