LTC2413 LINER [Linear Technology], LTC2413 Datasheet - Page 32

• Manufacturer Part Number: LTC2413
• Description: 24-Bit No Latency ADC, with Simultaneous 50Hz/60Hz Rejection
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2413
Input Bandwidth
The combined effect of the internal sinc
of the analog and digital autocalibration circuits deter-
mines the LTC2413 input bandwidth. When the internal
oscillator is used (F
3.3Hz. If an external conversion clock generator of fre-
quency f
bandwidth is 0.236 • 10
Due to the complex filtering and calibration algorithms
utilized, the converter input bandwidth is not modeled very
accurately by a first order filter with the pole located at the
3dB frequency. When the internal oscillator is used, the
shape of the LTC2413 input bandwidth is shown in
Figure 36. When an external oscillator of frequency f
is used, the shape of the LTC2413 input bandwidth can be
derived from Figure 36, in which the horizontal axis is
scaled by f
The conversion noise (800nV
can be modeled as a white noise source connected to a
noise free converter. The noise spectral density is 63nV/ Hz
for an infinite bandwidth source and 77nV/ Hz for a single
0.5MHz pole source. From these numbers, it is clear that
particular attention must be given to the design of external
amplification circuits. Such circuits face the simultaneous
requirements of very low bandwidth (just a few Hz) in
order to reduce the output referred noise and relatively
high bandwidth (at least 500kHz) necessary to drive the
input switched-capacitor network. A possible solution is a
32
Figure 36. Input Signal Bandwidth Using the Internal Oscillator
EOSC
EOSC
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
0.0
is connected to the F
/139800.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
U
O
= LOW), the 3dB input bandwidth is
–6
U
• f
EOSC
RMS
.
typical for V
W
O
pin, the 3dB input
4
digital filter and
2413 F36
U
REF
= 5V)
EOSC
high gain, low bandwidth amplifier stage followed by a
high bandwidth unity-gain buffer.
When external amplifiers are driving the LTC2413, the
ADC input referred system noise calculation can be simpli-
fied by Figure 37. The noise of an amplifier driving the
LTC2413 input pin can be modeled as a band limited white
noise source. Its bandwidth can be approximated by the
bandwidth of a single pole lowpass filter with a corner
frequency f
From Figure 37, using f
on the y-axis the noise equivalent bandwidth freq
input driving amplifier. This bandwidth includes the band
limiting effects of the ADC internal calibration and filtering.
The noise of the driving amplifier referred to the converter
input and including all these effects can be calculated as
N = n
LTC2413 input) can now be obtained by summing as
square root of sum of squares the three ADC input referred
noise sources: the LTC2413 internal noise (800nV), the
noise of the IN
driving amplifier.
If the F
f
x-axis is scaled by f
ratio f
decrease, but in the same time the LTC2413 noise floor
rises and the noise contribution of the driving amplifiers
lose significance.
EOSC
Figure 37. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source
, Figure 37 can still be used for noise calculation if the
i
EOSC
O
• freq
pin is driven by an external oscillator of frequency
/139800, the Figure 37 plot accuracy begins to
1000
i
. The amplifier noise spectral density is n
100
0.1
10
1
i
0.1
+
. The total system noise (referred to the
driving amplifier and the noise of the IN
INPUT NOISE SOURCE SINGLE POLE
1
EOSC
EQUIVALENT BANDWIDTH (Hz)
i
10
as the x-axis selector, we can find
/139800. For large values of the
100
F
O
= LOW
1k
10k 100k
2413 F37
1M
sn2413 2413fs
i
of the
i
–
.