LTC2487 datasheet(13/32 Pages) LINER

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LTC2487

2487fa

amplifiers. The remaining common mode input current

is eliminated by either balancing the differential input im-

pedances or setting the common mode input equal to the

common mode reference (see the Automatic Differential

Input Current Cancellation section). This unique architec-

ture does not require on-chip buffers, thereby enabling

signals to swing beyond ground and VCC. Moreover, the

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 LTC2487 automatically enters an internal reset state

when the power supply voltage, VCC, drops below a

threshold of approximately 2.0V. This feature guarantees

the integrity of the conversion result and input channel

selection.

WhenVCCrisesabovethisthreshold,theconvertercreates

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 channels IN+ = CH0 and

IN– = CH1 with simultaneous 50Hz/60Hz rejection, 1x

output rate, and gain = 1. 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, temperature selection or

gain can be programmed into the device during this first

data input/output cycle.

Reference Voltage Range

Thisconverteracceptsatrulydifferentialexternalreference

voltage.Theabsolute/commonmodevoltagerangeforthe

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 LTC2487 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,

commonmoderangefortheCH0-CH3andCOMinputpins

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

limits, the ESD protection devices begin to turn on and the

errorsduetoinputleakagecurrentincreaserapidly.Within

these limits, the LTC2487 converts the bipolar differential

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

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

to +FS = 0.5 • VREF/Gain where VREF = REF+ – REF–.

Outside this range, the converter indicates the overrange

or the underrange condition using distinct output codes

(see Table 1).

In order to limit any fault current, resistors of up to 5k

may be added in series with the input. The effect of series

resistanceontheconverteraccuracycanbeevaluatedfrom

the curves presented in the Input Current/Reference Cur-

rent 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.

applicaTions inForMaTion

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