Datasheet

LTC2450-1
17
24501fc
APPLICATIONS INFORMATION
noise contribution of the external drive circuit would be
V
n
= n
i
• √π/2 • F
i
. Then, the total system noise level can
be estimated as the square root of the sum of (V
n
2
) and
the square of the LTC2450-1 noise fl oor (≈2μV
2
).
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 fi lter 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 R
S
and C
IN
components shown in Figure 15 sized such that τ = R
S
• C
IN
> 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 sinc
1
type digital fi lter with the
fi rst notch located at f
0
= 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 1.4μV
RMS
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 V
IN
drive circuit can
be modeled as a single pole equivalent circuit character-
ized by a pole location F
i
and a noise spectral density n
i
.
If the converter has an unlimited bandwidth or at least
a bandwidth substantially larger than F
i
, then the total
INPUT SIGNAL FREQUENCY (Hz)
0
INPUT SIGNAL ATTENUATIOIN (dB)
–20
–10
0
480
24501 F18
–30
–40
–25
–15
–5
–35
–45
–50
12060
240180
360 420 540
300
600
INPUT SIGNAL FREQUENCY (MHz)
0
INPUT SIGNAL ATTENUATION (dB)
–40
0
10.0 12.5 15.0
24501 F19
–60
–80
–20
–100
2.5
5.0 7.5