LTC2499_1 Datasheet, PDF (14/34 Pages) Linear Technology – 24-Bit 8-/16-Channel ΔΣ ADC with Easy Drive Input Current Cancellation and I2C Interface

LTC2499
APPLICATIONS INFORMATION
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 V_CC drops below approxi-
mately 2.0V. This feature guarantees the integrity of the
conversion result and input channel selection.
When V_CC 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 V_CC). For correct converter operation,
V_REF must be positive (REF⁺ > REF^–).
The LTC2499 differential reference input range is 0.1V to
V_CC. For the simplest operation, REF⁺ can be shorted to V_CC
and REF ^– can be shorted to GND. The converter output 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 effective 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 V_CC + 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 V_IN = IN⁺ – IN^– (where IN⁺ and IN^– are
the selected input channels), from – FS = – 0.5 • V_REF
to + FS = 0.5 • V_REF where V_REF = 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 V_CC. 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 V_REF = 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.
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