LTC2435-1IGN datasheet(30/40 Pages) LINER

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LTC2435/LTC2435-1

24351fa

APPLICATIO S I FOR ATIO

Output Data Rate

When using its internal oscillator, the LTC2435 can pro-

duce up to 15 readings per second with a notch frequency

of 60Hz (FO = LOW) and 12.5 readings per second with a

notch frequency of 50Hz (FO = HIGH) and the LTC2435-1

can produce up to 13.6 readings per second with FO =

LOW. 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 insignifi-

cantly short. When operated with an external conversion

clock (FOconnectedtoanexternaloscillator),theLTC2435/

LTC2435-1 output data rate can be increased as desired.

The duration of the conversion phase is 10278/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 LTC2435/LTC2435-1 perfor-

mance 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 LTC2435/LTC2435-1’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 LTC2435/

LTC2435-1 typical performance can be inferred from

Figures 16, 17, 21 and 22 in which the horizontal axis is

scaled by 153600/fEOSC.

Third, the internal analog circuits are optimized for normal

operation; therefore an increase in the frequency of the

external oscillator will start to decrease the effectiveness

of the internal analog 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 200 readings per second are shown in Figures 26 to

33. The degradation becomes more obvious above output

data rate of 150Hz, which corresponds to an external os-

cillator of 1.536MHz. In order to obtain the highest possible

level of accuracy from this converter at output data rates

above 150 readings per second, the user is advised to

maximize the power supply voltage used and to limit the

maximum ambient operating temperature. In certain cir-

cumstances, a reduction of the differential reference volt-

age may be beneficial.

Figure 27. + FS Error vs Output Data Rate and Temperature

Figure 26. Offset Error vs Output Data Rate and Temperature

OUTPUT DATA RATE (READINGS/SEC)

–300

–310

–320

–330

–340

–350

80 100

2435 F26

120 140 160 180 200

TA = 25°C

TA = 85°C

VINCM = VREFCM

VCC = VREF = 5V

VIN = 0V

FO = EXT OSC

OUTPUT DATA RATE (READINGS/SEC)

–300

–320

–340

–360

–380

–400

–420

–440

–460

–480

–500

2435 F27

204060 80 100 120 140 160 180 200

VINCM = VREFCM

VCC = VREF = 5V

FO = EXT OSC

TA = 25°C

TA = 85°C

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