LTC6911-1/LTC6911-2
16
sn691112 691112fs
APPLICATIO S I FOR ATIO
WUUU
of 1, or 1.5mV at a gain setting of 2. At high gains, V
OS(IN)
approaches V
OS(OA)
. (Offset voltage is random and can
have either polarity centered on 0V.) The MOS input
circuitry of the internal op amp in Figure 1 draws negligible
input currents (unlike some op amps), so only V
OS(OA)
and
G affect the overall amplifier’s offset.
AC-Coupled Operation
Adding capacitors in series with the INA and INB pins
convert the LTC6911-X into a dual AC-coupled inverting
amplifier, suppressing the input signal’s DC level (and also
adding the additional benefit of reducing the offset voltage
from the LTC6911-X’s amplifier itself). No further compo-
nents are required because the input of the LTC6911-X
biases itself correctly when a series capacitor is added.
The INA and INB analog input pins connect internally to a
resistor whose nominal value varies between 10k and 1k
depending on the version of LTC6911 used (see the
rightmost column of Tables 1 and 2). Therefore, the low
frequency cutoff will vary with capacitor and gain setting.
For example, if a low frequency corner of 1kHz or lower on
the LTC6911-1 is desired, use a series capacitor of 0.16µF
or larger. A 0.16µF capacitor has a reactance of 1kΩ at
1kHz, giving a 1kHz lower –3dB frequency for gain settings
of 10V/V through 100V/V. If the LTC6911-1 is operated at
lower gain settings with an 0.16µF capacitor, the higher
input resistance will reduce the lower corner frequency
down to 100Hz at a gain setting of 1V/V. These frequencies
scale inversely with the value of the input capacitor used.
Note that operating the LTC6911 family in “zero” gain
mode (digital inputs 000) open circuits the INA and INB
pins and this demands some care if employed with a series
AC-coupled input capacitor. When the chip enters the zero
gain mode, the opened INA or INB pin tends to sample and
freeze the voltage across the capacitor to the value it held
just before the zero gain state. This can place the INA or
INB pin at or near the DC potential of a supply rail (the INA
or INB pin may also drift to a supply potential in this state
due to small junction leakage currents). To prevent driving
the INA or INB pin outside the supply limit and potentially
damaging the chip, avoid AC input signals in the zero gain
state with an AC-coupled capacitor. Also, switching later
to a nonzero gain value will cause a transient pulse at the
output of the LTC6911-1 (with a time constant set by the
capacitor value and the new LTC6911-1 input resistance
value). This occurs because the INA and INB pins return to
the AGND potential forcing transient current sourced by
the amplifier output to charge the AC-coupling capacitor to
its proper DC blocking value.
SNR and Dynamic Range
The term “dynamic range” is much used (and abused)
with signal paths. Signal-to-noise ratio (SNR) is an unam-
biguous comparison of signal and noise levels, measured
in the same way and under the same operating conditions.
In a variable gain amplifier, however, further characteriza-
tion is useful because both noise and maximum signal
level in the amplifier will vary with the gain setting, in
general. In the LTC6911-X, maximum output signal is
independent of gain (and is near the full power supply
voltage, as detailed in the Swing sections of the Electrical
Characteristics table). The maximum input level falls with
increasing gain, and the input-referred noise falls as well
(as also listed in the table). To summarize the useful signal
range in such an amplifier, we define Dynamic Range (DR)
as the ratio of maximum input (at unity gain) to minimum
input-referred noise (at maximum gain). This DR has a
physical interpretation as the range of signal levels that
will experience an SNR above unity V/V or 0dB. At a 10V
total power supply, DR in the LTC6911-X (gains 0V/V to
100V/V) is typically 120dB (the ratio of a nominal 9.9V
P-P
,
or 3.5V
RMS
(maximum input), to the 3.8µV
RMS
(high gain
input noise). The SNR of an amplifier is the ratio of input
level to input-referred noise, and can be 110dB with the
LTC6911 family at unity gain.
