LTC6910-1
LTC6910-2/LTC6910-3
19
6910123fa
Analog Input and DC Levels
As described in Tables 1, 2 and 3 and under Pin Functions,
the IN pin presents a variable input resistance returned
internally to a potential equal to that at the AGND pin
(within a small offset-voltage error). This input resistance
varies with digital gain setting, becoming infinite (open
circuit) at “zero” gain (digital input 000), and as low as 1kΩ
at high gain settings. It is important to allow for this
input-resistance variation with gain, when driving the
LTC6910-X from other circuitry. Also, as the gain in-
creases above unity, the DC linear input-voltage range
(corresponding to rail-to-rail swing at the OUT pin) shrinks
toward the AGND potential. The output swings positive or
negative around the AGND potential (in the opposite
direction from the input, because the gain is inverting).
AC-Coupled Operation
Adding a capacitor in series with the IN pin makes the
LTC6910-X into an AC-coupled amplifier, suppressing the
source’s DC level (and even minimizing the offset voltage
from the LTC6910-X itself). No further components are
required because the input of the LTC6910-X biases itself
correctly when a series capacitor is added. The IN pin
connects to an internal variable resistor (and floats when
DC open-circuited to a well defined voltage equal to the
AGND input voltage at nonzero gain settings). The value of
this internal input resistor varies with gain setting over a
total range of about 1k to 10k, depending on version (the
rightmost columns of Table 1, Table 2 and Table 3).
Therefore, with a series input capacitor the low frequency
cutoff will also vary with gain. For example, for a low
frequency corner of 1kHz or lower, 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 in the LTC6910-1. If the
LTC6910-1 is operated at lower gain settings with an
0.16µF input 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.
Note that operating the LTC6910-X in zero gain mode
(digital inputs 000) open circuits the IN pin and this
demands some care if employed with a series input
capacitor. When the chip enters the zero gain mode, the
opened IN pin tends to freeze the voltage across the
capacitor to the value it held just before the zero gain state.
This can place the IN pin at or near the DC potential of a
supply rail (the IN pin may also drift to a supply potential
in this state due to small junction leakage currents). To
prevent driving the IN pin outside the supply limit and
potentially damaging the chip, avoid AC input signals in
the zero gain state with a series capacitor. Also, switching
later to a nonzero gain value will cause a transient pulse at
the output of the LTC6910-X (with a time constant set by
the capacitor value and the new LTC6910-X input resis-
tance value). This occurs because the IN pin returns to the
AGND potential and transient current flows to charge the
capacitor to a new DC drop.
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 LTC6910-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 listed also 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). (These two num-
bers are measured commensurately, in RMS Volts.
For deterministic signals such as sinusoids, 1V
RMS
=
2.828V
P-P
.) 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
LTC6910-1 (gains 0V to 100V/V) is typically 120dB (the
ratio of a nominal 9.9V
P-P
, or 3.5V
RMS
, maximum input to
the 3.4µV
RMS
high gain input noise). The corresponding
DR for the LTC6910-2 (gains 0V to 64V) is also 120dB; for
APPLICATIO S I FOR ATIO
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