LTC6409
16
6409fa
applicaTions inForMaTion
between the R
L
• 4 = 200Ω differential resistance seen at
location B and the 200Ω formed by the two 100Ω match-
ing resistors at the LTC6409 output. Thus, the differential
power at location B is 10 – 6 = 4dBm. Since the transformer
ratio is 4:1 and it has an insertion loss of about 1dB, the
power at location C (across R
L
) is calculated to be 4 – 6
– 1 = –3dBm. This means that IMD3 should be measured
while the power at the output of the demo board is –3dBm
which is equivalent to having 2V
P-P
differential peak (or
10dBm) at the output of the LTC6409.
GBW vs f
–3dB
Gain-bandwidth product (GBW) and –3dB frequency (f
–3dB
)
have been both specified in the Electrical Characteristics
table as two different metrics for the speed of the LTC6409.
GBW is obtained by measuring the gain of the amplifier
at a specific frequency (f
TEST
) and calculate gain • f
TEST
.
To measure gain, the feedback factor (i.e. b = R
I
/(R
I
+
R
F
)) is chosen sufficiently small so that the feedback loop
does not limit the available gain of the LTC6409 at f
TEST
,
ensuring that the measured gain is the open loop gain of
the amplifier. As long as this condition is met, 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 practi-
cal interest in different applications and is by definition
the frequency at which the gain is 3dB lower than its low
frequency value. The value of f
–3dB
depends on the speed
of the amplifier as well as the feedback factor. Since the
LTC6409 is designed to be stable in a differential signal
gain of 1 (where R
I
= R
F
or b = 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 frequen-
cies before crossover frequency and the GBW and f
–3dB
numbers are close to each other. However, the LTC6409 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 input signal frequencies typically lie,
e.g. 100MHz) the amplifier’s gain and the thus the feedback
loop gain is larger. This has the important advantage of
further linearizing the amplifier and improving distortion
at those frequencies.
Looking at the Frequency Response vs Closed Loop Gain
graph in the Typical Performance Characteristics section
of this data sheet, one sees that for a closed loop gain
(A
V
) of 1 (where R
I
= R
F
= 150Ω), f
–3dB
is about 2GHz.
However, for A
V
= 400 (where R
I
= 25Ω and R
F
= 10kΩ),
the gain at 100MHz is close to 40dB = 100V/V, implying
a GBW value of 10GHz.
Feedback Capacitors
When the LTC6409 is configured in low differential gains,
it is often advantageous to utilize a feedback capacitor (C
F
)
in parallel with each feedback resistor (R
F
). The use of C
F
implements a pole-zero pair (in which the zero frequency
is usually smaller than the pole frequency) and adds posi-
tive phase to the feedback loop gain around the amplifier.
Therefore, if properly chosen, the addition of C
F
boosts
the phase margin and improves the stability response of
the feedback loop. For example, with R
I
= R
F
= 150Ω, it is
recommended for most general applications to use C
F
=
1.3pF across each R
F
. This value has been selected to
maximize f
–3dB
for the LTC6409 while keeping the peaking
of the closed loop gain versus frequency response under
a reasonable level (<1dB). It also results in the highest
frequency for 0.1dB gain flatness (f
0.1dB
).
However, other values of C
F
can also be utilized and tailored
to other specific applications. In general, a larger value
for C
F
reduces the peaking (overshoot) of the amplifier in
both frequency and time domains, but also decreases the
closed loop bandwidth (f
–3dB
). For example, while for a
closed loop gain (A
V
) of 5, C
F
= 0.8pF results in maximum
f
–3dB
(as previously shown in the Frequency Response vs
Closed Loop Gain graph of this data sheet), if C
F
= 1.2pF
is used, the amplifier exhibits no overshoot in the time
domain which is desirable in certain applications. Both the
circuits discussed in this section have been shown in the
Typical Applications section of this data sheet.
