Datasheet

LTC6362
13
6362fa
The LTC6362’s input referred voltage noise contributes the
equivalent noise of a 920Ω resistor. When the feedback
network is comprised of resistors whose values are larger
than this, the output noise is resistor noise and amplifier
current noise dominant. For feedback networks consisting
of resistors with values smaller than 920Ω, the output
noise is voltage noise dominant.
Lower resistor values always result in lower noise at the
penalty of increased distortion due to increased loading of
the feedback network on the output. Higher resistor values
will result in higher output noise, but typically improved
distortion due to less loading on the output. For this rea-
son, when LTC6362 is configured in a differential gain of
1, using feedback resistors of at least 1k is recommended.
GBW vs f
–3dB
Gain-bandwidth product (GBW) and –3dB frequency
(f
–3dB
) have been specified in the Electrical Characteristics
table as two different metrics for the speed of the LTC6362.
GBW is obtained by measuring the open-loop gain of the
amplifier at a specific frequency (f
TEST
), then calculating
gain • f
TEST
. GBW is a parameter that depends only on the
internal design and compensation of the amplifier and is
a suitable metric to specify the inherent speed capability
of the amplifier.
f
–3dB
, on the other hand, is a parameter of more practical
interest in different applications and is by definition the
frequency at which the closed-loop gain is 3dB lower than
its low frequency value. The value of f
–3dB
depends on the
APPLICATIONS INFORMATION
Figure 4. Simplified Noise Model
–
+
e
no
2
R
F
e
nRI
2
R
F
R
I
R
I
e
nRF
2
e
nRI
2
e
ni
2
e
nRF
2
i
n+
2
i
n–
2
6362 F04
speed of the amplifier as well as the feedback factor. Since
the LTC6362 is designed to be stable in a differential signal
gain of 1 (where R
I
= R
F
or β = 1/2), the maximum f
–3dB
is obtained and measured in this gain setting, as reported
in the Electrical Characteristics table.
In most amplifiers, the open-loop gain response exhibits a
conventional single-pole roll-off for most of the frequencies
before the unity-gain crossover frequency, and the GBW and
unity-gain frequency are close to each other. However, the
LTC6362 is intentionally compensated in such a way that
its GBW is significantly larger than its f
–3dB
. This means
that at lower frequencies where the amplifier inputs gen-
erally operate, the amplifier’s gain and thus the feedback
loop gain is larger. This has the important advantage of
further linearizing the amplifier and improving distortion
at those frequencies.
Feedback Capacitors
In cases where the LTC6362 is connected such that the
combination of parasitic capacitances (device + PCB) at the
inverting input forms a pole whose frequency lies within
the closed-loop bandwidth of the amplifier, a capacitor
(C
F
) can be added in parallel with the feedback resistor
(R
F
) to cancel the degradation on stability. C
F
should be
chosen such that it generates a zero at a frequency close
to the frequency of the pole.
In general, a larger value for C
F
reduces the peaking (over-
shoot) of the amplifier in both frequency and time domains,
but also decreases the closed-loop bandwidth (f
–3dB
).
Board Layout and Bypass Capacitors
For single supply applications, it is recommended that
high quality 0.1µF ceramic bypass capacitors be placed
directly between the V
+
and the V
–
pin with short con-
nections. The V
–
pins (including the exposed pad in the
DD8 package) should be tied directly to a low impedance
ground plane with minimal routing. For dual (split) power
supplies, it is recommended that additional high quality
0.1µF ceramic capacitors be used to bypass V
+
to ground
and V
–
to ground, again with minimal routing. Small
geometry (e.g., 0603) surface mount ceramic capacitors
have a much higher self-resonant frequency than leaded
capacitors, and perform best with LTC6362.