Datasheet

LTC2480
21
2480fd
Input signals applied to IN
+
and IN
–
pins may extend by
300mV below ground and above V
CC
. In order to limit any
fault current, resistors of up to 5k may be added in series
with the IN
+
and IN
–
pins without affecting the performance
of the devices. The effect of the series resistance on the
converter accuracy can be evaluated from the curves
presented in the Input Current/Reference Current sections.
In addition, series resistors will introduce a temperature
dependent offset error due to the input leakage current.
A 1nA input leakage current will develop a 1ppm offset
error on a 5k resistor if V
REF
= 5V. This error has a very
strong temperature dependency.
Serial Interface Timing Modes
The LTC2480’s 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 or continuous conversion. 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 f
O
= HIGH)
or an external oscillator connected to the f
O
pin. Refer to
Table 6 for a summary.
External Serial Clock, Single Cycle Operation
(SPI/MICROWIRE Compatible)
This timing mode uses an external serial clock to shift
out the conversion result and a CS signal to monitor and
control the state of the conversion cycle, see Figure 5.
The serial clock mode is selected on the falling edge
of CS. To select the external serial clock mode, the
serial clock pin (SCK) must be LOW during each CS
falling edge.
applicaTions inForMaTion
Table 6. LTC2480 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, 3-Wire I/O External SCK SCK Figure 7
Internal SCK, Single Cycle Conversion Internal
CS↓ CS↓
Figures 8, 9
Internal SCK, 3-Wire I/O, Continuous Conversion Internal Continuous Internal Figure 10
EOC
BIT 23
SDO
SCK
(EXTERNAL)
CS
EN GS2 GS1 GS0 IM FA FB SPD
SDI*
DON’T CARE
TEST EOC
IMMSBSIG
BIT 0
LSB
BIT 4BIT 19 BIT 18 BIT 17 BIT 16BIT 20BIT 21BIT 22
SLEEPSLEEP
DATA OUTPUT CONVERSION
2480 F05
CONVERSION
Hi-ZHi-ZHi-Z
TEST EOC
V
CC
F
O
V
REF
IN
+
IN
–
SCK
SDI
SDO
CS
GND
2 10
INT/EXT CLOCK
3
4
5
9
7
8
6
1
REFERENCE
VOLTAGE
0.1V TO V
CC
ANALOG
INPUT
1µF
2.7V TO 5.5V
LTC2480
4-WIRE
SPI INTERFACE
DON’T CARE
TEST EOC
(OPTIONAL)
Figure 5. External Serial Clock, Single Cycle Operation