Datasheet

LTC2446/LTC2447
24
24467fa
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 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:
33nV/√Hz • √1.8MHz = 44.3µV.
APPLICATIO S I FOR ATIO
WUU
U
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:
33nV/√Hz • √787Hz = 0.93µV.
The total noise is the RMS sum of this noise with the 2µV
noise of the ADC at OSR=256.
√(0.93µV)
2
+ (2uV)
2
= 2.2µV.
Increasing the oversample ratio to 32768 reduces the
noise bandwidth of the ADC to 6.2Hz which reduces the
total amplifier noise to:
33nV/√Hz • √6.2Hz = 82nV.
The total noise is the RMS sum of this noise with the 200nV
noise of the ADC at OSR = 32768.
√(82nV)
2
+ (200nV)
2
= 216nV.
In this way, the digital filter with its variable oversampling
ratio can greatly reduce the effects of external noise
sources.
Table 7. Performance vs Over-Sample Ratio
MAXIMUM FIRST NOTCH EFFECTIVE –3dB
OVER-
CONVERSION RATE FREQUENCY NOISE BW POINT (Hz)
SAMPLE *RMS *RMS ENOB INTERNAL INTERNAL INTERNAL INTERNAL
RATIO NOISE NOISE (V
REF
= 5V) 9MHz EXTERNAL 9MHz EXTERNAL 9MHz EXTERNAL 9MHz EXTERNAL
(OSR) LTC2446 LTC2447 LTC2446 LTC2447 CLOCK f
O
CLOCK f
O
CLOCK f
O
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
*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.