LTC2489CDEPBF datasheet(20/24 Pages) LINER

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LTC2489

2489f

by the user and can be made insigniﬁcantly short. When

operating with an external conversion clock (fO connected

to an external oscillator), the LTC2489 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 fEOSC over the nominal 307.2kHz will trans-

late 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 effective resolution as well as a shift in frequency

rejection.

A change in fEOSC results in a proportional change in the

internal notch position. This leads to reduced differential

mode rejection of line frequencies. The common mode

APPLICATIONS INFORMATION

rejection of line frequencies remains unchanged, thus fully

differential input signals with a high degree of symmetry

on both the IN+ and IN– pins will continue to reject line

frequency noise.

An increase in fEOSC also increases the effective dynamic

input and reference current. External RC networks will

continue to have zero differential input current, but the

time required for complete settling (580ns for fEOSC =

307.2kHz) is reduced, proportionally.

Once the external oscillator frequency is increased above

1MHz (a more than 3X increase in output rate) the effective-

ness of internal auto calibration circuits begins to degrade.

This results in larger offset errors, full-scale errors, and

decreased resolution, as shown in Figures 19 to 26.

INPUT FREQUENCY (Hz)

12.5

37.5

62.5

87.5 100 112.5 125 137.5 150 162.5 175 187.5 200

2489 F17

–20

–40

–60

–80

–100

–120

VCC = 5V

VREF = 5V

VIN(CM) = 2.5V

VIN(P-P) = 5V

TA = 25°C

MEASURED DATA

CALCULATED DATA

INPUT FREQUENCY (Hz)

105 120 135 150 165 180 195 210 225 240

2489 F18

–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)

Figure 17. Input Normal Mode Rejection vs Input Frequency with

Input Perturbation of 100% (50Hz/60Hz Notch)

Figure 18. Measure Input Normal Mode Rejection vs Input

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

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