LT1217
6
care to minimize the stray capacitance between the output
and the inverting input. Capacitance on the inverting input
to ground will cause peaking in the frequency response
(and overshoot in the transient response), but it does not
degrade the stability of the amplifier. The amount of
capacitance that is necessary to cause peaking is a func-
tion of the closed loop gain taken.
The higher the gain, the more capacitance is required to
cause peaking. We can add capacitance from the inverting
input to ground to increase the bandwidth in high gain
applications. For example, in this gain of 100 application,
the bandwidth can be increased from 1MHz to 2MHz by
adding a 2200pF capacitor.
LT1229 • TA03
–
+
C
G
R
G
30Ω
R
F
3k
V
OUT
LT1217
V
IN
Boosting Bandwidth of High Gain Amplifier with
Capacitance on Inverting Input
FREQUENCY (Hz)
100k
35
GAIN (dB)
37
38
40
42
43
45
1M 10M
LT1217 • TA04
36
39
41
44
C
G
= 0
C
G
= 2200pF
C
G
= 4700pF
Capacitive Loads
The LT1217 can be isolated from capacitive loads with a
small resistor (10Ω to 20Ω) or it can drive the capacitive
load directly if the feedback resistor is increased. Both
techniques lower the amplifier’s bandwidth about the
same amount. The advantage of resistive isolation is that
the bandwidth is only reduced when the capacitive load is
present. The disadvantage of resistor isolation is that
resistive loading causes gain errors. Because the DC
accuracy is not degraded with resistive loading, the de-
sired way of driving capacitive loads, such as flash
converters, is to increase the feedback resistor. The Maxi-
mum Capacitive Load versus Feedback Resistor curve
shows the value of feedback resistor and capacitive load
that gives 5dB of peaking. For less peaking, use a larger
feedback resistor.
Power Supplies
The LT1217 may be operated with single or split supplies
as low as ±4.5V (9V total) to as high as ±18V (36V total).
It is not necessary to use equal value split supplies,
however, the offset voltage will degrade about 350µV per
volt of mismatch. The internal compensation capacitor
decreases with increasing supply voltage. The –3dB Band-
width versus Supply Voltage curves show how this affects
the bandwidth for various feedback resistors. Generally,
the bandwidth at ±5V supplies is about half the value it is
at ±15V supplies for a given feedback resistor.
The LT1217 is very stable even with minimal supply
bypassing, however, the transient response will suffer if
the supply rings. It is recommended for good slew rate and
settling time that 4.7µF tantalum capacitors be placed
within 0.5 inches of the supply pins.
Input Range
The non-inverting input of the LT1217 looks like a 100MΩ
resistor in parallel with a 3pF capacitor until the common
mode range is exceeded. The input impedance drops
somewhat and the input current rises to about 10µA when
the input comes too close to the supplies. Eventually,
when the input exceeds the supply by one diode drop, the
base collector junction of the input transistor forward
biases and the input current rises dramatically. The input
current should be limited to 10mA when exceeding the
supplies. The amplifier will recover quickly when the input
is returned to its normal common mode range unless the
input was over 500mV beyond the supplies, then it will
take an extra 100ns.
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