LTC2492 Linear Technology, LTC2492 Datasheet - Page 15

• Manufacturer Part Number: LTC2492
• Description: 24-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
Easy Drive Input Current Cancellation
The LTC2492 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 sensors
to directly interface to the LTC2492 without external
amplifi ers. 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 architecture
does not require on-chip buffers, thereby enabling
signals to swing beyond ground and V
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 LTC2492 automatically enters an internal reset
state when the power supply voltage V
approximately 2V. This feature guarantees the integrity of
the conversion result, input channel selection, and serial
clock mode.
When V
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
CH1, simultaneous 50Hz/60Hz rejection and 1× output
rate. The fi rst conversion following a POR cycle is accurate
within the specifi cation 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 fi rst data input/output cycle.
CC
rises above this threshold, the converter creates
CC
CC
. Moreover, the
+
drops below
= CH0, IN
–
=
Reference Voltage Range
This converter accepts a truly differential external reference
voltage. The absolute/common mode voltage range for
REF
the device (GND to V
V
The LTC2492 differential reference input range is 0.1V to
V
and REF
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
signifi cantly 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 to CH3 and COM input
pins extending from GND – 0.3V to V
these limits, the ESD protection devices begin to turn
on and the errors due to input leakage current increase
rapidly. Within these limits, the LTC2492 converts the
bipolar differential input signal V
IN
–0.5 • V
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 V
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
temperature dependency.
REF
CC
+
. For the simplest operation, REF
and IN
+
must be positive (REF
and REF
–
REF
can be shorted to GND. The converter output noise
–
to +FS = 0.5 • V
are the selected input channels), from –FS =
–
pins covers the entire operating range of
REF
CC
). For correct converter operation,
= 5V. This error has a very strong
REF
+
> REF
where V
IN
+
–
).
can be shorted to V
= IN
REF
CC
LTC2492
+ 0.3V. Outside
+
= REF
CC
– IN
. In order to
+
–
– REF
15
(where
2492fb
CC
–
.