LTC2442
14
2442fb
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Chip Select Input (CS)
The active LOW chip select, CS (Pin 35), is used to test the
conversion status and to enable the data output transfer
as described in the previous sections.
In addition, the CS signal can be used to trigger a new
conversion cycle before the entire serial data transfer has
been completed. The LTC2442 will abort any serial data
transfer in progress and start a new conversion cycle
anytime a LOW-to-HIGH transition is detected at the CS
pin after the converter has entered the data output state.
Serial Data Input (SDI)
The serial data input (SDI, Pin 33) is used to select the
speed/resolution and input channel of the LTC2442. SDI
is programmed by a serial input data stream under the
control of SCK during the data output cycle, see Figure 3.
Initially, after powering up, the device performs a conver
-
sion with SEL
+
= CH0, SEL
–
= CH1, OSR = 256 (output
rate nominally 879Hz), and 1X speedup mode (no latency).
Once this first conversion is complete, the device enters
the sleep state and is ready to output the conversion result
and receive the serial data input stream programming the
speed/resolution and input channel for the
next conversion.
At
the conclusion of each conversion cycle, the device
enters this state.
In order to change the speed/resolution or input channel,
the first three bits shifted into the device are 101. This is
compatible with the programming sequence of all LTC
multichannel differential input DS ADCs. If the sequence
is set to 000 or 100, the following input data is ignored
(don’t care) and the previously selected speed/resolution
and channel remain valid for the next conversion. Combi
-
nations other
than 101, 100, and 000 of the three control
bits should be avoided.
If the first three bits shifted into the device are 101, then
the following five bits select the input channel for the fol
-
lowing conversion (
see Tables 3 and 4). The next five bits
select the speed/resolution and mode 1X (no latency) 2X
(double output rate with one conversion latency), see Table
4. If these five bits are set to all 0’s, the previous speed
remains selected for the next conversion. This is useful
in applications requiring a fixed output rate/resolution but
need to change the input channel.
When an update operation is initiated the first three bits
are 101. The following five bits are the channel address.
The first bit
, SGL, determines if the input selection is
differential (SGL = 0) or single-ended (SGL = 1). For SGL
= 0, two adjacent channels can be selected to form a dif
-
ferential input. For SGL = 1, one of 4 channels is selected
as the positive input. The negative input is COM for all
single ended operations. The next 4-bits (ODD, A2, A1,
A0) determine which channel is selected and its polarity,
(see Table 3). In order to remain software compatible with
LTCs other multi-channel DS ADCs, A2 and A1 are unused
and should be set low.
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 4)
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
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