LTC1400CS8 datasheet(12/20 Pages) LINER

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LTC1400

1400fa

Input signal leads to AIN and signal return leads from GND

(Pin 4) should be kept as short as possible to minimize

noisecoupling.Inapplicationswherethisisnotpossible,a

shielded cable between source and ADC is recommended.

Also,sinceanypotentialdifferenceingroundsbetweenthe

signalsourceandADCappearsasanerrorvoltageinseries

with the input signal, attention should be paid to reducing

the ground circuit impedance as much as possible.

APPLICATIO S I FOR ATIO

ANALOG SUPPLY

–5V

GND

LTC1400

VSS

VCC

GND

DIGITAL SUPPLY

GND

DIGITAL CIRCUITRY

VCC

GND

1400 F11

Figure11showstherecommendedsystemgroundconnec-

tions. All analog circuitry grounds should be terminated at

theLTC1400GNDpin.ThegroundreturnfromtheLTC1400

Pin 4 to the power supply should be low impedance for

noise free operation. Digital circuitry grounds must be

connected to the digital supply common.

InapplicationswheretheADCdataoutputsandcontrolsig-

nalsareconnectedtoacontinuouslyactivemicroprocessor

bus, it is possible to get errors in the conversion results.

These errors are due to feedthrough from the micropro-

cessor to the successive approximation comparator. The

problem can be eliminated by forcing the microprocessor

into a Wait state during conversion or by using three-state

buffers to isolate the ADC data bus.

Power-Down Mode

Upon power-up, the LTC1400 is initialized to the active

state and is ready for conversion. However, the chip can

be easily placed into the Nap or Sleep mode by exercising

the right combination of CLK and CONV signal. In the Nap

modeallpowerisoffexcepttheinternalreference,whichis

still active and provides 2.42V output voltage to the other

circuitry. In this mode, the ADC draws only 6mW of power

instead of 75mW (for minimum power, the logic inputs

must be within 500mV of the supply rails). The wake-up

time from the Nap mode to the active mode is 350ns.

Figure 11. Power Supply Connection

CLK

CONV

NAP

SLEEP

VREF

t11

REFRDY

NOTE: NAP AND SLEEP ARE INTERNAL SIGNALS. REFRDY APPEARS AS A BIT IN THE DOUT WORD.

1400 F12

Figure 12. Nap Mode and Sleep Mode Waveforms

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