LT1395 Linear Technology, LT1395 Datasheet - Page 8

• Manufacturer Part Number: LT1395
• Description: Single/Dual/Quad 400MHz Current Feedback Amplifier
• Manufacturer: Linear Technology

LT1395/LT1396/LT1397
Capacitance on the Inverting Input
Current feedback amplifiers require resistive feedback
from the output to the inverting input for stable operation.
Take care to minimize the stray capacitance between the
output and the inverting input. Capacitance on the invert-
ing input to ground will cause peaking in the frequency
response (and overshoot in the transient response).
Capacitive Loads
The LT1395/LT1396/LT1397 can drive many capacitive
loads directly when the proper value of feedback resistor
is used. The required value for the feedback resistor will
increase as load capacitance increases and as closed-loop
gain decreases. Alternatively, a small resistor (5 to 35 )
can be put in series with the output to isolate the capacitive
load from the amplifier output. This has the advantage that
the amplifier bandwidth is only reduced when the capaci-
tive load is present. The disadvantage is that the gain is a
function of the load resistance. See the Typical Perfor-
mance Characteristics curves.
Power Supplies
The LT1395/LT1396/LT1397 will operate from single or
split supplies from 2V (4V total) to 6V (12V total). It
is not necessary to use equal value split supplies, how-
ever the offset voltage and inverting input bias current
will change. The offset voltage changes about 2.5mV per
volt of supply mismatch. The inverting bias current will
typically change about 10 A per volt of supply mismatch.
Slew Rate
Unlike a traditional voltage feedback op amp, the slew rate
of a current feedback amplifier is not independent of the
amplifier gain configuration. In a current feedback ampli-
fier, both the input stage and the output stage have slew rate
limitations. In the inverting mode, and for gains of 2 or more
in the noninverting mode, the signal amplitude between the
input pins is small and the overall slew rate is that of the
output stage. For gains less than 2 in the noninverting mode,
the overall slew rate is limited by the input stage.
The input slew rate of the LT1395/LT1396/LT1397 is
approximately 600V/ s and is set by internal currents and
capacitances. The output slew rate is set by the value of
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the feedback resistor and internal capacitance. At a gain
of 2 with 255
supplies, the output slew rate is typically 800V/ s. Larger
feedback resistors will reduce the slew rate as will lower
supply voltages.
Differential Input Signal Swing
To avoid any breakdown condition on the input transis-
tors, the differential input swing must be limited to 5V. In
normal operation, the differential voltage between the
input pins is small, so the 5V limit is not an issue.
Buffered RGB to Color-Difference Matrix
An LT1397 can be used to create buffered color-differ-
ence signals from RGB inputs (Figure 1). In this applica-
tion, the R input arrives via 75 coax. It is routed to the
noninverting input of LT1397 amplifier A1 and to a 845
resistor R8. There is also an 82.5 termination resistor
R11, which yields a 75 input impedance at the R input
when considered in parallel with R8. R8 connects to the
inverting input of a second LT1397 amplifier (A2), which
also sums the weighted G and B inputs to create a
–0.5 • Y output. LT1397 amplifier A3 then takes the
–0.5 • Y output and amplifies it by a gain of –2, resulting
in the Y output. Amplifier A1 is configured in a noninvert-
ing gain of 2 with the bottom of the gain resistor R2 tied
to the Y output. The output of amplifier A1 thus results in
the color-difference output R-Y.
The B input is similar to the R input. It arrives via 75
coax, and is routed to the noninverting input of LT1397
amplifier A4, and to a 2320 resistor R10. There is also
a 76.8 termination resistor R13, which yields a 75
input impedance when considered in parallel with R10.
R10 also connects to the inverting input of amplifier A2,
adding the B contribution to the Y signal as discussed
above. Amplifier A4 is configured in a noninverting gain
of 2 configuration with the bottom of the gain resistor R4
tied to the Y output. The output of amplifier A4 thus
results in the color-difference output B-Y.
The G input also arrives via 75
contribution to the Y signal via a 432 resistor R9, which
is tied to the inverting input of amplifier A2. There is also
a 90.9 termination resistor R12, which yields a 75
feedback and gain resistors and 5V
coax and adds its