Datasheet

LTC2413
32
sn2413 2413fs
APPLICATIO S I FOR ATIO
WUU
U
Input Bandwidth
The combined effect of the internal sinc
4
digital filter and
of the analog and digital autocalibration circuits deter-
mines the LTC2413 input bandwidth. When the internal
oscillator is used (F
O
= LOW), the 3dB input bandwidth is
3.3Hz. If an external conversion clock generator of fre-
quency f
EOSC
is connected to the F
O
pin, the 3dB input
bandwidth is 0.236 • 10
–6
• f
EOSC
.
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
EOSC
is used, the shape of the LTC2413 input bandwidth can be
derived from Figure␣ 36, in which the horizontal axis is
scaled by f
EOSC
/139800.
The conversion noise (800nV
RMS
typical for V
REF
= 5V)
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
Figure 36. Input Signal Bandwidth Using the Internal Oscillator
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
INPUT SIGNAL ATTENUATION (dB)
2413 F36
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
–4.0
–4.5
–5.0
–5.5
–6.0
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
i
. The amplifier noise spectral density is n
i
.
From Figure␣ 37, using f
i
as the x-axis selector, we can find
on the y-axis the noise equivalent bandwidth freq
i
of the
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
i
• √freq
i
. The total system noise (referred to the
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 and the noise of the IN
–
driving amplifier.
If the F
O
pin is driven by an external oscillator of frequency
f
EOSC
, Figure 37 can still be used for noise calculation if the
x-axis is scaled by f
EOSC
/139800. For large values of the
ratio f
EOSC
/139800, the Figure 37 plot accuracy begins to
decrease, but in the same time the LTC2413 noise floor
rises and the noise contribution of the driving amplifiers
lose significance.
1
10
0.1
100
1000
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
0.1 1 10 100 1k 10k 100k 1M
2413 F37
F
O
= LOW
Figure 37. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source