LTC2445 datasheet(23/28 Pages) LINER

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LTC2444/LTC2445/

LTC2448/LTC2449

sn2444589 2444589fs

clock applied to FO. Combining a large OSR with a reduced

sample rate leads to notch frequencies fN near DC while

maintaining simple antialiasing requirements. A 100kHz

clock applied to FO results in a NULL at 0.6Hz plus all

harmonics up to 20kHz, see Figure 13. This is useful in

applications requiring digitalization of the DC component

of a noisy input signal and eliminates the need of placing

a 0.6Hz filter in front of the ADC.

An external oscillator operating from 100kHz to 20MHz

can be implemented using the LTC1799 (resistor set

SOT-23 oscillator), see Figure 16. By floating pin 4 (DIV)

of the LTC1799, the output oscillator frequency is:

MHz

OSC

SET

⎛

⎝

⎜

⎞

⎠

⎟

•

The normal mode rejection characteristic shown in

Figure 13 is achieved by applying the output of the LTC1799

(with RSET = 100k) to the FO pin on the LTC2444/LTC2445/

LTC2448/LTC2449 with SDI tied HIGH (OSR = 32768).

Figure 13. LTC2444/LTC2445/LTC2448/LTC2449 Normal

Mode Rejection (External Oscillator at 90kHz)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–40

–20

2440 F14

–60

–80

246

–100

–120

–140

Figure 12. LTC2444/LTC2445/LTC2448/LTC2449

Normal Mode Rejection (Internal Oscillator)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–60

–40

1440 F13

–80

–100

1000000

2000000

–120

1.8MHz

–140

–20

REJECTION > 120dB

If FO is grounded, fS is set by the on-chip oscillator at

1.8MHz ±5% (over supply and temperature variations). At

an OSR of 32,768, the first NULL is at fN = 55Hz and the no

latency output rate is fN/8 = 6.9Hz. At the maximum OSR,

the noise performance of the device is 280nVRMS(LTC2444/

LTC2448) and 200nVRMS (LTC2445/LTC2449) with better

than 80dB rejection of 50Hz ±2% and 60Hz ±2%. Since the

OSR is large (32,768) the wide band rejection is extremely

large and the antialiasing requirements are simple. The

first multiple of fS occurs at 55Hz • 32,768 = 1.8MHz, see

Figure 12.

The first NULL becomes fN = 7.04kHz with an OSR of 256

(an output rate of 880Hz) and FO grounded. While the

NULL has shifted, the sample rate remains constant. As a

result of constant modulator sampling rate, the linearity,

offset and full-scale performance remains unchanged as

does the first multiple of fS.

The sample rate fS and NULL fN, may also be adjusted by

driving the FO pin with an external oscillator. The sample

rate is fS = fEOSC/5, where fEOSC is the frequency of the

APPLICATIO S I FOR ATIO

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