LTC1404CS8 datasheet(15/24 Pages) LINER

15 / 24 page

LTC1404

APPLICATIONS INFORMATION

BOARD LAYOUT AND BYPASSING

To obtain the best performance from the LTC1404, a

printed circuit board is required. Layout for the printed

circuit board should ensure that digital and analog signal

lines are separated as much as possible. In particular, care

should be taken not to run any digital traces alongside an

analog signal trace or underneath the ADC. The analog

input should be screened by GND.

High quality 10

µF surface mount AVX capacitor with a

0.1

µF ceramic should be used at the VCC, VSS and VREF

pins. For better results, another 10

µFAVXcapacitorcanbe

added to the VCC pin. At 600ksps, the CLK frequency can

be as high as 9.6MHz. A poor quality capacitor can lose

more than 80% of its capacitance at this frequency range.

Therefore, it is important to consult the manufacturer’s

data sheet before the capacitor is used. For the LTC1404,

at 600ksps, every bit decision must be determined within

104ns (9.6MHz). During this short time interval, the

supply disturbance due to a CLK transition needs to settle.

The ADC must update its DAC, make a comparator deci-

sion based on sub-mV overdrive, latch the new DAC

information and output the serial data. This ADC provides

one power supply, VCC, which is connected to both the

internal analog and digital circuitry. Any ringing due to

poor supply or reference bypassing, inductive trace runs,

CLK and CONV over- or undershoot, or unnecessary DOUT

loading can cause ADC errors. Therefore, the bypass

capacitors must be located as close to the pins as possible.

The traces connecting the pins and the bypass capacitors

must be kept short and should be made as wide as

possible. In unipolar mode operation, VSS must be con-

nected to the GND pin directly.

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 the analog input signal and the

ADC is recommended. Also, any potential difference in

grounds between the analog signal and the ADC appears

as an error voltage in series with the analog input signal.

Attention should be paid to reducing the ground circuit

impedance as much as possible.

Figure 11 shows the recommended system ground con-

nections. All analog circuitry grounds should be termi-

nated at the LTC1404 GND pin. The ground return from the

LTC1404 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. As an

alternative, instead of a direct short between the digital and

analog circuitry, a 10

Ω or a ferrite bead jumper helps

reduce the digital noise.

ANALOG SUPPLY

–5V

GND

LTC1404

VSS

VCC

GND

DIGITAL SUPPLY

GND

DIGITAL CIRCUITRY

VCC

GND

1404 F11

Figure 11. Power Supply Connection

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 LTC1404 is initialized to the active

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

easily placed into Nap or Sleep mode by exercising the

right combination of CLK and CONV signals. In Nap mode,

all power is off except for the internal reference, which is

still active and provides 2.43V output voltage to the other

circuitry. In this mode, the ADC draws only 7.5mW of

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

LTC1404CS8 датащи(PDF) 15 Page - Linear Technology

LTC1404CS8 датащи(HTML) 15 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL