LTC2486IDE datasheet(30/36 Pages) LINER

30 / 36 page

LTC2486

2486f

APPLICATIONS INFORMATION

A 1V difference in between common mode input and com-

mon mode reference results in a 6.7ppm INL error for every

100

Ω 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 100

Ω reference

resistance will create a 0.5µV full scale error.

Normal Mode Rejection and Anti-aliasing

One of the advantages delta-sigma ADCs offer over

conventional ADCs is on-chip digital ﬁltering. Combined

with a large oversample ratio, the LTC2486 signiﬁcantly

simpliﬁes anti-aliasing ﬁlter requirements. Additionally,

the input current cancellation feature allows external low

pass ﬁltering without degrading the DC performance of

the device.

The SINC4 digital ﬁlter provides excellent normal mode

rejection at all frequencies except DC and integer multiples

of the modulator sampling frequency (fS) (see Figures

18 and 19). The modulator sampling frequency is fS =

15,360Hz while operating with its internal oscillator and

fS = FEOSC/20 when operating with an external oscillator

of frequency FEOSC.

When using the internal oscillator, the LTC2486 is designed

to reject line frequencies. As shown in Figure 20, rejec-

tion nulls occur at multiples of frequency fN, where fN is

determined by the input control bits FA and FB (fN = 50Hz

or 60Hz or 55Hz for simultaneous rejection). Multiples of

the modulator sampling rate (fS = fN • 256) only reject noise

to 15dB (see Figure 21); if noise sources are present at

these frequencies anti-aliasing will reduce their effects.

Figure 18. Input Normal Mode Rejection, Internal Oscillator

and 50Hz Rejection Mode

Figure 19. Input Normal Mode Rejection, Internal Oscillator

and 60Hz Rejection Mode

Figure 20. Input Normal Mode Rejection at DC

Figure 21. Input Normal Mode Rejection at fS = 256 • fN

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS

2486 F18

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

0fS

2486 F19

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS

INPUT SIGNAL FREQUENCY (Hz) 2486 F20

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

2fN 3fN 4fN 5fN 6fN 7fN 8fN

fN = fEOSC/5120

INPUT SIGNAL FREQUENCY (Hz)

250fN 252fN 254fN 256fN 258fN 260fN 262fN

2486 F21

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

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