LTC2414IGN datasheet(30/48 Pages) LINER

30 / 48 page

LTC2414/LTC2418

241418fa

In addition to this gain error, the converter INL perfor-

mance is degraded by the reference source impedance.

When FO = LOW (internal oscillator and 60Hz notch), every

100

Ω of source resistance driving REF+ or REF– translates

into about 1.34ppm additional INL error. When FO = HIGH

(internal oscillator and 50Hz notch), every 100

Ωof source

resistance driving REF+ or REF– translates into about

1.1ppm additional INL error. When FO is driven by an

external oscillator with a frequency fEOSC, every 100Ω of

source resistance driving REF+ or REF– translates into

about 8.73 • 10–6 • fEOSCppm additional INL error.

Figure 22 shows the typical INL error due to the source

resistance driving the REF+ or REF– pins when large CREF

values are used. The effect of the source resistance on the

two reference pins is additive with respect to this INL error.

In general, matching of source impedance for the REF+

and REF– pins does not help the gain or the INL error. The

user is thus advised to minimize the combined source

impedance driving the REF+ and REF– pins rather than to

try to match it.

The magnitude of the dynamic reference current depends

upon the size of the very stable internal sampling capaci-

tors and upon the accuracy of the converter sampling

clock. The accuracy of the internal clock over the entire

temperature and power supply range is typical better than

0.5%. Such a specification can also be easily achieved by

an external clock. When relatively stable resistors

(50ppm/

°C) are used for the external source impedance

seen by REF+ and REF–, the expected drift of the dynamic

current gain error will be insignificant (about 1% of its

value over the entire temperature and voltage range). Even

for the most stringent applications a onetime calibration

operation may be sufficient.

In addition to the reference sampling charge, the reference

pins ESD protection diodes have a temperature dependent

leakage current. This leakage current, nominally 1nA

(

±10nA max), results in a small gain error. A 100Ω source

resistance will create a 0.05

µV typical and 0.5µV maxi-

mum full-scale error.

Output Data Rate

When using its internal oscillator, the LTC2414/LTC2418

can produce up to 7.5 readings per second with a notch

frequency of 60Hz (FO = LOW) and 6.25 readings per

second with a notch frequency of 50Hz (FO = HIGH). The

actual output data rate will depend upon the length of the

sleep and data output phases which are controlled by the

user and which can be made insignificantly short. When

operated with an external conversion clock (FO connected

to an external oscillator), the LTC2414/LTC2418 output

data rate can be increased as desired up to that determined

by the maximum fEOSC frequency of 2000kHz. The dura-

tion of the conversion phase is 20510/fEOSC. If fEOSC =

153600Hz, the converter behaves as if the internal oscil-

lator is used and the notch is set at 60Hz. There is no

significant difference in the LTC2414/LTC2418 perfor-

mance between these two operation modes.

An increase in fEOSC over the nominal 153600Hz will

translate into a proportional increase in the maximum

output data rate. This substantial advantage is neverthe-

less accompanied by three potential effects, which must

be carefully considered.

First, a change in fEOSC will result in a proportional change

in the internal notch position and in a reduction of the

converter differential mode rejection at the power line

frequency. In many applications, the subsequent perfor-

mance degradation can be substantially reduced by rely-

ing upon the LTC2414/LTC2418’s exceptional common

APPLICATIO S I FOR ATIO

Figure 22. INL vs Differential Input Voltage (VIN = IN+ – IN–)

and Reference Source Resistance (RSOURCE at REF+ and REF– for

Large CREF Values (CREF ≥ 1µF)

VINDIF/VREFDIF

–0.5 –0.4–0.3–0.2–0.1 0

0.1 0.2 0.3 0.4 0.5

–3

–6

–9

–12

–15

VCC = 5V

REF+ = 5V

REF– = GND

VINCM = 0.5 • (IN

+ + IN–) = 2.5V

FO = GND

CREF = 10µF

TA = 25°C

RSOURCE = 1000Ω

RSOURCE = 500Ω

RSOURCE = 100Ω

2414/18 F22

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