Datasheet

LTC2497
11
2497fa
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 V
CC
. 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 LTC2497 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. 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 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 LTC2497 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.
Since the transition noise is well below 1LSB (0.02LSB), a
decrease in reference voltage will proportionally improve
the converter resolution and improve INL.
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. Within
these limits, the LTC2497 converts the bipolar differen-
tial 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 due to input ESD leakage current, resis-
tors 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 LTC2497
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.
applicaTions inForMaTion