LTC2494 Linear Technology, LTC2494 Datasheet - Page 14

• Manufacturer Part Number: LTC2494
• Description: 16-Bit 8-/16-Channel ADC
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
LTC2494
transparently removes the differential input current. This
enables external RC networks and high impedance sen-
sors to directly interface to the LTC2494 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 architec-
ture does not require on-chip buffers, thereby enabling
signals to swing beyond ground or up to V
the cancellation does not interfere with the transparent
offset and full-scale auto-calibration and the absolute ac-
curacy (full scale + offset + linearity + drift) is maintained
even with external RC networks.
Power-Up Sequence
The LTC2494 automatically enters an internal reset state
when the power supply voltage, V
proximately 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 channels IN
IN
rate, and gain = 1. 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, temperature selection, or
gain can be programmed into the device during this fi rst
data input/output cycle.
Reference Voltage Range
This converter accepts a truly differential external reference
voltage. The absolute/common mode voltage range for the
REF
the device (GND to V
V
14
REF
–
= CH1 with simultaneous 50Hz/60Hz rejection 1x output
+
must be positive (REF
and REF
CC
rises above this threshold, the converter creates
–
pins covers the entire operating range of
CC
). For correct converter operation,
+
> REF
–
CC
).
, drops below ap-
CC
+
. Moreover,
= CH0 and
The LTC2494 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 nano volts is nearly
constant with reference voltage. A decrease in reference
voltage will not signifi cantly 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 V
limits, the ESD projection devices begin to turn on and the
errors due to input leakage current increase rapidly. Within
these limits, the LTC2494 converts the bipolar differential
input signal V
selected input channels), from –FS = –0.5 • V
+FS = 0.5 • V
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 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
perature 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
amplifi er is used, the LTC2494 automatically calibrates
both the offset and drift of this circuit and the Easy Drive
sampling scheme enables a wide variety of amplifi ers to
be used.
CC
CC
. For the simplest operation, REF
and REF
REF
–
REF
IN
can be shorted to GND. The converter out-
/Gain where V
= 5V. This error has a very strong tem-
= IN
+
+ IN
–
(where IN
REF
CC
= REF
+ 0.3V. Outside these
+
can be shorted to
+
+
– REF
and IN
CC
. In order to
REF
–
–
. Outside
/Gain to
are the
2494fb