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LTC6912

6912fa

PI FU CTIO S

APPLICATIO S I FOR ATIO

Functional Description

The LTC6912-X is a small outline, wideband, inverting

two-channel amplifier with voltage gains that are indepen-

dently programmable. Each delivers a choice of eight

voltage gains, configurable through a 3-wire serial digital

interface, which accepts TTL or CMOS logic levels (See

Figure 5). Tables 1 and 2 list the nominal gains for the

LTC6912-1 and LTC6912-2 respectively. Gain control

within the amplifier occurs by switching resistors from a

matched array in or out of a closed-loop op amp circuit

using MOS analog switches (Figure 1). The bandwidths of

the individual amplifiers depend on gain setting. The

Typical Performance Characteristics section shows mea-

sured frequency responses.

Description of the 3-Wire SPI Interface

Gain control of each amplifier is independently program-

mable using the 3-wire SPI interface (see Figure 5). Logic

levels for the LTC6912 3-wire serial interface are TTL/

CMOS compatible. When CS/LD is low, the serial data on

DIN is shifted into an 8-bit shift-register on the rising edge

of the clock, with the MSB transferred first. Serial data on

DOUT is shifted out on the clock’s falling edge. A rising edge

on CS/LD will latch the shift-register’s contents into an 8-

bit D-latch and disable the clock internally on the IC. The

upper nibble of the D-latch (4 most significant bits),

configure the gain for the B-channel amplifier. The lower

nibble of the D-latch (4 least significant bits), configures

the gain for the A-channel amplifier. Tables 1 and 2 detail

the nominal gains and respective gain codes. Care must be

taken to ensure CLK is taken low before CS/LD is pulled

low to avoid an extra internal clock pulse to the input of the

8-bit shift-register (See Figure 5).

DOUT is active in all states, therefore DOUT cannot be

“wire-OR’d” to other SPI outputs.

An LTC6912 may be daisy-chained with other LTC6912s

or other devices having serial interfaces by connecting the

DOUT to the DIN of the next chip while CLK and CS/LD

remain common to all chips in the daisy chain. The serial

data is clocked to all the chips then the CS/LD signal is

pulled high to update all of them simultaneously. Figure 6

shows an example of two LTC6912s in a daisy chained SPI

Figure 5. Serial Digital Interface Block Diagram

CLK

CS/LD

SHDN

6912 F05

LOWER NIBBLE UPPER NIBBLE

8-BIT LATCH

8-BIT

SHIFT-REGISTER

CHANNEL A

CHANNEL B

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7

DOUT

LSB

MSB

RESET

DIN

OUT A, OUT B: Analog Output. These pins are the output

of the A and B channel amplifiers respectively. Each

operational amplifier can swing rail-to-rail (V+ to V–) as

specified in the Electrical Characteristics table. For best

performance, loading the output as lightly as possible will

minimize signal distortion and gain error. The Electrical

Characteristics table shows performance at output cur-

rents up to 10mA, and the current limits which occur when

the output is shorted midsupply at 2.7V and

±5V supplies.

Output current above 10mA is possible but current-limit-

ing circuitry will begin to affect amplifier performance at

approximately 20mA. Long-term operation above 20mA

output is not recommended. Do not exceed maximum

junction temperature of 150

°C for a GN and 125°C for a

DFN package. The output will drive capacitive loads up to

50pF. Capacitances higher than 50pF should be isolated

by a series resistor (10

Ω or higher).

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