LTC2489
11
2489fb
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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 LTC2489 automatically enters an internal reset state
when the power supply voltage, V
CC
, drops below ap-
proximately 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 channels IN
+
= CH0 and
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 refer
-
ence voltage. The absolute/common mode voltage range
for the 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 LTC2489 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 out-
put 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 proportion
-
ally improve the converter resolution and improve INL.
Input Voltage Range
The
LTC2489 input measurement range is –0.5•V
REF
to
+0.5•V
REF
in both differential and single-ended configura-
tions as shown in Figure 27. Highest linearity is achieved with
Fully Differential drive and a constant common-mode voltage
(Figure 27b). Other drive schemes may incur an INL error
of approximately 50ppm. This error can be calibrated out
using a three point calibration and a second-order curve fit.
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
CC
+ 0.3V. Within
these limits, the LTC2489 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 under-
range condition using distinct output codes (see Table 1).
In order to limit any fault current due to input ESD leakage
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.
I
2
C INTERFACE
The LTC2489 communicates through an I
2
C interface. The
I
2
C interface is a 2-wire open-drain interface supporting
multiple devices and multiple masters on a single bus. The
connected devices can only pull the data line (SDA) low
and can never drive it high. SDA is required to be externally
connected to the supply through a pull-up resistor. When
the data line is not being driven, it is high. Data on the
I
2
C bus can be transferred at rates up to 100kbits/s in the
standard mode and up to 400kbits/s in the fast mode. The
V
CC
power should not be removed from the device when
the I
2
C bus is active to avoid loading the I
2
C bus lines
through the internal ESD protection diodes.
Each device on the I
2
C bus is recognized by a unique
address stored in that device and can operate either as a
transmitter or receiver, depending on the function of the
device. In addition to transmitters and receivers, devices
can also be considered as masters or slaves when perform
-
ing data transfers. A master is the device which initiates a
data transfer on the bus and generates the clock signals
to permit that transfer. Devices addressed by the master
are considered a slave.
applicaTions inForMaTion
