Datasheet
LTC2412
22
2412f
APPLICATIO S I FOR ATIO
WUU
U
PRESERVING THE CONVERTER ACCURACY
The LTC2412 is designed to reduce as much as possible
the conversion result sensitivity to device decoupling,
PCB layout, antialiasing circuits, line frequency perturba-
tions and so on. Nevertheless, in order to preserve the
extreme accuracy capability of this part, some simple
precautions are desirable.
Digital Signal Levels
The LTC2412’s digital interface is easy to use. Its digital
inputs (F
O
, CS and SCK in External SCK mode of operation)
accept standard TTL/CMOS logic levels and the internal
hysteresis receivers can tolerate edge rates as slow as
100µs. However, some considerations are required to take
advantage of the exceptional accuracy and low supply
current of this converter.
The digital output signals (SDO and SCK in Internal SCK
mode of operation) are less of a concern because they are
not generally active during the conversion state.
While a digital input signal is in the range 0.5V to
(V
CC
␣ –␣ 0.5V), the CMOS input receiver draws additional
current from the power supply. It should be noted that,
when any one of the digital input signals (F
O
, CS and SCK
in External SCK mode of operation) is within this range, the
LTC2412 power supply current may increase even if the
signal in question is at a valid logic level. For micropower
operation, it is recommended to drive all digital input
signals to full CMOS levels [V
IL
< 0.4V and V
OH
>
(V
CC
– 0.4V)].
During the conversion period, the undershoot and/or
overshoot of a fast digital signal connected to the LTC2412
pins may severely disturb the analog to digital conversion
process. Undershoot and overshoot can occur because of
the impedance mismatch at the converter pin when the
transition time of an external control signal is less than
twice the propagation delay from the driver to LTC2412.
For reference, on a regular FR-4 board, signal propagation
velocity is approximately 183ps/inch for internal traces
and 170ps/inch for surface traces. Thus, a driver gener-
ating a control signal with a minimum transition time of
1ns must be connected to the converter pin through a
trace shorter than 2.5 inches. This problem becomes
particularly difficult when shared control lines are used
and multiple reflections may occur. The solution is to
carefully terminate all transmission lines close to their
characteristic impedance.
Figure 10. Internal Serial Clock, Continuous Operation
SDO
SCK
(INTERNAL)
CS
LSB
24
MSBSIGCH0/CH1
BIT 5 BIT 0BIT 27 BIT 26BIT 28BIT 29BIT 30
EOC
BIT 31
DATA OUTPUT CONVERSIONCONVERSION
2412 F10
= 50Hz REJECTION
= EXTERNAL OSCILLATOR
= 60Hz REJECTION
V
CC
V
CC
F
O
REF
+
SCK
CH1
+
CH1
–
SDO
GND
CS
114
2
3
13
6
7
12
8, 9, 10, 15, 16
11
REFERENCE
VOLTAGE
0.1V TO V
CC
CH0
+
CH0
–
4
5
ANALOG INPUT RANGE
–0.5V
REF
TO 0.5V
REF
1µF
2.7V TO 5.5V
LTC2412
2-WIRE
INTERFACE
REF
–