Datasheet

LTC2440
25
2440fe
For more information www.linear.com/LTC2440
APPLICATIONS INFORMATION
Table 6 lists the properties of the LTC2440 with various
combinations of oversample ratio and clock frequency.
Understanding these properties is the key to fine tuning
the characteristics of the LTC2440 to the application.
Maximum Conversion Rate
The maximum conversion rate is the fastest possible rate
at which conversions can be performed.
First Notch Frequency
This is the first notch in the SINC
4
portion of the digital filter
and depends on the f
O
clock frequency and the oversample
ratio. Rejection at this frequency and its multiples (up to
the modulator sample rate of 1.8MHz) exceeds 120dB.
This is 8 times the maximum conversion rate.
Effective Noise Bandwidth
The LTC2440 has extremely good input noise rejection from
the first notch frequency all the way out to the modulator
sample rate (typically 1.8MHz). Effective noise bandwidth
is a measure of how the ADC will reject wideband input
noise up to the modulator sample rate. The example on
the following page shows how the noise rejection of the
LTC2440 reduces the effective noise of an amplifier driv
-
ing its input.
Table 6. Performance vs Oversample Ratio
Oversample
Ratio
(OSR)
ADC
Noise*
ENOB
(V
REF
= 5V)*
Maximum Conversion Rate
(sps)
First Notch Frequency
(Hz)
Effective Noise BW
(Hz)
–3dB Point
(Hz)
Internal
Clock
External f
O
(f
O
/x)
Internal
Clock
External f
O
(f
O
/x)
Internal
9MHz clock
External
(f
O
/x)
Internal
Clock
External f
O
(f
O
/x)
64 23µV 17 2816.35 f
O
/2738 28125 f
O
/320 3148 f
O
/2860 1696 f
O
/5310
128 3.5µV 20 1455.49 f
O
/5298 14062.5 f
O
/640 1574 f
O
/5720 848 f
O
/10600
256 2µV 21.3 740.18 f
O
/10418 7031.3 f
O
/1280 787 f
O
/11440 424 f
O
/21200
512 1.4µV 21.8 373.28 f
O
/20658 3515.6 f
O
/2560 394 f
O
/22840 212 f
O
/42500
1024 1µV 22.4 187.45 f
O
/41138 1757.8 f
O
/5120 197 f
O
/45690 106 f
O
/84900
2048 750nV 22.9 93.93 f
O
/82098 878.9 f
O
/10200 98.4 f
O
/91460 53 f
O
/170000
4096 510nV 23.4 47.01 f
O
/164018 439.5 f
O
/20500 49.2 f
O
/183000 26.5 f
O
/340000
8192 375nV 24 23.52 f
O
/327858 219.7 f
O
/41000 24.6 f
O
/366000 13.2 f
O
/679000
16384 250nV 24.4 11.76 f
O
/655538 109.9 f
O
/81900 12.4 f
O
/731000 6.6 f
O
/1358000
32768 200nV 24.6 5.88 f
O
/1310898 54.9 f
O
/163800 6.2 f
O
/1463000 3.3 f
O
/2717000
*ADC noise increases by approximately √2 when OSR is decreased by a factor of 2 for OSR 32768 to OSR 256. The ADC noise at OSR 128 and OSR 64
include effects from internal modulator quantization noise.
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