LTC2486IDE datasheet(27/36 Pages) LINER

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LTC2486

2486f

Digital Signal Levels

The LTC2486’s digital interface is easy to use. Its digital

inputs SDI, FO, ⎯C⎯S, and SCK (in external serial clock mode)

accept standard CMOS logic levels. Internal hysteresis

circuits can tolerate edge transition times as slow as

100µs.

The digital input signal range is 0.5V to VCC – 0.5V. During

transitions, the CMOS input circuits draw dynamic cur-

rent. For optimal performance, application of signals to

the serial data interface should be reserved for the sleep

and data output periods.

During the conversion period, overshoot and undershoot

of fast digital signals applied to both the serial digital in-

terface and the external oscillator pin (FO) may degrade

the converter performance. Undershoot and overshoot

occur due to impedance mismatch of the circuit board

trace at the converter pin when the transition time of an

external control signal is less than twice the propagation

delay from the driver to the input pin. For reference, on a

regular FR-4 board, the propagation delay is approximately

183ps/inch. In order to prevent overshoot, a driver with

a 1ns transition time must be connected to the converter

through a trace shorter than 2.5 inches. This becomes

difﬁcult when shared control lines are used and multiple

reﬂections occur.

APPLICATIONS INFORMATION

Parallel termination near the input pin of the LTC2486 will

eliminate this problem, but will increase the driver power

dissipation. A series resistor from 27

Ω to 54Ω (depend-

ing on the trace impedance and connection) placed near

the driver will also eliminate over/under shoot without

additional driver power dissipation.

For many applications, the serial interface pins (SCK, SDI,

⎯C⎯S, F

O) remain static during the conversion cycle and

no degradation occurs. On the other hand, if an external

oscillator is used (FO driven externally) it is active during

the conversion cycle. Moreover, the digital ﬁlter rejection

is minimal at the clock rate applied to FO. Care must be

taken to ensure external inputs and reference lines do not

cross this signal or run near it. These issues are avoided

when using the internal oscillator.

Driving the Input and Reference

The input and reference pins of the LTC2486 are connected

directly to a switched capacitor network. Depending on

the relationship between the differential input voltage and

the differential reference voltage, these capacitors are

switched between these four pins. Each time a capacitor

is switched between two of these pins, a small amount

of charge is transferred. A simpliﬁed equivalent circuit is

shown in Figure 12.

Figure 12. LTC2486 Equivalent Analog Input Circuit

IN+

IN–

10k

INTERNAL

SWITCH

NETWORK

10k

CEQ

12pF

10k

IIN

–

REF+

IREF

IIN

IREF

–

2486 F12

SWITCHING FREQUENCY

fSW = 123kHz INTERNAL OSCILLATOR

fSW = 0.4 • fEOSC EXTERNAL OSCILLATOR

REF–

10k

100

Ω

INPUT

MULTIPLEXER

100

Ω

IIN

AVG

IN CM

REF CM

() = () =

−

•

–

()

IIREF

AVG

REF

REF CM

IN CM

() ≈

()

.–

.•

()

REF

REF CM

where

REF

–

•

(

=−

+−

))

–

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

=−

+−

REF

WHERE IN AN

D IN ARE THE SELECTED INPUT CHANNELS

IN CM

−

()

–

M INTERNAL OSCILLATOR

−

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

271

Ω

0Hz MODE

2.98M INTERNAL OSCILLATOR 50Hz/60

EQ =Ω

Hz MODE

0.833 10

EXTERNAL OSCIL

EOSC

=•

()

LATOR

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