LTC2489 datasheet(18/24 Pages) LINER

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LTC2489

2489f

In cases where large bypass capacitors are required on

the reference inputs (CREF > .01μF), full-scale and linear-

ity errors are proportional to the value of the reference

resistance. Every ohm of reference resistance produces

a full-scale error of approximately 0.5ppm (while operat-

ing with the internal oscillator) (see Figures 12 and 13). If

the input common mode voltage is equal to the reference

common mode voltage, a linearity error of approximately

0.67ppm per 100Ω of reference resistance results (see

Figure 14). In applications where the input and reference

common mode voltages are different, the errors increase.

A 1V difference in between common mode input and

common mode reference results in a 6.7ppm INL error

for every 100Ω of reference resistance.

In addition to the reference sampling charge, the reference

ESD protection diodes have a temperature dependent leak-

age current. This leakage current, nominally 1nA (±10nA

max) results in a small, gain error. A 100Ω reference

resistance will create a 0.5μV full-scale error.

Normal Mode Rejection and Antialiasing

One of the advantages delta-sigma ADCs offer over

conventional ADCs is on-chip digital ﬁltering. Combined

with a large oversample ratio, the LTC2489 signiﬁcantly

simpliﬁes antialiasing ﬁlter requirements. Additionally,

the input current cancellation feature allows external low

pass ﬁltering without degrading the DC performance of

the device.

APPLICATIONS INFORMATION

Figure 14. INL vs Differential Input Voltage and

Reference Source Resistance for CREF > 1μF

VIN/VREF

– 0.5

0.3

2489 F14

–2

–6

–4

–8

–10

– 0.3

– 0.1

0.1

0.5

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

TA = 25°C

CREF = 10μF

R = 1k

R = 100

R = 500

Figure 12. +FS Error vs RSOURCE at VREF (Large CREF)

Figure 13. –FS Error vs RSOURCE at VREF (Large CREF)

RSOURCE (Ω)

300

400

500

800

2489 F12

200

100

200

400

600

1000

VCC = 5V

VREF = 5V

VIN

+ = 3.75V

VIN

– = 1.25V

FO = GND

TA = 25°C

CREF = 1μF, 10μF

CREF = 0.1μF

CREF = 0.01μF

RSOURCE (Ω)

–200

–100

800

2489 F13

–300

–400

–500

200

400

600

1000

VCC = 5V

VREF = 5V

VIN

+ = 1.25V

VIN

– = 3.75V

FO = GND

TA = 25°C

CREF = 1μF, 10μF

CREF = 0.1μF

CREF = 0.01μF

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