LTC2436-1IGN datasheet(23/28 Pages) LINER

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LTC2436-1

24361f

Larger values of reference capacitors (CREF > 0.01µF) may

be required as reference filters in certain configurations.

Such capacitors will average the reference sampling charge

and the external source resistance will see a quasi con-

stant reference differential impedance. When FO = LOW

(internal oscillator and 50Hz/60Hz notch), the typical

differential reference resistance is 1.4M

Ω which will gen-

erate a gain error of approximately 1LSB full scale for each

Ω of source resistance driving REF+ or REF–. When FO

is driven by an external oscillator with a frequency fEOSC

(external conversion clock operation), the typical differen-

tial reference resistance is 0.20 • 1012/fEOSCΩ and each

ohm of source resistance drving REF+ or REF– will result

in 1.54 • 10–7 • fEOSCLSB gain error at full scale. The effect

APPLICATIO S I FOR ATIO

Figure 19. +FS Error vs RSOURCE at REF+ or REF– (Small CIN)

Figure 20. –FS Error vs RSOURCE at REF+ or REF– (Small CIN)

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

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

RSOURCE (Ω)

100

10k

100k

24361 F19

–3

–2

–1

VCC = 5V

REF+ = 5V

REF – = GND

IN+ = 5V

IN – = 2.5V

FO = GND

TA = 25°C

CREF = 0.01µF

CREF = 0.001µF

CREF = 100pF

CREF = 0pF

RSOURCE (Ω)

100

10k

100k

2412 F19

VCC = 5V

REF+ = 5V

REF – = GND

IN+ = GND

IN – = 2.5V

FO = GND

TA = 25°C

CREF = 0.01µF

CREF = 0.001µF

CREF = 100pF

CREF = 0pF

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

of the source resistance on the two reference pins is

additive with respect to this gain error. The typical +FS and

–FS errors for various combinations of source resistance

seen by the REF+ and REF– pins and external capacitance

CREF connected to these pins are shown in Figures 19, 20,

21 and 22.

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

mance is degraded by the reference source impedance.

When FO = LOW (internal oscillator and 50Hz/60Hz notch),

every 1000

Ω of source resistance driving REF+ or REF–

translates into about 1LSB additional INL error. When FO

is driven by an external oscillator with a frequency fEOSC,

every 100

Ω of source resistance driving REF+ or REF–

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