Datasheet

LTC2485
14
2485fc
APPLICATIONS INFORMATION
The LTC2485 powers up in a default mode commonly
used for most measurements. The device will remain in
this mode until a valid write cycle is performed. In this
default mode, the measured input is external, the digital
fi lter simultaneously rejects 50Hz and 60Hz line frequency
noise, and the speed mode is 1x (offset automatically,
continuously calibrated).
The I
2
C serial interface grants access to any or all special
functions contained within the LTC2485. In order to change
the mode of operation, a valid write address followed by 8
bits of data are shifted into the device (see Table 1). The
fi rst 4 bits are reserved and should be low. The 5th bit
(IM) is used to select the internal temperature sensor as
the conversion input, while the 6th and 7th bits (FA, FB)
combine to determine the line frequency rejection mode.
The 8th bit (SPD) is used to double the output rate by
disabling the offset auto calibration.
Table 1. Selecting Special Modes
IM FA FB SPD COMMENTS
0 0 0 0 External Input, 50Hz and 60Hz Rejection,
Autocalibration
0010
External Input, 50Hz Rejection,
Autocalibration
0100 External Input, 60Hz Rejection,
Autocalibration
0 0 0 1 External Input, 50Hz and 60Hz Rejection,
2x Speed
0 0 1 1 External Input, 50Hz Rejection, 2x Speed
0 1 0 1 External Input, 60Hz Rejection, 2x Speed
1 0 0 0 Temperature Input, 50Hz and 60Hz
Rejection, Autocalibration
1 0 1 X Temperature Input, 50Hz Rejection,
Autocalibration
1 1 0 X Temperature Input, 60Hz Rejection,
Autocalibration
X 1 1 X Reserved, Do Not Use
Temperature Sensor (IM)
The LTC2485 includes an on-chip temperature sensor.
The temperature sensor is selected by setting IM = 1 in
the serial input data stream. Conversions are performed
directly on the temperature sensor by the converter. While
operating in this mode, the device behaves as a temperature
to bits converter. The digital reading is proportional to the
absolute temperature of the device. This feature allows
the converter to linearize temperature sensors or continu-
ously remove temperature effects from external sensors.
Several applications leveraging this feature are presented
in more detail in the applications section. While operating
in this mode, the speed is set to normal independent of
the control bit (SPD).
Rejection Mode (FA, FB)
The LTC2485 includes a high accuracy on-chip oscilla-
tor with no required external components. Coupled with
a 4th order digital lowpass fi lter, the LTC2485 rejects
line frequency noise. In the default mode, the LTC2485
simultaneously rejects 50Hz and 60Hz by at least 87dB.
The LTC2485 can also be confi gured to selectively reject
50Hz or 60Hz to better than 110dB.
Speed Mode (SPD)
The LTC2485 continuously performs offset calibrations.
Every conversion cycle, two conversions are automatically
performed (default) and the results combined. This result
is free from offset and drift. In applications where the offset
is not critical, the autocalibration feature can be disabled
with the benefi t of twice the output rate.
Linearity, full-scale accuracy and full-scale drift are identi-
cal for both 2x and 1x speed modes. In both the 1x and
2x speed there is no latency. This enables input steps or
multiplexer channel changes to settle in a single conver-
sion cycle easing system overhead and increasing the
effective conversion rate.