LTC2439-1IGN datasheet(25/28 Pages) LINER

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LTC2439-1

24391f

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 one-time 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 LTC2439-1 can

produce up to 6.8 readings per second. 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 LTC2439-1 output data rate can be in-

creased as desired. The duration of the conversion phase

is 20510/fEOSC. If fEOSC = 139,800Hz, the converter be-

haves as if the internal oscillator is used with simultaneous

50Hz/60Hz. There is no significant difference in the

LTC2439-1 performance between these two operation

modes.

An increase in fEOSC over the nominal 139,800Hz 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.

RSOURCE (Ω)

0 100 200 300 400 500 600 700 800 900 1000

24361 F21

VCC = 5V

REF+ = 5V

REF– = GND

IN+ = 3.75V

IN – = 1.25V

FO = GND

TA = 25°C

CREF = 0.01µF

CREF = 0.1µF

CREF = 1µF, 10µF

RSOURCE (Ω)

0 100 200 300 400 500 600 700 800 900 1000

24361 F22

VCC = 5V

REF+ = 5V

REF– = GND

IN+ = 1.25V

IN – = 3.75V

FO = GND

TA = 25°C

CREF = 0.01µF

CREF = 0.1µF

CREF = 1µF, 10µF

VINDIF/VREFDIF

–0.5 –0.4–0.3–0.2–0.1 0

0.1 0.2 0.3 0.4 0.5

–1

VCC = 5V

REF+ = 5V

REF– = GND

VINCM = 0.5 • (IN

+ + IN–) = 2.5V

FO = GND

CREF = 10µF

TA = 25°C

RSOURCE = 1000Ω

24361 F23

Figure 21. +FS Error vs RSOURCE at REF+ and REF– (Large CREF)

Figure 22. –FS Error vs RSOURCE at REF+ and REF– (Large CREF)

Figure 23. INL vs Differential Input Voltage (VIN = IN

+ – IN–)

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

for Large CREF Values (CREF ≥ 1µF)

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