LTC2435-1IGN datasheet(13/40 Pages) LINER

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LTC2435/LTC2435-1

24351fa

APPLICATIO S I FOR ATIO

The LTC2435/LTC2435-1 perform a full-scale calibration

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 full-scale readings with respect to time,

supply voltage change and temperature drift.

Unlike the LTC2430, the LTC2435 and LTC2435-1 do not

perform an offset calibration every conversion cycle. This

enables the LTC2435/LTC2435-1 to double their output

rate while maintaining line frequency rejection. The initial

offset of the LTC2435/LTC2435-1 is within 5mV indepen-

dent of VREF.BasedontheLTC2435/LTC2435-1newmodu-

lator architecture, the temperature drift of the offset is less

than 100nV/°C. More information on the LTC2435/

LTC2435-1 offset is described in the Offset Accuracy and

Drift section of this data sheet.

Power-Up Sequence

The LTC2435/LTC2435-1 automatically enter an internal

reset state when the power supply voltage VCC drops

below approximately 2.2V. This feature guarantees the

integrity of the conversion result and of the serial interface

mode selection. (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 a duration of approximately 1ms. The POR

signal clears all internal registers. Following the POR

signal, the LTC2435/LTC2435-1 start a normal conversion

cycle and follow the succession of states described above.

The first conversion result following POR is accurate

within the specifications of the device if the power supply

voltage is restored within the operating range (2.7V to

5.5V) before the end of the POR time interval.

Reference Voltage Range

These converters accept a truly differential external refer-

ence voltage. The absolute/common mode voltage speci-

fication for the REF+ and REF– pins covers the entire range

from GND to VCC. For correct converter operation, the

REF+ pin must always be more positive than the REF– pin.

The LTC2435/LTC2435-1 can accept a differential refer-

ence voltage from 0.1V to VCC. The converter output noise

is determined by the thermal noise of the front-end cir-

cuits, and as such, its value is nearly constant with

reference voltage. A decrease in reference voltage will not

significantly improve the converter’s effective resolution.

On the other hand, a reduced reference voltage will im-

prove the converter’s overall INL performance. A reduced

reference voltage will also improve the converter perfor-

mance when operated with an external conversion clock

(external FO signal) at substantially higher output data

rates (see the Output Data Rate section).

Input Voltage Range

The analog input is truly differential with an absolute/

common mode range for the IN+ and IN– input pins

extending from GND – 0.3V to VCC + 0.3V. Outside

these limits, the ESD protection devices begin to turn on

and the errors due to input leakage current increase

rapidly. Within these limits, the LTC2435/LTC2435-1 con-

vert the bipolar differential input signal, VIN = IN+ – IN–,

from – FS = – 0.5 • VREF to +FS = 0.5 • VREF where VREF =

REF+ – REF–. Outside this range, the converters indicate

the overrange or the underrange condition using distinct

output codes.

Input signals applied to IN+ and IN– pins may extend by

300mV below ground and above VCC. In order to limit any

fault current, resistors of up to 5k may be added in series

with the IN+ and IN– pins without affecting the perfor-

mance of the device. In the physical layout, it is important

to maintain the parasitic capacitance of the connection

between these series resistors and the corresponding pins

as low as possible; therefore, the resistors should be

located as close as practical to the pins. The effect of the

series resistance on the converter accuracy can be evalu-

ated from the curves presented in the Input Current/

Reference Current sections. In addition, series resistors

will introduce a temperature dependent offset error due to

the input leakage current. A 1nA input leakage current will

develop a 1ppm offset error on a 5k resistor if VREF = 5V.

This error has a very strong temperature dependency.

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