LTC2487 Linear Technology, LTC2487 Datasheet - Page 13

• Manufacturer Part Number: LTC2487
• Description: 16-Bit 2-/4-Channel ADC
• Manufacturer: Linear Technology

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applicaTions inForMaTion
amplifiers. The remaining common mode input current
is eliminated by either balancing the differential input im-
pedances or setting the common mode input equal to the
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
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 LTC2487 automatically enters an internal reset state
when the power supply voltage, V
threshold of approximately 2.0V. This feature guarantees
the integrity of the conversion result and input channel
selection.
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 channels IN
IN
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
–
= CH1 with simultaneous 50Hz/60Hz rejection, 1x
CC
rises above this threshold, the converter creates
CC
CC
, drops below a
. Moreover, the
+
= CH0 and
REF
the device (GND to V
V
The LTC2487 differential reference input range is 0.1V to
V
V
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-CH3 and COM input pins
extending from GND – 0.3V to V
limits, the ESD protection devices begin to turn on and the
errors due to input leakage current increase rapidly. Within
these limits, the LTC2487 converts the bipolar differential
input signal V
selected input channels), from –FS = –0.5 • V
to +FS = 0.5 • V
Outside this range, the converter indicates the overrange
or the underrange condition using distinct output codes
(see Table 1).
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 Cur-
rent 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
strong temperature dependency.
REF
CC
CC
. For the simplest operation, REF
+
and REF
must be positive (REF
and REF
–
IN
–
can be shorted to GND. The converter out-
pins covers the entire operating range of
= IN
REF
CC
+
/Gain where V
). For correct converter operation,
– IN
REF
–
+
> REF
(where IN
= 5V. This error has a very
CC
–
+ 0.3V. Outside these
REF
).
+
can be shorted to
+
LTC2487
= REF
and IN
+
–
REF
– REF

are the
/Gain
2487fa
–
.