LTC2483 datasheet(17/20 Pages) LINER

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LTC2450-1

24501fb

APPLICATIONS INFORMATION

noise contribution of the external drive circuit would be

Vn = ni • √π/2 • Fi. Then, the total system noise level can

be estimated as the square root of the sum of (Vn2) and

the square of the LTC2450-1 noise ﬂoor (≈2μV2).

Aliasing

The LTC2450-1 signal acquisition circuit is a sampled

data system and as such suffers from input signal alias-

ing. As can be seen from Figure 19, due to the very high

over-sample ratios the high frequency input signal attenu-

ation is reasonably good. Nevertheless a continuous time

antialiasing ﬁlter connected at the input will preserve

the converter accuracy when the input signal includes

undesirable high frequency components. The antialias-

ing function can be accomplished using the RS and CIN

components shown in Figure 15 sized such that τ = RS

• CIN > 450ns.

Figure 18. Input Signal Attenuation vs Frequency

(Low Frequencies)

Figure 19. Input Signal Attenuation vs Frequency

Signal Bandwidth and Noise Equivalent Input

Bandwidth

The LTC2450-1 includes a sinc1 type digital ﬁlter with the

ﬁrst notch located at f0 = 60Hz. As such the 3dB input

signal bandwidth is 26.54Hz. The calculated LTC2450-1

input signal attenuation with frequency at low frequencies

is shown in Figure 18.

The LTC2450-1 input signal attenuation with frequency

over a wide frequency range is shown in Figure 19.

The converter noise level is about 1.4μVRMS and can be

modeled by a white noise source connected at the input

of a noise free converter.

For a simple system noise analysis the VIN drive circuit can

be modeled as a single pole equivalent circuit character-

ized by a pole location Fi and a noise spectral density ni.

If the converter has an unlimited bandwidth or at least

a bandwidth substantially larger than Fi, then the total

INPUT SIGNAL FREQUENCY (Hz)

–20

–10

480

24501 F18

–30

–40

–25

–15

–5

–35

–45

–50

120

240

180

360 420

540

300

600

INPUT SIGNAL FREQUENCY (MHz)

–40

10.0

12.5

15.0

24501 F19

–60

–80

–20

–100

2.5

5.0

7.5

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