LTC2440

25

sn2440, 2440fas

APPLICATIO S I FOR ATIO

combinations of oversample ratio and clock frequency.

Understanding these properties is the key to fine 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

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 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

LTC2440 reduces the effective noise of an amplifier driv-

ing its input.

Table 6

Oversample Ratio

ADC

ENOB

Maximum Conversion Rate

First Notch Frequency

Effective Noise BW

–3dB point(Hz)

(OSR)

Noise*

Internal

External

Internal

External

Internal

External

Internal

External

9MHz clock

9MHz clock

9MHz clock

9MHz clock

64

23

µV

17

3515.6

28125

3148

1696

128

3.5

µV

20

1757.8

14062.5

1574

848

256

2

µV

21.3

878.9

7031.3

787

424

512

1.4

µV

21.8

439.5

3515.6

394

212

1024

1

µV

22.4

219.7

1757.8

197

106

2048

750nV

22.9

109.9

878.9

98.4

53

4096

510nV

23.4

54.9

439.5

49.2

26.5

8192

375nV

24

27.5

219.7

24.6

13.2

16384

250nV

24.4

13.7

109.9

12.4

6.6

32768

200nV

24.6

6.9

54.9

6.2

3.3

*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.