11
LTC2404/LTC2408
Converter Operation Cycle
The LTC2404/LTC2408 are low power, 4-/8-channel delta-
sigma analog-to-digital converters with easy-to-use
4-wire interfaces. Their operation is simple and made up
of four states. The converter operation begins with the
conversion, followed by a low power sleep state and
concluded with the data output (see Figure 1). Channel
selection may be performed while the device is in the sleep
state or at the conclusion of the data output state. The
interface consists of serial data output (SDO), serial clock
(CLK/SCK), chip select (CSADC/CSMUX) and data input
(D
IN
). By tying SCK to CLK and CSADC to CSMUX, the
interface requires only four wires.
Initially, the LTC2404 or LTC2408 performs a conversion.
Once the conversion is complete, the device enters the
sleep state. While in the sleep state, power consumption
is reduced by an order of magnitude. The part remains in
the sleep state as long as CSADC is logic HIGH. The
conversion result is held indefinitely in a static shift
register while the converter is in the sleep state.
Channel selection for the next conversion cycle is per-
formed while the device is in the sleep state or at the end
of the data output state. A specific channel is selected by
applying a 4-bit serial word to the D
IN
pin on the rising edge
of CLK while CSMUX is HIGH, see Figure 3 and Table 3. The
channel is selected based on the last four bits clocked into
the D
IN
pin before CSMUX goes low. If D
IN
is all 0’s, the
previous channel remains selected.
In the example, Figure 3, the MUX channel is selected
during the sleep state, just before the data output state
begins. Once the channel selection is complete, the device
remains in the sleep state as long as CSADC remains
HIGH.
Once CSADC is pulled low, the device begins outputting
the conversion result. There is no latency in the conversion
result. Since there is no latency, the first conversion
following a change in input channel is valid and corre-
sponds to that channel. The data output corresponds to
the conversion just performed. This result is shifted out on
the serial data output 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
CSADC is brought HIGH. The device automatically initiates
a new conversion and the cycle repeats.
Through timing control of the CSADC and SCK pins, the
LTC2404/LTC2408 offer two modes of operation: internal
or external SCK. These 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 LTC2404/LTC2408 incorporate an on-chip
highly accurate oscillator. This eliminates the need for
external frequency setting components such as crystals or
oscillators. Clocked by the on-chip oscillator, the LTC2404/
LTC2408 reject line frequencies (50 or 60Hz ±2%) a
minimum of 110dB.
APPLICATIONS INFORMATION
WUU
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Figure 1. LTC2408 State Transition Diagram
CONVERT
SLEEP
CHANNEL SELECT
(SLEEP)
DATA OUTPUT
(CHANNEL SELECT)
24048 F01
0
1
CSADC
AND
SCK
