Datasheet
LTC2444/LTC2445/
LTC2448/LTC2449
25
2444589fb
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 LTC2444/LTC2445/LTC2448/LTC2449 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 modu-
lator sample rate. The example on the following page
shows how the noise rejection of the LTC2444/LTC2445/
LTC2448/LTC2449 reduces the effective noise of an am-
plifier driving its input.
Table 8
Over- *RMS *RMS ENOB Maximum First Notch Effective –3dB
sample Noise Noise (V
REF
= 5V) Conversion Rate Frequency Noise BW point (Hz)
Ratio LTC2444/ LTC2445/ LTC2444/ LTC2445/ Internal External Internal External Internal External Internal External
(OSR) LTC2448 LTC2449 LTC2449 LTC2449 9MHz clock f
o
9MHz clock f
o
9MHz clock f
o
9MHz clock f
o
64 23µV23µV 17 17 3515.6 f
o
/2560 28125 f
o
/320 3148 f
o
/5710 1696 f
o
/5310
128 4.5µV 3.5µV 20.1 20 1757.8 f
o
/5120 14062.5 f
o
/640 1574 f
o
/2860 848 f
o
/10600
256 2.8µV2µV 20.8 21.3 878.9 f
o
/10240 7031.3 f
o
/1280 787 f
o
/1140 424 f
o
/21200
512 2µV 1.4µV 21.3 21.8 439.5 f
o
/20480 3515.6 f
o
/2560 394 f
o
/2280 212 f
o
/42500
1024 1.4µV1µV 21.8 22.4 219.7 f
o
/40960 1757.8 f
o
/5120 197 f
o
/4570 106 f
o
/84900
2048 1.1µV 750nV 22.1 22.9 109.9 f
o
/81920 878.9 f
o
/1020 98.4 f
o
/9140 53 f
o
/170000
4096 720nV 510nV 22.7 23.4 54.9 f
o
/163840 439.5 f
o
/2050 49.2 f
o
/18300 26.5 f
o
/340000
8192 530nV 375nV 23.2 24 27.5 f
o
/327680 219.7 f
o
/4100 24.6 f
o
/36600 13.2 f
o
/679000
16384 350nV 250nV 23.8 24.4 13.7 f
o
/655360 109.9 f
o
/8190 12.4 f
o
/73100 6.6 f
o
/1358000
32768 280nV 200nV 24.1 24.6 6.9 f
o
/1310720 54.9 f
o
/16380 6.2 f
o
/146300 3.3 f
o
/2717000
Input Bandwidth and Frequency Rejection
The combined effect of the internal SINC
4
digital filter and
the digital and analog autocalibration circuits determines
the LTC2444/LTC2445/LTC2448/LTC2449 input band-
width and rejection characteristics. The digital filter’s
response can be adjusted by setting the oversample ratio
(OSR) through the SPI interface or by supplying an exter-
nal conversion clock to the f
o
pin.
Table 8 lists the properties of the LTC2444/LTC2445/
LTC2448/LTC2449 with various combinations of
oversample ratio and clock frequency. Understanding
these properties is the key to fine tuning the characteris-
tics of the LTC2444/LTC2445/LTC2448/LTC2449 to the
application.
Maximum Conversion Rate
The maximum conversion rate is the fastest possible rate
at which conversions can be performed.
APPLICATIO S I FOR ATIO
WUU
U
*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.