LTC2444/LTC2445/
LTC2448/LTC2449
16
2444589fc
For more information www.linear.com/LTC2444
Table 6. LTC2444/LTC2445/LTC2448/LTC2449 Interface Timing Modes
CONFIGURATION
SCK
SOURCE
CONVERSION
CYCLE
CONTROL
DATA
OUTPUT
CONTROL
CONNECTION
AND
WAVEFORMS
External SCK, Single Cycle Conversion External CS and SCK CS and SCK Figures 5, 6
External SCK, 2-Wire I/O External SCK SCK Figure 7
Internal SCK, Single Cycle Conversion Internal
CS ↓ CS ↓
Figures 8, 9
Internal SCK, 2-Wire I/O, Continuous Conversion Internal Continuous Internal Figure 10
applicaTions inForMaTion
Speed Multiplier Mode
In addition to selecting the speed/resolution, a speed
multiplier mode is used to double the output rate while
maintaining the selected resolution. The last bit of the
5-bit speed/resolution control word (TWOX, see Table 5)
determines if the output rate is 1X (no speed increase) or
2X (double the selected speed).
While operating in the 1X mode, the device combines two
internal conversions for each conversion result in order
to remove the ADC offset. Every conversion cycle, the
offset and offset drift are transparently calibrated greatly
simplifying the user interface. The resulting conversion
result has no latency. The first conversion following a
newly selected speed/resolution and input channel is
valid. This is identical to the operation of the LTC2440,
LTC2414 and LTC2418.
While operating in the 2X mode, the device performs a
running average of the last two conversion results. This
automatically removes the offset and drift of the device
while increasing the output rate by 2X. The resolution
(noise) remains the same. If a new channel is selected,
the conversion result is valid for all conversions after
the first conversion (one cycle latency). If a new speed/
resolution is selected, the first conversion result is valid
but the resolution (noise) is a function of the running av-
erage. All subsequent conversion results are valid. If the
mode is changed from either 1X to 2X or 2X to 1X without
changing the resolution or channel, the first conversion
result is valid.
If an external buffer/amplifier circuit is used for the
LTC2445/LTC2449, the 2X mode can be used to increase
the settling time of the amplifier between readings. While
operating in the 2X mode, the multiplexer output (input
to the external buffer/amplifier) is switched at the end of
each conversion cycle. Prior to concluding the data out/
in cycle, the analog multiplexer output is switched. This
occurs at the end of the conversion cycle (just prior to
the data output cycle) for auto calibration. The time re
-
quired to read the conversion enables more settling time
for the external buffer/amplifier. The offset/offset drift of
the external amplifier is automatically removed by the
converter’s auto calibration sequence for both the 1X and
2X speed modes.
While operating in the 1X mode, if a new input channel
is selected the multiplexer is switched on the falling edge
of the 14th SCK (once the complete data input word is
programmed). The remaining data output sequence time
can be used to allow the external buffer/amplifier to settle.
BUSY
The BUSY output (Pin 2) is used to monitor the state of
conversion, data output and sleep cycle. While the part is
converting, the BUSY pin is HIGH. Once the conversion is
complete, BUSY goes LOW indicating the conversion is
complete and data out is ready. The part now enters the
LOW power sleep state. BUSY remains LOW while data is
shifted out of the device and SDI is shifted into the device. It
goes HIGH at the conclusion of the data input/output cycle
indicating a new conversion has begun. This rising edge
may be used to flag the completion of the data read cycle.
Serial Interface Timing Modes
The LTC2444/LTC2445/LTC2448/LTC2449’s 3- or 4-wire
interface is SPI and MICROWIRE compatible. This interface
offers several flexible modes of operation. These include
internal/external serial clock, 3- or 4-wire I/O, single cycle
conversion and autostart. The following sections describe
each of these serial interface timing modes in detail. In all
these cases, the converter can use the internal oscillator
(F
O
= LOW) or an external oscillator connected to the F
O
pin. Refer to Table6 for a summary.
