LTC2440IGN datasheet(21/28 Pages) LINER

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LTC2440

sn2440, 2440fas

APPLICATIO S I FOR ATIO

The sample rate fS and NULL fN, my 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

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 14. 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 22. By floating pin 4 (DIV)

of the LTC1799, the output oscillator frequency is:

fMHz

OSC

SET

⎛

⎝⎜

⎞

⎠⎟

•

The normal mode rejection characteristic shown in Fig-

ure 14 is achieved by applying the output of the LTC1799

(with RSET = 100k) to the FO pin on the LTC2440 with SDI

tied HIGH (OSR = 32768).

Reduced Power Operation

In addition to adjusting the speed/resolution of the

LTC2440, the speed/resolution/power dissipation may

also be adjusted using the automatic sleep mode. During

the conversion cycle, the LTC2440 draws 8mA supply

current independent of the programmed speed. Once the

conversion cycle is completed, the device automatically

enters a low power sleep state drawing 8

µA. The device

remains in this state as long as CS is HIGH and data is not

shifted out. By adjusting the duration of the sleep state

(hold CS HIGH longer) and the duration of the conversion

cycle (programming OSR) the DC power dissipation can

be reduced, see Figure 16.

For example, if the OSR is programmed at the fastest rate

(OSR = 64, tCONV = 0.285ms) and the sleep state is 10ms,

the effective output rate is approximately 100Hz while the

average supply current is reduced to 240

µA. By further

extending the sleep state to 100ms, the effective output

rate of 10Hz draws on average 30

µA. Noise, power, and

speed can be optimized by adjusting the OSR (Noise/

Speed) and sleep mode duration (Power).

Figure 14. LTC2440 Normal Mode Rejection

(External Oscillator at 90kHz)

DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)

–40

–20

2440 F14

–60

–80

246

–100

–120

–140

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