10
LTC2400
an order of magnitude. The part remains in the sleep state
as long as CS is logic 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 begins outputting the
conversion result. There is no latency in the conversion
result. The data output corresponds to the conversion just
performed. This result is shifted out on the serial data out
pin (SDO) under the control of the serial clock (SCK). Data
is updated on the falling edge of SCK allowing the user to
reliably latch data on the rising edge of SCK, see Figure 3.
The data output state is concluded once 32 bits are read
out of the ADC or when CS is brought HIGH. The device
automatically initiates a new conversion cycle and the
cycle repeats.
Through timing control of the CS and SCK pins, the
LTC2400 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.
Conversion Clock
A major advantage delta-sigma converters offer over
conventional type converters is an on-chip digital filter
(commonly known as Sinc or Comb filter). For high
resolution, low frequency applications, this filter is typi-
cally designed to reject line frequencies of 50 or 60Hz plus
their harmonics. In order to reject these frequencies in
excess of 110dB, a highly accurate conversion clock is
required. The LTC2400 incorporates an on-chip highly
accurate oscillator. This eliminates the need for external
frequency setting components such as crystals or oscilla-
tors. Clocked by the on-chip oscillator, the LTC2400
rejects line frequencies (50 or 60Hz ±2%) a minimum of
110dB.
Ease of Use
The LTC2400 data output has no latency, filter settling or
redundant data associated with the conversion cycle.
There is a one-to-one correspondence between the
conversion and the output data. Therefore, multiplexing
an analog input voltage is easy.
The LTC2400 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 LTC2400 automatically enters an internal reset state
when the power supply voltage V
CC
drops below approxi-
mately 2.2V. This feature guarantees the integrity of the
conversion result and of the serial interface mode selec-
tion which is performed at the initial power-up. (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 duration of approximately 0.5ms. The POR
signal clears all internal registers. Following the POR
signal, the LTC2400 starts a normal conversion cycle and
follows the normal succession of states described above.
The first conversion result following POR is accurate
within the specifications of the device.
Reference Voltage Range
The LTC2400 can accept a reference voltage from 0V to
V
CC
. The converter output noise is determined by the
thermal noise of the front-end circuits, and as such, its
value in microvolts is nearly constant with reference
voltage. A decrease in reference voltage will not signifi-
cantly improve the converter’s effective resolution. On the
other hand, a reduced reference voltage will improve the
overall converter INL performance. The recommended
range for the LTC2400 voltage reference is 100mV to V
CC
.
Input Voltage Range
The converter is able to accommodate system level offset
and gain errors as well as system level overrange situa-
tions due to its extended input range, see Figure 2. The
LTC2400 converts input signals within the extended input
range of –0.125 • V
REF
to 1.125 • V
REF
.
APPLICATIONS INFORMATION
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