LTC2421 LINER [Linear Technology], LTC2421 Datasheet - Page 10

• Manufacturer Part Number: LTC2421
• Description: 1-/2-Channel 20-Bit UPower No Latency ADCs in MSOP-10
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC2421/LTC2422
The LTC2421/LTC2422 are pin compatible with the
LTC2401/LTC2402. The devices are designed to allow the
user to incorporate either device in the same design with
no modifications. While the LTC2421/LTC2422 output word
length is 24 bits (as opposed to the 32-bit output of the
LTC2401/LTC2402), its output clock timing can be identi-
cal to the LTC2401/LTC2402. As shown in Figure 1, the
LTC2421/LTC2422 data output is concluded on the falling
edge of the 24th serial clock (SCK). In order to maintain
drop-in compatibility with the LTC2401/LTC2402, it is
possible to clock the LTC2421/LTC2422 with an additional
8 serial clock pulses. This results in 8 additional output bits
which are always logic HIGH.
Converter Operation Cycle
The LTC2421/LTC2422 are low power, delta-sigma ana-
log-to-digital converters with an easy to use 3-wire serial
interface. Their operation is simple and made up of three
states. The converter operating cycle begins with the con-
version, followed by the sleep state and concluded with the
data output (see Figure 2). The 3-wire interface consists of
serial data output (SDO), a serial clock (SCK) and a chip
select (CS).
Initially, the LTC2421/LTC2422 perform a conversion. Once
the conversion is complete, the device enters the sleep
state. While in this sleep state, power consumption is re-
duced by an order of magnitude if CS is HIGH. The part
remains in the sleep state as long as CS is logic HIGH. The
conversion result is held indefinitely in a static shift regis-
ter while the converter is in the sleep state.
10
SDO
SCK
CS
U
CONVERSION
EOC = 1
U
Figure 1. LTC2421/LTC2422 Compatible Timing with the LTC2401/LTC2402
W
SLEEP
EOC = 0
U
8
4 STATUS BITS 20 DATA BITS
Once CS is pulled LOW and SCK rising edge is applied, the
device begins outputting the conversion result. There is no
latency in the conversion result. The data output corre-
sponds 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 4. The data output
state is concluded once 24 bits are read out of the ADC or
when CS is brought HIGH. The device automatically
initiates a new conversion and the cycle repeats.
Through timing control of the CS and SCK pins, the
LTC2421/LTC2422 offer several flexible modes of opera-
tion (internal or external SCK and free-running conver-
sion modes). These various modes do not require
programming configuration registers; moreover, they do
not disturb the cyclic operation described above. These
modes of operation are described in detail in the Serial
Interface Timing Modes section.
DATA OUTPUT
DATA OUT
8
Figure 2. LTC2421/LTC2422 State Transition Diagram
8
1
DATA OUTPUT
LAST 8 BITS ALWAYS 1
EOC = 1
CONVERT
8 (OPTIONAL)
CS AND
SCK
SLEEP
CONVERSION
0
24212 F02
24212 F01
24212f