Datasheet
LTC2430/LTC2431
29
24301f
an infinite bandwidth source and 216nV/√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 high gain, low bandwidth amplifier stage
followed by a high bandwidth unity-gain buffer.
When external amplifiers are driving the LTC2430/
LTC2431, the ADC input referred system noise calculation
can be simplified by Figure 29. The noise of an amplifier
driving the LTC2430/LTC2431 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 spec-
tral density is n
i
. From Figure␣ 29, 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 band-
width 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 LTC2430/LTC2431 input) can now
be obtained by summing as square root of sum of squares
the three ADC input referred noise sources: the LTC2430/
LTC2431 internal noise (2.8µV), the noise of the IN
+
driving amplifier and the noise of the IN
–
driving amplifier.
APPLICATIO S I FOR ATIO
WUUU
Figure 28. Input Signal Bandwidth Using the Internal Oscillator
Figure 26. Resolution (Noise
RMS
≤ 1LSB)
vs Output Data Rate and V
CC
OUTPUT DATA RATE (READINGS/SEC)
0
15
RESOLUTION (BITS)
16
18
19
20
22
10
50
70
2430 F26
17
21
40
90
100
20
30
60 80
V
INCM
= V
REFCM
V
IN
= 0V
F
O
= EXT OSC
REF
–
= GND
T
A
= 25°C
RES = LOG
2
(V
REF
/NOISE
RMS
)
V
CC
= V
REF
= 5V
V
CC
= 2.7V
V
REF
= 2.5V
Figure 27. Resolution (INL
MAX
≤ 1LSB)
vs Output Data Rate and V
CC
OUTPUT DATA RATE (READINGS/SEC)
0
15
RESOLUTION (BITS)
16
18
19
20
22
10
50
70
2430 F27
17
21
40
90
100
20
30
60 80
V
INCM
= V
REFCM
V
IN
= 0V
F
O
= EXT OSC
REF
–
= GND
T
A
= 25°C
RES = LOG
2
(V
REF
/INL
MAX
)
V
CC
= V
REF
= 5V
V
CC
= 2.7V
V
REF
= 2.5V
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0
INPUT SIGNAL ATTENUATION (dB)
–3
–2
–1
0
4
2431 F28
–4
–5
–6
1
2
3
5
F
O
= HIGH F
O
= LOW
INPUT NOISE SOURCE SINGLE POLE
EQUIVALENT BANDWIDTH (Hz)
1
INPUT REFERRED NOISE
EQUIVALENT BANDWIDTH (Hz)
10
100
1000
10 100 1k 10k 100k 1M
2431 G29
0.1
0.1 1
F
O
= LOW
F
O
= HIGH
Figure 29. Input Referred Noise Equivalent Bandwidth
of an Input Connected White Noise Source