LTC2446 LINER [Linear Technology], LTC2446 Datasheet - Page 24

• Manufacturer Part Number: LTC2446
• Description: 24-Bit High Speed 8-Channel ?? ADCs with Selectable Multiple Reference Inputs
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2446/LTC2447
Table 7. Performance vs Over-Sample Ratio
SAMPLE
*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.
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
and depends on the f
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 LTC2446/LTC2447 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 LTC2446/LTC2447 reduces the effective
noise of an amplifier driving its input.
Example:
If an amplifier (e.g. LT1219) driving the input of an
LTC2446/LTC2447 has wideband noise of 33nV/√Hz,
band-limited to 1.8MHz, the total noise entering the
ADC input is:
24
OVER-
RATIO
(OSR) LTC2446 LTC2447 LTC2446
16384
32768
1024
2048
4096
8192
128
256
512
33nV/√Hz • √1.8MHz = 44.3µV.
64
NOISE
720nV
530nV
350nV
280nV
*RMS
4.5µV
2.8µV
1.4µV
1.1µV
23µV
2µV
NOISE
750nV
510nV
375nV
250nV
200nV
*RMS
3.5µV
1.4µV
23µV
2µV
1µV
U
o
clock frequency and the oversample
U
20.1
20.8
21.3
21.8
22.1
22.7
23.2
23.8
24.1
17
(V
REF
ENOB
4
portion of the digital filter
= 5V)
LTC2447
21.3
21.8
22.4
22.9
23.4
24.4
24.6
W
17
20
24
INTERNAL
CONVERSION RATE
CLOCK
3515.6
1757.8
9MHz
878.9
439.5
219.7
109.9
54.9
27.5
13.7
6.9
U
MAXIMUM
f
EXTERNAL
f
f
f
O
f
f
f
f
O
O
O
f
f
O
O
O
O
/1310720
/163840
/327680
/655360
O
O
/10240
/20480
/40960
/81920
/2560
/5120
f
O
When the ADC digitizes the input, its digital filter rejects the
wideband noise from the input signal. The noise reduction
depends on the oversample ratio which defines the effec-
tive bandwidth of the digital filter.
At an oversample of 256, the noise bandwidth of the ADC
is 787Hz which reduces the total amplifier noise to:
The total noise is the RMS sum of this noise with the 2µV
noise of the ADC at OSR=256.
Increasing the oversample ratio to 32768 reduces the
noise bandwidth of the ADC to 6.2Hz which reduces the
total amplifier noise to:
The total noise is the RMS sum of this noise with the 200nV
noise of the ADC at OSR = 32768.
In this way, the digital filter with its variable oversampling
ratio can greatly reduce the effects of external noise
sources.
INTERNAL
14062.5
CLOCK
7031.3
3515.6
1757.8
28125
33nV/√Hz • √787Hz = 0.93µV.
√
33nV/√Hz • √6.2Hz = 82nV.
√
9MHz
878.9
439.5
219.7
109.9
54.9
(0.93µV)
(82nV)
FIRST NOTCH
FREQUENCY
EXTERNAL
f
f
f
f
f
f
f
f
O
2
f
f
O
O
O
O
O
O
O
O
O
/16380
/1280
/2560
/5120
/1020
/2050
/4100
/8190
+ (200nV)
/320
/640
2
f
O
+ (2uV)
INTERNAL INTERNAL
CLOCK
9MHz
3148
1574
98.4
49.2
24.6
12.4
787
394
197
6.2
2
2
EFFECTIVE
NOISE BW
= 2.2µV.
= 216nV.
EXTERNAL
f
f
f
f
O
f
f
f
f
f
f
O
O
O
/146300
O
O
O
O
O
O
/18300
/36600
/73100
/5710
/2860
/1140
/2280
/4570
/9140
f
O
CLOCK
9MHz
1696
26.5
13.2
848
424
212
106
6.6
3.3
53
POINT (Hz)
–3dB
f
f
EXTERNAL
f
f
f
O
O
f
f
f
f
O
O
O
f
O
O
O
O
/1358000
/2717000
/170000
/340000
/679000
O
/10600
/21200
/42500
/84900
/5310
f
O
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