LTC2486IDE datasheet(15/36 Pages) LINER

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LTC2486

2486f

APPLICATIONS INFORMATION

Easy Drive Input Current Cancellation

The LTC2486 combines a high precision, delta-sigma ADC

with an automatic, differential, input current cancellation

front end. A proprietary front end passive sampling network

transparently removes the differential input current. This

enables external RC networks and high impedance sen-

sors to directly interface to the LTC2486 without external

ampliﬁers. The remaining common mode input current

is eliminated by either balancing the differential input

impedances or setting the common mode input equal to

the common mode reference (see 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 LTC2486 automatically enters an internal reset state

when the power supply voltage, VCC, drops below ap-

proximately 2V. This feature guarantees the integrity of

the conversion result, input channel selection, and serial

clock mode.

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 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

This converter accepts a truly differential external reference

voltage. The absolute/common mode voltage range for the

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 LTC2486 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 to CH3 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 LTC2486 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).

Signals applied to the input (CH0 to CH3, COM) may

extend 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 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 tem-

perature dependency.

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