LTC6912IGN-2 datasheet(16/24 Pages) LINER

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LTC6912

6912fa

INA, INB: Analog Inputs. The input signal to the A channel

amplifier of the LTC6912-X is the voltage difference be-

tween the INA pin and AGND pin. Likewise, the input signal

to the B channel amplifier of the LTC6912-X is the voltage

difference between the INB pin and AGND pin. The INA (or

INB) pin connects internally to a digitally controlled resis-

tance whose other end is a current summing point at the

same potential as the AGND pin (Figure 1). At unity gain,

the value of this input resistance is approximately 10k

Ω

and the INA (or INB) pin voltage range is rail-to-rail (V+ to

V–). At gain settings above unity, the input resistance falls.

The linear input range at INA and INB also falls inversely

proportional to the programmed gain. Tables 1 and 2

summarize this behavior. The higher gains are designed to

boost lower level signals with good noise performance. In

the “zero” gain state (state = 0), or in software shutdown

(state = 8) analog switches disconnect the INA or INB pin

internally and this pin presents a very high input resis-

tance. In the “zero” gain state (state = 0), the input may

vary from rail to rail but the output is insensitive to it and

is forced to the AGND potential. Circuitry driving the INA

and INB pins must consider the LTC6912-X’s input resis-

tance, its process variance, and the variation of this

resistance from gain setting to gain setting. Signal sources

with significant output resistance may introduce a gain

error as the source’s output resistance and the LTC6912-

X’s input resistance forms a voltage divider. This is espe-

cially true at higher gain settings where the input resis-

tance is the lowest.

PI FU CTIO S

Figure 1. GN-16 Block Diagram

–

MOS INPUT

OP AMP

MOS INPUT

OP AMP

V+

V–

SHDN

CS/LD

DATA

CLK

6912 BD

LOWER

NIBBLE

UPPER

NIBBLE

8-BIT

LATCH

8-BIT

SHIFT-REGISTER

INPUT R ARRAY

FEEDBACK R ARRAY

INPUT R ARRAY

FEEDBACK R ARRAY

CHANNEL A

CHANNEL B

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7

INA

AGND

V–

OUT A

OUT B

V+

DGND

DOUT

INB

100k

In single supply voltage applications, the LTC6912-X’s DC

ground reference for both input and output is AGND, not

V–. With increasing gains, the LTC6912-X’s input voltage

range for an unclipped output is no longer rail-to-rail but

diminishes inversely to gain, centered about the AGND

potential.

TYPICAL PERFOR A CE CHARACTERISTICS

LTC6912-2 Gain Shift vs

Temperature (Heavy Load)

TEMPERATURE (°C)

–50

–25

100

125

0.25

–0.25

–0.50

–0.75

–1.00

6912 G26

VS = 5V

RL = 500

GAIN = 1

GAIN = 8

GAIN = 64

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