LT1221(Rev_A) 查看數據表（PDF） - Linear Technology
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LT1221 Datasheet PDF : 8 Pages
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LT1221
APPLICATIONS INFORMATION
offset current have identical specifications. For this reason,
matching the impedance on the inputs to reduce bias
current errors is not necessary.
Capacitive Loading
The LT1221 is stable with capacitive loads. This is accom-
plished 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. There will be peaking in the frequency domain as
shown in the curve of Frequency Response vs Capacitive
Load. The small-signal transient response will have more
overshoot as shown in the photo of the small-signal
response with 1000pF load. The large-signal response with
a 10,000pF load shows the output slew rate being limited
to 4V/µs by the short-circuit current. The LT1221 can drive
coaxial cable directly, but for best pulse fidelity a resistor of
value equal to the characteristic impedance of the cable
(i.e., 75Ω) should be placed in series with the output. The
other end of the cable should be terminated with the same
value resistor to ground.
Compensation
The LT1221 has a typical gain-bandwidth product of
150MHz which allows it to have wide bandwidth in high
gain configurations (i.e., in a gain of 10, it will have a
bandwidth of about 15MHz). The amplifier is stable in a
noise gain of 4 so the ratio of the signal at the inverting input
to the output must be 1/4 or less. Straightforward gain
configurations of 4 or –3 are stable, but there are several
others that allow the amplifier to be stable for lower signal
gains (the noise gain, however, remains 4 or more). One
example is the summing amplifier on the first page of this
data sheet. Each input signal has a gain of –1 to the output,
but it is easily seen that this configuration is equivalent to
a gain of –3 as far as the amplifier is concerned. Another
circuit is shown below with a DC gain of 1, but an AC gain
of 5. The break frequency of the R-C combination across
the amplifier inputs should be approximately a factor of 10
less than the gain-bandwidth of the amplifier divided by the
high frequency gain (in this case 1/10 of 150MHz/5 or
3MHz).
TYPICAL APPLICATIONS N
20MHz, AV = 50 Instrumentation Amplifier
+
+
V–IN
10k
LT1221
–
1k
250Ω
1k
+
200pF 250Ω
1k
LT1221
1k
–
10k
VOUT
–
LT1221
+
LT1221 • TA03
Lag Compensation
VIN
+
500Ω
LT1221
–
100pF
2k
AV = 1, f < 3MHz
VOUT
LT1221 • TA04
Cable Driver
VIN
+
LT1221
–
1.5k
75Ω 75Ω CABLE
VOUT
75Ω
510Ω
LT1221 • TA05
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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