LTC6912

22

6912fa

SNR from an amplifier is the ratio of input level to input-

referred noise, and can be 108dB with the LTC6912 family

at unity gain.

Construction and Instrumentation Cautions

Electrically clean construction is important in applications

seeking the full dynamic range of the LTC6912 family of

dual amplifiers. It is absolutely critical to have AGND either

AC bypassed or wired directly using the shortest possible

wiring, to a low impedance ground return for best channel-

to-channel isolation. Short, direct wiring minimizes para-

sitic capacitance and inductance. High quality supply

bypass capacitors of 0.1

µF near the chip provide good

APPLICATIO S I FOR ATIO

decoupling from a clean, low inductance power source.

But several centimeters of wire (i.e., a few

µH of induc-

tance) from the power supplies, unless decoupled by

substantial capacitance (>10

µF) near the chip, can create

a parasitic high-Q LC resonant circuit in the hundreds of

kHz range in the chip’s supplies or ground reference. This

may impair circuit performance at those frequencies. A

compact, carefully laid out printed circuit board with a

good ground plane makes a significant difference in mini-

mizing distortion. Finally, equipment to measure perfor-

mance can itself introduce distortion or noise floors.

Checking for these limits with wired shorts from INA to

OUTA and INB to OUTB in place of the chip is a prudent

routine procedure.

TYPICAL APPLICATIO

Low Noise AC Amplifier with Programmable Gain and

Bandwidth

Analog data acquisition can exploit band limiting as well as

gain to suppress unwanted signals or noise. Tailoring an

analog front end to both the level and bandwidth of each

source maximizes the resulting SNR. Figure 7 shows a

block diagram for a low noise amplifier with gain and

bandwidth independently programmable over a 100:1

range. Channels A and B of the LTC6912-1 are used to

independently control the gain and bandwidth respec-

tively over a 100:1 range. The LT1884 dual op amp forms

an integrating lowpass loop with capacitor C2 to set the

programmable upper corner frequency. The LT1884 also

supports rail-to-rail output swings over the total supply

voltage range of 2.7V to 10.5V. AC coupling through

capacitor C1 establishes a fixed low frequency corner of

1Hz, which can be adjusted by changing C1. Alternatively,

shorting C1 makes the amplifier DC coupled. If DC gain is

not needed, the AC coupling cap C1 serves to suppress

several error sources: any shift in DC levels, low frequency

noise, and DC offset voltages (not including the LT1884’s

low internal offset).

Figure 7. Block Diagram of an AC Amplifier with Programmable Gain and Bandwidth

–

+

–

+

GAINA

GAINB

C1

10

µF

C2

1

µF

R1

15.8k

R2

15.8k

1M

R

R

1/2 LT1884

1/2 LT1884

1/2 LT1884

LTC6912-1

CHANNEL A

LTC6912-1

CHANNEL B

GAIN

CONTROL

PGA

BANDWIDTH

CONTROL

PGA

R2

R1

6912 F07

INA

OUTA

INB

OUTB

1

2

πR1C1

1

R2

GAINB

2

–3dB BANDWIDTH RANGE IS FROM

TO

≤