LTC2430IGN datasheet(27/40 Pages) LINER

27 / 40 page

LTC2430/LTC2431

24301f

APPLICATIO S I FOR ATIO

Output Data Rate

When using the internal oscillator, the LTC2430/LTC2431

can produce up to 7.5 readings per second with a notch

frequency of 60Hz (FO = LOW) and 6.25 readings per

second with a notch frequency of 50Hz (FO = HIGH). The

actual output data rate will depend upon the length of the

sleep and data output phases which are controlled by the

user and which can be made insignificantly short. When

operated with an external conversion clock (FO connected

to an external oscillator), the LTC2430/LTC2431 output

data rate can be increased as desired. The duration of the

conversion phase is 20510/fEOSC. If fEOSC = 153600Hz, the

converter behaves as if the internal oscillator is used and

the notch is set at 60Hz. There is no significant difference

in the LTC2430/LTC2431 performance between these two

operation modes.

An increase in fEOSC over the nominal 153600Hz will

translate into a proportional increase in the maximum

output data rate. This substantial advantage is nevertheless

accompanied by three potential effects, which must be

carefully considered.

First, a change in fEOSC will result in a proportional change

in the internal notch position and in a reduction of the

converter differential mode rejection at the power line

frequency. In many applications, the subsequent perfor-

mance degradation can be substantially reduced by rely-

ing upon the LTC2430/LTC2431’s exceptional common

mode rejection and by carefully eliminating common

mode to differential mode conversion sources in the input

circuit. The user should avoid single-ended input filters

and should maintain a very high degree of matching and

symmetry in the circuits driving the IN+ and IN– pins.

Second, the increase in clock frequency will increase

proportionally the amount of sampling charge transferred

through the input and the reference pins. If large external

input and/or reference capacitors (CIN, CREF) are used, the

previous section provides formulae for evaluating the

effect of the source resistance upon the converter perfor-

mance for any value of fEOSC. If small external input and/

or reference capacitors (CIN, CREF) are used, the effect of

the external source resistance upon the LTC2430/LTC2431

typical performance can be inferred from Figures 13, 14

and 17 in which the horizontal axis is scaled by

153600/fEOSC.

Third, an increase in the frequency of the external oscilla-

tor above 1.6MHz (a more than 10

× increase in the output

data rate) will start to decrease the effectiveness of the

internal autocalibration circuits. This will result in a progres-

sive degradation in the converter accuracy and linearity.

Typical measured performance curves for output data rates

up to 100 readings per second are shown in Figures 20 to

27. In order to obtain the highest possible level of accuracy

from this converter at output data rates above 50 readings

per second, the user is advised to maximize the power

supply voltage used and to limit the maximum ambient

operating temperature. The accuracy is also sensitive to the

clock signal levels and edge rate as discussed in the sec-

tion Digital Signal Levels. In certain circumstances, a re-

duction of the differential reference voltage may be

beneficial.

Input Bandwidth

The combined effect of the internal sinc4 digital filter and

of the analog and digital autocalibration circuits deter-

mines the LTC2430/LTC2431 input bandwidth. When the

internal oscillator is used, the 3dB input bandwidth of the

LTC2430/LTC2431 is 3.63Hz for 60Hz notch frequency

(FO = LOW) and 3.02Hz for 50Hz notch frequency

(FO = HIGH). If an external conversion clock generator of

frequency fEOSC is connected to the FO pin, the 3dB input

bandwidth is 2.36 • 10–5 • fEOSC.

Due to the complex filtering and calibration algorithms

utilized, the converter input bandwidth is not modeled very

accurately by a first order filter with the pole located at the

3dB frequency. When the internal oscillator is used, the

shape of the LTC2430/LTC2431 input bandwidth is shown

in Figure 28. When an external oscillator of frequency

fEOSC is used, the shape of the LTC2430/LTC2431 input

bandwidth can be derived from Figure 28, FO = LOW curve

of the LTC2411 in which the horizontal axis is scaled by

fEOSC/153600.

The conversion noise (2.8

µVRMS typicalforVREF =5V)can

be modeled as a white noise source connected to a noise

free converter. The noise spectral density is 67nV/

√Hz for

LTC2430IGN Datasheet(PDF) 27 Page - Linear Technology

LTC2430IGN Datasheet(HTML) 27 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL