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LTC2482

2482fb

APPLICATIONS INFORMATION

(referred to the LTC2482 input) can now be obtained by

summing as square root of sum of squares the three ADC

input referred noise sources: the LTC2482 internal noise,

the noise of the IN+ driving ampliﬁer and the noise of the

IN– driving ampliﬁer.

If the fO pin is driven by an external oscillator of frequency

fEOSC, Figure 26 can still be used for noise calculation if

the x-axis is scaled by fEOSC/307200. For large values of

the ratio fEOSC/307200, the Figure 26 plot accuracy begins

to decrease, but at the same time the LTC2482 noise ﬂoor

rises and the noise contribution of the driving ampliﬁers

lose signiﬁcance.

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 oversampling ratio, the LTC2482 signiﬁcantly

simpliﬁes antialiasing ﬁlter requirements. Additionally,

the input current cancellation feature of the LTC2482 al-

lows external lowpass ﬁltering without degrading the DC

performance of the device.

The SINC4 digital ﬁlter provides greater than 120dB normal

mode rejection at all frequencies except DC and integer

multiples of the modulator sampling frequency (fS). The

LTC2482’s autocalibration circuits further simplify the

antialiasing requirements by additional normal mode

signal ﬁltering both in the analog and digital domain.

Independent of the operating mode, fS = 256 • fN = 2048

• fOUTMAX where fN is the notch frequency and fOUTMAX

is the maximum output data rate. In the internal oscilla-

tor mode with 50Hz/60Hz rejection, fS = 13960Hz. In the

external oscillator mode, fS = fEOSC/20.

The regions of low rejection occurring at integer multiples

of fS have a very narrow bandwidth. Magniﬁed details of

the normal mode rejection curves are shown in Figure 27

(rejection near DC) and Figure 28 (rejection at fS = 256fN)

where fN represents the notch frequency. These curves

have been derived for the external oscillator mode but

they can be used in all operating modes by appropriately

selecting the fN value.

The user can expect to achieve this level of performance

using the internal oscillator as it is demonstrated by

Figure 29. Typical measured values of the normal mode

rejection of the LTC2482 operating with an internal oscil-

lator (50Hz/60Hz rejection) is shown in Figure 29.

As a result of these remarkable normal mode speciﬁca-

tions, minimal (if any) antialias ﬁltering is required in front

of the LTC2482. If passive RC components are placed in

front of the LTC2482, the input dynamic current should

be considered (see Input Current section). In this case,

the differential input current cancellation feature of the

LTC2482 allows external RC networks without signiﬁcant

degradation in DC performance.

Traditional high order delta-sigma modulators, while

providing very good linearity and resolution, suffer

from potential instabilities at large input signal levels.

The proprietary architecture used for the LTC2482 third

INPUT SIGNAL FREQUENCY (Hz)

2482 F27

–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

2482 F28

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

Figure 27. Input Normal Mode Rejection at DC

Figure 28. Input Normal Mode Rejection at fS = 256fN

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