LTC2485 datasheet(23/40 Pages) LINER

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LTC2485

2485fa

voltage). Table 6 summarizes the effects of mismatched

source impedance and differences in reference/input com-

mon mode voltages.

Table 6. Suggested Input Configuration for LTC2485

BALANCED INPUT

UNBALANCED INPUT

RESISTANCES

Constant

CEXT > 1nF at Both

CEXT > 1nF at Both IN+

VIN(CM) – VREF(CM)

IN+ and IN–. Can Take

and IN–. Can Take Large

Large Source Resistance

Source Resistance.

with Negligible Error

Unbalanced Resistance

Results in an Offset

Which Can be Calibrated

Varying

CEXT > 1nF at Both IN+

Minimize IN+ and IN–

VIN(CM) – VREF(CM)

and IN–. Can Take Large

Capacitors and Avoid

Source Resistance with

Large Source Impedance

Negligible Error

(< 5k Recommended)

The magnitude of the dynamic input current depends upon

the size of the very stable internal sampling capacitors and

upon the accuracy of the converter sampling clock. The

accuracy of the internal clock over the entire temperature

and power supply range is typically 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 IN+ and

IN–, the expected drift of the dynamic current and offset

will be insignificant (about 1% of their respective values

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 input sampling charge, 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

µV typical and 10µV maximum offset voltage.

Figure 13. An RC Network at IN+ and IN–

Figure 14. +FS Error vs RSOURCE at IN+ and IN–

Figure 15. –FS Error vs RSOURCE at IN+ and IN–

CEXT

2485 F13

VINCM + 0.5VIN

RSOURCE

IN+

LTC2485

CPAR

≅20pF

CEXT

VINCM – 0.5VIN

RSOURCE

IN –

CPAR

≅20pF

RSOURCE (Ω)

–20

100k

2485 F14

–40

–60

–80

100

10k

VCC = 5V

VREF = 5V

VIN

+ = 3.75V

VIN

– = 1.25V

TA = 25°C

CEXT = 0pF

CEXT = 100pF

CEXT = 1nF, 0.1µF, 1µF

RSOURCE (Ω)

–20

100k

2485 F15

–40

–60

–80

100

10k

VCC = 5V

VREF = 5V

VIN

+ = 1.25V

VIN

– = 3.75V

TA = 25°C

CEXT = 0pF

CEXT = 100pF

CEXT = 1nF, 0.1µF, 1µF

APPLICATIO S I FOR ATIO

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