15
LTC1196/LTC1198
U
S
A
O
PPLICATI
U
U
Connection to a microprocessor or a DSP serial port is
quite simple (see Data Transfer section). It requires no
additional hardware, but the speed will be limited by the
clock rate of the microprocessor or the DSP which limits
the conversion time of the LTC1196/LTC1198.
Data Transfer
Data transfer differs slightly between the LTC1196 and the
LTC1198. The LTC1196 interfaces over 3 lines: CS, CLK
and D
OUT
. A falling CS initiates data transfer as shown in
the LTC1196 Operating Sequence. After CS falls, the first
CLK pulse enables D
OUT
. After two null bits, the A/D
conversion result is output on the D
OUT
line. Bringing CS
high resets the LTC1196 for the next data exchange.
The LTC1198 can transfer data with 3 or 4 wires. The
additional input, D
IN
, is used to select the 2-channel MUX
configuration.
The data transfer between the LTC1198 and the digital
systems can be broken into two sections: Input Data Word
and A/D Conversion Result. First, each bit of the input data
word is captured on the rising CLK edge by the LTC1198.
Second, each bit of the A/D conversion result on the D
OUT
line is updated on the rising CLK edge by the LTC1198.
This bit should be captured on the next rising CLK edge by
the digital systems (see A/D Conversion Result section).
Data transfer is initiated by a falling chip select (CS) signal
as shown in the LTC1198 Operating Sequence. After CS
falls the LTC1198 looks for a start bit. After the start bit is
received, the 4-bit input word is shifted into the D
IN
input.
The first two bits of the input word configure the LTC1198.
The last two bits of the input word allow the ADC to acquire
the input voltage by 2.5 clocks before the conversion
starts. After the conversion starts, two null bits and the
conversion result are output on the D
OUT
line. At the end
of the data exchange CS should be brought high. This
resets the LTC1198 in preparation for the next data ex-
change.
Input Data Word
The LTC1196 requires no D
IN
word. It is permanently
configured to have a single differential input. The conver-
sion result is output on the D
OUT
line in an MSB-first
sequence, followed by zeros indefinitely if clocks are
continuously applied with CS low.
The LTC1198 clocks data into the D
IN
input on the rising
edge of the clock. The input data word is defined as follows:
Start Bit
The first “l(fā)ogical one” clocked into the D
IN
input after CS
goes low is the start bit. The start bit initiates the data
transfer. The LTC1198 will ignore all leading zeros which
precede this logical one. After the start bit is received, the
remaining bits of the input word will be clocked in. Further
inputs on the D
IN
pin are then ignored until the next CS cycle.
Multiplexer (MUX) Address
The 2 bits of the input word following the START bit assign
the MUX configuration for the requested conversion. For
a given channel selection, the converter will measure the
voltage between the two channels indicated by the “+” and
“–” signs in the selected row of the following table. In
single-ended mode, all input channels are measured with
respect to GND.
D
IN1
D
IN2
D
OUT1
D
OUT2
CS
SHIFT MUX
ADDRESS IN
2 NULL BITS
SHIFT A/D CONVERSION
RESULT OUT
1196/98 AI01
SGL/
DIFF
ODD/
SIGN
DUMMY
START
MUX
ADDRESS
DUMMY
BITS
119698 AI02
DUMMY
LTC1198 Channel Selection
MUX ADDRESS
SGL/DIFF
1
1
0
0
ODD/SIGN
0
1
0
1
CHANNEL #
0
+
+
–
1
+
–
+
GND
–
–
SINGLE-ENDED
MUX MODE
DIFFERENTIAL
MUX MODE
1196/98 AI03