Datasheet

LTC2440
25
2440fd
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 fi ne 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 fi rst notch in the SINC
4
portion of the digital fi lter
and depends on the fo 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 fi rst 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 amplifi er driv-
ing its input.
Table 6
Oversample
Ratio
(OSR)
ADC
Noise*
ENOB
(V
REF
= 5V)*
Maximum Conversion Rate First Notch Frequency Effective Noise BW –3dB Point (Hz)
Internal
9MHz clock
External
f
O
Internal
9MHz clock
External
f
O
Internal
9MHz clock
External
f
O
Internal
9MHz clock
External
f
O
64 23μV 17 3515.6 f
O
/2560 28125 f
O
/320 3148 f
O
/2850 1696 f
O
/5310
128 3.5μV 20 1757.8 f
O
/5120 14062.5 f
O
/640 1574 f
O
/5700 848 f
O
/10600
256 2μV 21.3 878.9 f
O
/10240 7031.3 f
O
/1280 787 f
O
/11400 424 f
O
/21200
512 1.4μV 21.8 439.5 f
O
/20480 3515.6 f
O
/2560 394 f
O
/22800 212 f
O
/42500
1024 1μV 22.4 219.7 f
O
/40960 1757.8 f
O
/5120 197 f
O
/45700 106 f
O
/84900
2048 750nV 22.9 109.9 f
O
/81920 878.9 f
O
/1020 98.4 f
O
/91400 53 f
O
/170000
4096 510nV 23.4 54.9 f
O
/163840 439.5 f
O
/2050 49.2 f
O
/183000 26.5 f
O
/340000
8192 375nV 24 27.5 f
O
/327680 219.7 f
O
/4100 24.6 f
O
/366000 13.2 f
O
/679000
16384 250nV 24.4 13.7 f
O
/655360 109.9 f
O
/8190 12.4 f
O
/731000 6.6 f
O
/1358000
32768 200nV 24.6 6.9 f
O
/1310720 54.9 f
O
/16380 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.