LTC2400CS8 datasheet(25/40 Pages) LINER

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LTC2400

APPLICATIONS INFORMATION

INPUT FREQUENCY

–60

–40

2400 F26

–80

–100

fS/2

–120

–140

–20

Figure 26. Sinc4 Filter Rejection

RESISTANCE AT VREF (Ω)

–20

400

800

1000

160

2400 F25

200

600

100

120

140

CVREF = 0.01µF

CVREF = 0.1µF

CVREF = 1µF

CVREF = 10µF

VCC = 5V

VREF = 5V

TA = 25°C

Figure 25. INL Error vs RVREF (Large C)

In addition to the dynamic reference current, the VREF ESD

protection diodes have a temperature dependent leakage

current. This leakage current, nominally 1nA (

±10nAmax),

results in a fixed full-scale shift of 10

µV for a 10k source

resistance.

ANTIALIASING

One of the advantages delta-sigma ADCs offer over con-

ventional ADCs is on-chip digital filtering. Combined with

a large oversampling ratio, the LTC2400 significantly

simplifies antialiasing filter requirements.

The digital filter provides very high rejection except at

integer multiples of the modulator sampling frequency

(fS), see Figure 26. The modulator sampling frequency is

256 • FO, where FO is the notch frequency (typically 50Hz

or 60Hz). The bandwidth of signals not rejected by the

digital filter is narrow (

≈0.2%)comparedtothebandwidth

of the frequencies rejected.

As a result of the oversampling ratio (256) and the digital

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

of the LTC2400. If passive RC components are placed in

front of the LTC2400 the input dynamic current should be

considered (see Input Current section). In cases where

large effective RC time constants are used, an external

buffer amplifier may be required to minimize the effects of

input dynamic current.

The modulator contained within the LTC2400 can handle

large-signal level perturbations without saturating. Signal

levels up to 40% of VREF do not saturate the analog modu-

lator. These signals are limited by the input ESD protection

to 300mV below ground and 300mV above VCC.

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