LTC2420CS8 datasheet(11/36 Pages) LINER

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LTC2420

not disturb the cyclic operation described above. These

modes of operation are described in detail in the Serial

Interface Timing Modes section.

Conversion Clock

A major advantage delta-sigma converters offer over

conventional type converters is an on-chip digital filter

(commonly known as Sinc or Comb filter). For high

resolution, low frequency applications, this filter is typi-

cally designed to reject line frequencies of 50Hz or 60Hz

plus their harmonics. In order to reject these frequencies

in excess of 110dB, a highly accurate conversion clock is

required. The LTC2420 incorporates an on-chip highly

accurate oscillator. This eliminates the need for external

frequency setting components such as crystals or oscilla-

tors. Clocked by the on-chip oscillator, the LTC2420

rejects line frequencies (50Hz or 60Hz

±2%) a minimum

of 110dB.

Ease of Use

The LTC2420 data output has no latency, filter settling or

redundant data associated with the conversion cycle.

There is a one-to-one correspondence between the

conversion and the output data. Therefore, multiplexing

an analog input voltage is easy.

The LTC2420 performs offset and full-scale calibrations

every conversion cycle. This calibration is transparent to

the user and has no effect on the cyclic operation de-

scribed above. The advantage of continuous calibration is

extreme stability of offset and full-scale readings with re-

spect to time, supply voltage change and temperature drift.

Power-Up Sequence

The LTC2420 automatically enters an internal reset state

when the power supply voltage VCC drops below approxi-

mately 2.2V. This feature guarantees the integrity of the

conversion result and of the serial interface mode selec-

tion which is performed at the initial power-up. (See the

2-wire I/O sections in the Serial Interface Timing Modes

section.)

When the VCC voltage rises above this critical threshold,

the converter creates an internal power-on-reset (POR)

signal with duration of approximately 0.5ms. The POR

signal clears all internal registers. Following the POR

signal, the LTC2420 starts a normal conversion cycle and

follows the normal succession of states described above.

The first conversion result following POR is accurate

within the specifications of the device.

Reference Voltage Range

The LTC2420 can accept a reference voltage from 0V to

VCC. The converter output noise is determined by the

thermal noise of the front-end circuits, and as such, its

value in microvolts is nearly constant with reference

voltage. A decrease in reference voltage will not signifi-

cantly improve the converter’s effective resolution. On the

other hand, a reduced reference voltage will improve the

overall converter INL performance. The recommended

range for the LTC2420 voltage reference is 100mV to VCC.

Input Voltage Range

The converter is able to accommodate system level offset

and gain errors as well as system level overrange situa-

tions due to its extended input range, see Figure 3. The

LTC2420 converts input signals within the extended input

range of – 0.125 • VREF to 1.125 • VREF.

For large values of VREF, this range is limited by the

absolute maximum voltage range of – 0.3V to (VCC + 0.3V).

Beyond this range, the input ESD protection devices begin

to turn on and the errors due to the input leakage current

increase rapidly.

Input signals applied to VIN may extend below ground by

– 300mV and above VCC by 300mV. In order to limit any

APPLICATIO S I FOR ATIO

2420 F03

VCC + 0.3V

9/8VREF

VREF

1/2VREF

–0.3V

–1/8VREF

NORMAL

INPUT

RANGE

EXTENDED

INPUT

RANGE

ABSOLUTE

MAXIMUM

INPUT

RANGE

Figure 3. LTC2420 Input Range

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