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LTC2412

2412f

Frequency Rejection Selection (FO)

The LTC2412 internal oscillator provides better than 110dB

normal mode rejection at the line frequency and all its

harmonics for 50Hz

±2% or 60Hz ±2%. For 60Hz rejec-

tion, FO should be connected to GND while for 50Hz

rejection the FO pin should be connected to VCC.

The selection of 50Hz or 60Hz rejection can also be made

by driving FO to an appropriate logic level. A selection

change during the sleep or data output states will not

disturb the converter operation. If the selection is made

during the conversion state, the result of the conversion in

progress may be outside specifications but the following

conversions will not be affected.

When a fundamental rejection frequency different from

50Hz or 60Hz is required or when the converter must be

synchronized with an outside source, the LTC2412 can

operate with an external conversion clock. The converter

automatically detects the presence of an external clock

signal at the FO pin and turns off the internal oscillator. The

frequency fEOSC of the external signal must be at least

2560Hz (1Hz notch frequency) to be detected. The exter-

nal clock signal duty cycle is not significant as long as the

minimum and maximum specifications for the high and

low periods tHEO and tLEO are observed.

While operating with an external conversion clock of a

frequency fEOSC, the LTC2412 provides better than 110dB

normal mode rejection in a frequency range fEOSC/2560

±4% and its harmonics. The normal mode rejection as a

function of the input frequency deviation from fEOSC/2560

is shown in Figure 4.

Whenever an external clock is not present at the FO pin, the

converter automatically activates its internal oscillator and

enters the Internal Conversion Clock mode. The LTC2412

operation will not be disturbed if the change of conversion

clock source occurs during the sleep state or during the

data output state while the converter uses an external

serial clock. If the change occurs during the conversion

state, the result of the conversion in progress may be

Figure 4. LTC2412 Normal Mode Rejection When

Using an External Oscillator of Frequency fEOSC

APPLICATIO S I FOR ATIO

Table 2. LTC2412 Output Data Format

Differential Input Voltage

Bit 31

Bit 30

Bit 29

Bit 28

Bit 27

Bit 26

Bit 25

…

Bit 0

VIN*

EOC

CH0/CH1

SIG

MSB

VIN* ≥ 0.5 • VREF**

0/1

…

0.5 • VREF** – 1LSB

0/1

…

0.25 • VREF**

0/1

…

0.25 • VREF** – 1LSB

0/1

…

0/1

…

–1LSB

0/1

…

– 0.25 • VREF**

0/1

…

– 0.25 • VREF** – 1LSB

0/1

…

– 0.5 • VREF**

0/1

…

VIN* < –0.5 • VREF**

0/1

…

*The differential input voltage VIN = IN+ – IN–.

**The differential reference voltage VREF = REF+ – REF–.

DIFFERENTIAL INPUT SIGNAL FREQUENCY

DEVIATION FROM NOTCH FREQUENCY fEOSC/2560(%)

–12

–8

–4

2412 F04

–80

–85

–90

–95

–100

–105

–110

–115

–120

–125

–130

–135

–140

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