Datasheet

LTC2411/LTC2411-1
29
the internal oscillator is used, the 3dB input bandwidth of
the LTC2411 is 3.63Hz for 60Hz notch frequency (F
O
=
LOW) and 3.02Hz for 50Hz notch frequency (F
O
= HIGH).
The 3dB input bandwidth for the LTC2411-1 is 3.30Hz
(F
O
= LOW). If an external conversion clock generator of
frequency 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 LTC2411/LTC2411-1 input bandwidth is
shown in Figure␣ 28. When an external oscillator of fre-
quency f
EOSC
is used, the shape of the LTC2411/LTC2411-1
input bandwidth can be derived from Figure␣ 28, F
O
= LOW
curve of the LTC2411 in which the horizontal axis is scaled
by f
EOSC
/153600.
The conversion noise (1.45µV
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 70nV/√Hz
for an infinite bandwidth source and 126nV/√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
APPLICATIO S I FOR ATIO
WUUU
Figure 27. Resolution (INL
MAX
≤ 1LSB)
vs Output Data Rate and Reference Voltage
OUTPUT DATA RATE (READINGS/SEC)
10
RESOLUTION (BITS)
14
18
22
12
16
20
20 40 60 80
2411 F27
10010030507090
V
CC
= 5V
REF
–
= GND
V
INCM
= 2.5V
V
IN
= 0V
F
O
= EXT OSC
RES = LOG
2
(V
REF
/INL
MAX
)
T
A
= 25°C
V
REF
= 5VV
REF
= 2.5V
Figure 26. Resolution (Noise
RMS
≤ 1LSB)
vs Output Data Rate and Reference Voltage
OUTPUT DATA RATE (READINGS/SEC)
0
RESOLUTION (BITS)
20
21
80
2411 F26
19
18
20
40
50
100
22
60
10
30
90
70
V
CC
= 5V
REF
–
= GND
V
INCM
= 2.5V
V
IN
= 0V
F
O
= EXT OSC
RES = LOG
2
(V
REF
/NOISE
RMS
)
T
A
= 25°C
V
REF
= 2.5V
V
REF
= 5V
Figure 28. 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)
2411 F28
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
F
O
= HIGH
(LTC2411)
F
O
= LOW
(LTC2411)
F
O
= LOW
(LTC2411-1)
solution is a high gain, low bandwidth amplifier stage
followed by a high bandwidth unity-gain buffer.
When external amplifiers are driving the LTC2411/
LTC2411-1, the ADC input referred system noise calcula-
tion can be simplified by Figure 29. The noise of an
amplifier driving the LTC2411/LTC2411-1 input pin can be
modeled as a band-limited white noise source. Its band-
width 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␣ 29, using f
i
as the
x-axis selector, we can find on the y-axis the noise equiva-
lent bandwidth freq
i
of the input driving amplifier. This
bandwidth includes the band limiting effects of the ADC