LTC2444/LTC2445/

LTC2448/LTC2449

25

sn2444589 2444589fs

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 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

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

9MHz clock

9MHz clock

9MHz clock

64

23µV23µV

17

17

3515.6

28125

3148

1696

128

4.5µV

3.5µV

20.1

20

1757.8

14062.5

1574

848

256

2.8µV2µV

20.8

21.3

878.9

7031.3

787

424

512

2µV

1.4µV

21.3

21.8

439.5

3515.6

394

212

1024

1.4µV1µV

21.8

22.4

219.7

1757.8

197

106

2048

1.1µV

750nV

22.1

22.9

109.9

878.9

98.4

53

4096

720nV

510nV

22.7

23.4

54.9

439.5

49.2

26.5

8192

530nV

375nV

23.2

24

27.5

219.7

24.6

13.2

16384

350nV

250nV

23.8

24.4

13.7

109.9

12.4

6.6

32768

280nV

200nV

24.1

24.6

6.9

54.9

6.2

3.3

Input Bandwidth and Frequency Rejection

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-

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

*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.