LTC2499 datasheet(13/32 Pages) LINER

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LTC2499

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cancellation does not interfere with the transparent offset

and full-scale auto-calibration and the absolute accuracy

(full scale + offset + linearity + drift) is maintained even

with external RC networks.

Power-Up Sequence

The LTC2499 automatically enters an internal reset state

when the power supply voltage VCC drops below approxi-

mately 2.0V. This feature guarantees the integrity of the

conversion result and input channel selection.

When VCC rises above this threshold, the converter creates

an internal power-on-reset (POR) signal with a duration

of approximately 4ms. The POR signal clears all internal

registers. The conversion immediately following a POR

cycle is performed on the input channel IN+ = CH0, IN– =

CH1 with simultaneous 50Hz/60Hz rejection and 1x output

rate. The ﬁrst conversion following a POR cycle is accurate

within the speciﬁcation of the device if the power supply

voltage is restored to (2.7V to 5.5V) before the end of the

POR interval. A new input channel, rejection mode, speed

mode, or temperature selection can be programmed into

the device during this ﬁrst data input/output cycle.

Reference Voltage Range

This converter accepts a truly differential external reference

voltage. The absolute/common mode voltage range for

REF+ and REF– pins covers the entire operating range of

the device (GND to VCC). For correct converter operation,

VREF must be positive (REF+ > REF–).

The LTC2499 differential reference input range is 0.1V to

VCC. For the simplest operation, REF+ can be shorted to

VCC and REF– can be shorted to GND. The converter out-

put noise is determined by the thermal noise of the front

end circuits and, as such, its value in nanovolts is nearly

constant with reference voltage. A decrease in reference

voltage will not signiﬁcantly improve the converter’s effec-

tive resolution. On the other hand, a decreased reference

will improve the converter’s overall INL performance.

Input Voltage Range

The analog inputs are truly differential with an absolute,

common mode range for the CH0-CH15 and COM 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 LTC2499 converts the bipolar dif-

ferential input signal VIN = IN+ – IN– (where IN+ and IN– are

the selected input channels), from – FS = – 0.5 • VREF

to + FS = 0.5 • VREF where VREF = REF+ - REF–. Outside

this range, the converter indicates the overrange or the

underrange condition using distinct output codes (see

Table 1).

Signals applied to the input (CH0-CH15, COM) may extend

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

any fault current, resistors of up to 5kΩ may be added in

series with the input. The effect of series resistance on

the converter accuracy can be evaluated from the curves

presented in the Input Current/Reference Current sections.

In addition, series resistors will introduce a temperature

dependent error due to 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.

MUXOUT/ADCIN

The outputs of the multiplexer (MUXOUTP/MUXOUTN) and

the inputs to the ADC (ADCINP/ADCINN) can be used to

perform input signal conditioning on any of the selected

input channels or simply shorted together for direct

digitization. If an external ampliﬁer is used, the LTC2499

automatically calibrates both the offset and drift of this

circuit and the Easy Drive sampling scheme enables a

wide variety of ampliﬁers to be used.

In order to achieve optimum performance, if an external

ampliﬁer is not used, short these pins directly together

(ADCINP to MUXOUTP and ADCINN to MUXOUTN) and

minimize their capacitance to ground.

APPLICATIONS INFORMATION

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