LTC2436-1
9
24361f
cycle. There is a one-to-one correspondence between the
conversion and the output data. Therefore, multiplexing
multiple analog voltages is easy.
The LTC2436-1 performs offset and full-scale calibrations
every conversion cycle. This calibration is transparent to
the user and has no effect on the cyclic operation de-
scribed above. The advantage of continuous calibration is
extreme stability of offset and full-scale readings with re-
spect to time, supply voltage change and temperature drift.
Power-Up Sequence
The LTC2436-1 automatically enters an internal reset state
when the power supply voltage V
CC
drops below approxi-
mately 2V. This feature guarantees the integrity of the
conversion result and of the serial interface mode selec-
tion. (See the 2-wire I/O sections in the Serial Interface
Timing Modes section.)
When the V
CC
voltage rises above this critical threshold,
the converter creates an internal power-on-reset (POR)
signal with a typical duration of 1ms. The POR signal clears
all internal registers and selects channel 0. Following the
POR signal, the LTC2436-1 starts a normal conversion
cycle and follows the succession of states described above.
The first conversion result following POR is accurate within
the specifications of the device if the power supply voltage
is restored within the operating range (2.7V to 5.5V) be-
fore the end of the POR time interval.
Reference Voltage Range
This converter accepts a truly differential external refer-
ence voltage. The absolute/common mode voltage speci-
fication for the REF
+
and REF
–
pins covers the entire range
from GND to V
CC
. For correct converter operation, the
REF
+
pin must always be more positive than the REF
–
pin.
The LTC2436-1 can accept a differential reference voltage
from 0.1V to V
CC
. The converter output noise is deter-
mined 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
significantly improve the converter’s effective resolution,
since the thermal noise (800nV) is well below the quanti-
zation level of the device (75.6µV for a 5V reference). At the
minimum reference (100mV) the thermal noise
remains constant at 800nV RMS (or 4.8µV
P-P
), while the
quantization is reduced to 1.5µV per LSB. As a result,
lower the reference improves the effective resolution for
low level input voltages.
Input Voltage Range
The analog input is truly differential with an absolute/
common mode range for the CH0
+
/CH0
–
or CH1
+
/CH1
–
input pins extending from GND – 0.3V to V
CC
+ 0.3V.
Outside these limits, the ESD protection devices begin to
turn on and the errors due to input leakage current
increase rapidly. Within these limits, the LTC2436-1 con-
verts the bipolar differential input signal, V
IN
= IN
+
– IN
–
,
from –FS = –0.5 • V
REF
to +FS = 0.5 • V
REF
where V
REF
=
REF
+
– REF
–
, with the selected channel referred as IN
+
and
IN
–
. Outside this range, the converter indicates the
overrange or the underrange condition using distinct
output codes.
Input signals applied to the analog input pins may extend
by 300mV below ground and above V
CC
. In order to limit
any fault current, resistors of up to 5k may be added in
series with the pins without affecting the performance of
the device. In the physical layout, it is important to main-
tain the parasitic capacitance of the connection between
these series resistors and the corresponding pins as low
as possible; therefore, the resistors should be located as
close as practical to the pins. The effect of the 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 intro-
duce a temperature dependent offset error due to the input
leakage current. A 10nA input leakage current will develop
a 1LSB offset error on an 8k resistor if V
REF
= 5V. This error
has a very strong temperature dependency.
Output Data Format
The LTC2436-1 serial output data stream is 19 bits long.
The first 3 bits represent status information indicating the
conversion state, selected channel and sign. The next 16
bits are the conversion result, MSB first. The third and
fourth bit together are also used to indicate an underrange
condition (the differential input voltage is below –FS) or an
overrange condition (the differential input voltage is above
+FS).
APPLICATIO S I FOR ATIO
WUUU
