LT1226
6
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The LT1226 may be inserted directly into HA2541, HA2544,
AD847, EL2020 and LM6361 applications, provided that
the amplifier configuration is a noise gain of 25 or greater,
and the nulling circuitry is removed. The suggested nulling
circuit for the LT1226 is shown below.
Offset Nulling
Layout and Passive Components
As with any high speed operational amplifier, care must be
taken in board layout in order to obtain maximum perfor-
mance. Key layout issues include: use of a ground plane,
minimization of stray capacitance at the input pins, short
lead lengths, RF-quality bypass capacitors located close
to the device (typically 0.01µF to 0.1µF), and use of low
ESR bypass capacitors for high drive current applications
(typically 1µF to 10µF tantalum). Sockets should be
avoided when maximum frequency performance is
required, although low profile sockets can provide
reasonable performance up to 50MHz. For more details
see Design Note 50. Feedback resistors greater than 5kΩ
are not recommended because a pole is formed with the
input capacitance which can cause peaking. If feedback
resistors greater than 5kΩ are used, a parallel
capacitor of 5pF to 10pF should be used to cancel the input
pole and optimize dynamic performance.
Transient Response
The LT1226 gain bandwidth is 1GHz when measured at
1MHz. The actual frequency response in a gain of +25 is
considerably higher than 40MHz due to peaking caused by
a second pole beyond the gain of 25 crossover point. This
is reflected in the small signal transient response. Higher
noise gain configurations exhibit less overshoot as seen in
the inverting gain of 25 response.
Small Signal, A
V
= +25 Small Signal, A
V
= –25
The large signal response in both inverting and noninvert-
ing gain shows symmetrical slewing characteristics. Nor-
mally the noninverting response has a much faster rising
edge due to the rapid change in input common mode
voltage which affects the tail current of the input differen-
tial pair. Slew enhancement circuitry has been added to
the LT1226 so that the falling edge slew rate is enhanced
which balances the noninverting slew rate response.
Large Signal, A
V
= +25 Large Signal, A
V
= –25
Input Considerations
Resistors in series with the inputs are recommended for
the LT1226 in applications where the differential input
voltage exceeds ±6V continuously or on a transient basis.
An example would be in noninverting configurations with
high input slew rates or when driving heavy capacitive
loads. The use of balanced source resistance at each input
is recommended for applications where DC accuracy
must be maximized.
Capacitive Loading
The LT1226 is stable with all capacitive loads. This is
accomplished by sensing the load induced output pole
and adding compensation at the amplifier gain node. As
the capacitive load increases, both the bandwidth and
phase margin decrease so there will be peaking in the
LT1226 AI03
LT1226 AI02
–
+
3
2
1
8
5k
0.1µF
7
6
4
0.1µF
V
+
V
–
LT1226
LT1226 AI01
