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LTC2498

2498fc

APPLICATIONS INFORMATION

transparently removes the differential input current. This

enables external RC networks and high impedance sen-

sors to directly interface to the LTC2498 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 archi-

tecture does not require on-chip buffers thereby enabling

signals to swing beyond ground or up to 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 LTC2498 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, IN– =

CH1, 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 LTC2498 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 nano volts 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 input is truly differential with an absolute, com-

mon mode range for CH0 to CH15 and COM input pins

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

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

errors due to input leakage current increase rapidly. Within

these limits, the LTC2498 converts the bipolar differential

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.

Signals applied to the input (CH0 to CH15, 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

temperature dependency.

MUXOUT/ADCIN

The output of the multiplexer (MUXOUT) and the input

to the ADC (ADCIN) can be used to perform input signal

conditioning on any of the selected input channels or sim-

ply shorted together for direct digitization. If an external

ampliﬁer is used, the LTC2498 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.

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