Datasheet

LTC2450
17
2450fb
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 noise fl oor (≈2μV
2
).
Aliasing
The LTC2450 signal acquisition circuit is a sampled data
system and as such suffers from input signal aliasing. As
can be seen from Figure 19, due to the very high over-
sample ratios the high frequency input signal attenuation
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 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
input signal attenuation with frequency at low frequencies
is shown in Figure 18.
The LTC2450 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
2450 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
2450 F19
–60
–80
–20
–100
2.5
5.0 7.5