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LTC2412

2412f

APPLICATIO S I FOR ATIO

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 (FO, 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

100

µs.However,someconsiderationsarerequiredtotake

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

(VCC – 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 (FO, 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 [VIL < 0.4V and VOH >

(VCC – 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)

LSB24

MSB

SIG

CH0/CH1

BIT 5

BIT 0

BIT 27

BIT 26

BIT 28

BIT 29

BIT 30

EOC

BIT 31

DATA OUTPUT

CONVERSION

2412 F10

= 50Hz REJECTION

= EXTERNAL OSCILLATOR

= 60Hz REJECTION

VCC

REF+

SCK

CH1+

CH1–

SDO

GND

114

8, 9, 10, 15, 16

REFERENCE

VOLTAGE

0.1V TO VCC

CH0+

CH0–

ANALOG INPUT RANGE

–0.5VREF TO 0.5VREF

µF

2.7V TO 5.5V

LTC2412

2-WIRE

INTERFACE

REF–

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