LTC6945 datasheet(27/36 Pages) LINER

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2270f

LTC2270

Serial Programming Mode

To use the serial programming mode, PAR/SER should be

tied to ground. The CS, SCK, SDI and SDO pins become

a serial interface that program the A/D mode control

registers. Data is written to a register with a 16-bit serial

word. Data can also be read back from a register to verify

its contents.

Serial data transfer starts when CS is taken low. The data

on the SDI pin is latched at the first 16 rising edges of

SCK. Any SCK rising edges after the first 16 are ignored.

The data transfer ends when CS is taken high again.

The first bit of the 16-bit input word is the R/W bit. The

next seven bits are the address of the register (A6:A0).

The final eight bits are the register data (D7:D0).

If the R/W bit is low, the serial data (D7:D0) will be writ-

ten to the register set by the address bits (A6:A0). If the

R/W bit is high, data in the register set by the address bits

(A6:A0) will be read back on the SDO pin (see the timing

diagrams). During a read back command the register is

not updated and data on SDI is ignored.

The SDO pin is an open drain output that pulls to ground

with a 200Ω impedance. If register data is read back

through SDO, an external 2k pull-up resistor is required.

If serial data is only written and read back is not needed,

then SDO can be left floating and no pull-up resistor is

needed.

Table 3 shows a map of the mode control registers.

Software Reset

If serial programming is used, the mode control registers

should be programmed as soon as possible after the power

supplies turn on and are stable. The first serial command

must be a software reset which will reset all register data

bits to logic 0. To perform a software reset, bit D7 in the

reset register is written with a logic 1. After the reset SPI

write command is complete, bit D7 is automatically set

back to zero.

GROUNDING AND BYPASSING

The LTC2270 requires a printed circuit board with a clean

unbroken ground plane. A multilayer board with an in-

ternal ground plane in the first layer beneath the ADC is

recommended. 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 track alongside an analog signal track

or underneath the ADC.

High quality ceramic bypass capacitors should be used at

the VDD, OVDD, VCM, VREF, REFH and REFL pins. Bypass

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

Size 0402 ceramic capacitors are recommended. The traces

connecting the pins and bypass capacitors must be kept

short and should be made as wide as possible.

Of particular importance is the capacitor between REFH

and REFL. This capacitor should be on the same side of

the circuit board as the A/D, and as close to the device

as possible.

The analog inputs, encode signals, and digital outputs

should not be routed next to each other. Ground fill and

grounded vias should be used as barriers to isolate these

signals from each other.

HEAT TRANSFER

Most of the heat generated by the LTC2270 is transferred

from the die through the bottom-side exposed pad and

package leads onto the printed circuit board. For good

electrical and thermal performance, the exposed pad must

be soldered to a large grounded pad on the PC board. This

pad should be connected to the internal ground planes by

an array of vias.

APPLICATIONS INFORMATION

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