LTC2448 LINER [Linear Technology], LTC2448 Datasheet - Page 25

• Manufacturer Part Number: LTC2448
• Description: 24-Bit High Speed 8-/16-Channel ADCs with Selectable Speed/Resolution
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Table 8
*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.
Input Bandwidth and Frequency Rejection
The combined effect of the internal SINC
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
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.
sample
16384
32768
(OSR) LTC2448 LTC2449 LTC2449 LTC2449
Over-
Ratio LTC2444/ LTC2445/ LTC2444/ LTC2445/
1024
2048
4096
8192
128
256
512
64
720nV
530nV
350nV
280nV
*RMS
Noise
4.5µV
2.8µV
1.4µV
1.1µV
23µV
2µV
250nV
*RMS
750nV
510nV
375nV
200nV
Noise
3.5µV
1.4µV
23µV
2µV
1µV
U
U
20.1
20.8
21.3
21.8
22.1
22.7
23.2
23.8
24.1
17
(V
o
REF
ENOB
pin.
= 5V)
21.3
21.8
22.4
22.9
23.4
24.4
24.6
17
20
24
W
4
digital filter and
9MHz clock
Internal
3515.6
1757.8
878.9
439.5
219.7
109.9
Conversion Rate
54.9
27.5
13.7
6.9
Maximum
U
f
f
f
f
o
f
f
f
f
o
o
o
External
/1310720
f
f
o
o
o
o
/163840
/327680
/655360
o
o
/10240
/20480
/40960
/81920
/2560
/5120
f
o
First Notch Frequency
This is the first notch in the SINC
filter and depends on the f
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.
9MHz clock
14062.5
Internal
7031.3
3515.6
1757.8
28125
878.9
439.5
219.7
109.9
54.9
First Notch
Frequency
f
External
f
f
f
f
f
f
f
o
f
f
o
o
o
o
o
o
o
/16380
o
o
/1280
/2560
/5120
/1020
/2050
/4100
/8190
/320
/640
f
o
LTC2444/LTC2445/
9MHz clock
LTC2448/LTC2449
Internal
3148
1574
98.4
49.2
24.6
12.4
787
394
197
6.2
Noise BW
Effective
o
f
External
f
f
f
o
f
f
f
f
f
f
o
o
o
/146300
o
o
o
o
o
o
/18300
/36600
/73100
clock frequency and the
/5710
/2860
/1140
/2280
/4570
/9140
f
o
4
portion of the digital
9MHz clock
Internal
1696
26.5
13.2
848
424
212
106
6.6
3.3
53
point (Hz)
sn2444589 2444589fs
–3dB
f
f
f
f
f
o
o
External
f
f
f
f
o
o
o
/1358000
/2717000
f
o
o
o
o
25
/170000
/340000
/679000
o
/10600
/21200
/42500
/84900
/5310
f
o