ltc2484 Linear Technology Corporation, ltc2484 Datasheet - Page 13

• Manufacturer Part Number: ltc2484
• Description: 24-bit Delta Sigma Adc With Easy Drive Input Current Cancellation
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
While in this sleep state, power consumption is reduced by
two orders of magnitude. The part remains in the sleep
state as long as CS is HIGH. The conversion result is held
indefinitely in a static shift register while the converter is
in the sleep state.
Once CS is pulled LOW, the device exits the low power
mode and enters the data output state. If CS is pulled HIGH
before the first rising edge of SCK, the device returns to the
low power sleep mode and the conversion result is still
held in the internal static shift register. If CS remains LOW
after the first rising edge of SCK, the device begins
outputting the conversion result. Taking CS high at this
point will terminate the data input and output state and
start a new conversion. The conversion result is shifted
out of the device through the serial data output pin (SDO)
on the falling edge of the serial clock (SCK) (see Figure 2).
The LTC2484 includes a serial data input pin (SDI) in
which data is latched by the device on the rising edge of
SCK (Figure 2). The bit stream applied to this pin can be
used to select various features of the LTC2484, including
an on-chip temperature sensor, line frequency rejection
and output data rate. Alternatively, this pin may be tied to
ground and the part will perform conversions in a default
state. In the default state (SDI grounded) the device simply
performs conversions on the user applied input with
simultaneous rejection of 50Hz and 60Hz line frequencies.
Through timing control of the CS and SCK pins, the
LTC2484 offers several flexible modes of operation
(internal or external SCK and free-running conversion
modes). These various modes do not require program-
ming configuration registers; moreover, they do not dis-
turb the cyclic operation described above. These modes of
operation are described in detail in the Serial Interface
Timing Modes section.
Easy Drive Input Current Cancellation
The LTC2484 combines a high precision delta-sigma ADC
with an automatic differential input current cancellation
front end. A proprietary front-end passive sampling
U
U
W
U
network transparently removes the differential input cur-
rent. This enables external RC networks and high imped-
ance sensors to directly interface to the LTC2484 without
external amplifiers. 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
Input Current Cancellation section). This unique architec-
ture does not require on-chip buffers enabling input sig-
nals to swing all the way to ground and up to V
Furthermore, the cancellation does not interfere with the
transparent offset and full-scale autocalibration and the
absolute accuracy (full scale + offset + linearity) is main-
tained with external RC networks.
Accessing the Special Features of the LTC2484
The LTC2484 combines a high resolution, low noise ∆Σ
analog-to-digital converter with an on-chip selectable tem-
perature sensor, programmable digital filter and output
rate control. These special features are selected through a
single 8-bit serial input word during the data input/output
cycle (see Figure 2).
The LTC2484 powers up in a default mode commonly
used for most measurements. The device will remain in
this mode as long as the serial data input (SDI) is low. In
this default mode, the measured input is external, the
digital filter simultaneously rejects 50Hz and 60Hz line
frequency noise, and the speed mode is 1x (offset auto-
matically, continuously calibrated).
A simple serial interface grants access to any or all special
functions contained within the LTC2484. In order to
change the mode of operation, an enable bit (EN) followed
by up to 7 bits of data are shifted into the device
(see Table 1). The first 3 bits, in order to remain pin
compatible with the LTC2480, are DON’T CARE and can
be either HIGH or LOW. The 4th bit (IM) is used to select
the internal temperature sensor as the conversion input,
while the 5th and 6th bits (FA, FB) combine to determine
the line frequency rejection mode. The 7th bit (SPD) is
used to double the output rate by disabling the offset auto
calibration.
LTC2484
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2484fa
CC
.