LTC2498 datasheet(32/36 Pages) LINER

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LTC2498

2498fc

APPLICATIONS INFORMATION

noise. Figures 26 and 27 show measurement results for

the rejection of a 7.5V peak-to-peak noise source (150%

of full scale) applied to the LTC2498. From these curves,

it is shown that the rejection performance is maintained

even in extremely noisy environments.

Using the 2x speed mode of the LTC2498 alters the rejec-

tion characteristics around DC and multiples of fS. The

device bypasses the offset calibration in order to increase

the output rate. The resulting rejection plots are shown

in Figures 28 and 29. 1x type frequency rejection can be

achieved using the 2x mode by performing a running

average of the conversion results, see Figure 30.

Output Data Rate

When using its internal oscillator, the LTC2498 produces up

to 7.5 samples per second (sps) with a notch frequency of

60Hz. The actual output data rate depends upon the length

of the sleep and data output cycles which are controlled

by the user and can be made insigniﬁcantly short. When

operating with an external conversion clock (FO connected

to an external oscillator), the LTC2498 output data rate

can be increased. The duration of the conversion cycle is

41036/fEOSC. If fEOSC = 307.2kHz, the converter behaves

as if the internal oscillator is used.

An increase in fEOSCoverthenominal307.2kHzwilltranslate

into a proportional increase in the maximum output data

rate (up to a maximum of 100sps). The increase in output

rate leads to degradation in offset, full-scale error, and ef-

fective resolution as well as a shift in frequency rejection.

When using the integrated temperature sensor, the internal

oscillator should be used (fO = 0) or an external oscillator

applied to fO, fEOSC, should be set to 307.2kHz Max.

A change in fEOSC results in a proportional change in the

internal notch position. This leads to reduced differential

Figure 26. Measure Input Normal Mode Rejection vs Input

Frequency with Input Perturbation of 150% (60Hz Notch)

Figure 27. Measure Input Normal Mode Rejection vs Input

Frequency With input Perturbation of 150% (50Hz Notch)

Figure 28. Input Normal Mode Rejection 2x Speed Mode

Figure 29. Input Normal Mode Rejection 2x Speed Mode

INPUT FREQUENCY (Hz)

105 120 135 150 165 180 195 210 225 240

2498 F26

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

TA = 25°C

VIN(P-P) = 5V

VIN(P-P) = 7.5V

(150% OF FULL SCALE)

INPUT FREQUENCY (Hz)

2498 F27

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

TA = 25°C

VIN(P-P) = 5V

VIN(P-P) = 7.5V

(150% OF FULL SCALE)

12.5

37.5

62.5

87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

INPUT SIGNAL FREQUENCY (fN)

2498 F28

–20

–40

–60

–80

–100

–120

2fN 3fN 4fN 5fN 6fN 7fN 8fN

INPUT SIGNAL FREQUENCY (fN)

2498 F29

–20

–40

–60

–80

–100

–120

250

248

252 254 256 258 260 262 264

LTC2498 Datasheet(PDF) 32 Page - Linear Technology

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