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 0.1µF near the chip provide good
APPLICATIO S I FOR ATIO
WUUU
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 (>10µF) 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
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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
–
+
–
+
V
IN
V
OUT
GAINA
GAINB
C1
10µF
C2
1µF
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
V
OUT
= GAINA V
IN
R2
R1
6912 F07
INA OUTA
INB OUTB
1
2πR1C1
1
R2
GAINB
2π (
)
C2
–3dB BANDWIDTH RANGE IS FROM TO ≤
