LTC1400CS8 datasheet(12/20 Pages) LINER

12 / 20 page

LTC1400

APPLICATIONS INFORMATION

Input signal leads to AIN and signal return leads from GND

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

noise coupling. In applications where this is not possible,

a shielded cable between source and ADC is recom-

mended. Also, since any potential difference in grounds

between the signal source and ADC appears as an error

voltage in series with the input signal, attention should be

paid to reducing the ground circuit impedance as much as

possible.

nated at the LTC1400 GND pin. The ground return from the

LTC1400 Pin 4 to the power supply should be low imped-

ance for noise free operation. Digital circuitry grounds

must be connected to the digital supply common.

In applications where the ADC data outputs and control

signals are connected to a continuously active micropro-

cessor bus, it is possible to get errors in the conversion

results. These errors are due to feedthrough from the

microprocessor to the successive approximation com-

parator. 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

mode all power is off except the internal reference, which

is 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

Figure 11 shows the recommended system ground con-

nections. All analog circuitry grounds should be termi-

Figure 11. Power Supply Connection

ANALOG SUPPLY

–5V

GND

LTC1400

VSS

VCC

GND

DIGITAL SUPPLY

GND

DIGITAL CIRCUITRY

VCC

GND

LTC1400 • F11

CLK

CONV

NAP

SLEEP

VREF

REFRDY

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

LTC1400 • F08

Figure 12. Nap Mode and Sleep Mode Waveforms

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