7
LT1222
1222fc
APPLICATIONS INFORMATION
WUU
U
Output Clamping
Access to the internal compensation node at Pin 5 allows
the output swing of the LT1222 to be clamped. An example
is shown on the first page of this data sheet. The compen-
sation node is approximately one diode drop above the
output and can source or sink 1.2mA. Back-to-back Schot-
tky diodes clamp Pin 5 to a diode drop above ground so the
output is clamped to ±0.5V (the drop of the Schottkys at
1.2mA). The diode reference is bypassed for good AC
response. This circuit is useful for amplifying the voltage at
false sum nodes used in settling time measurements.
Capacitive Loading
The LT1222 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 LT1222 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 LT1222 has a typical gain-bandwidth product of
500MHz which allows it to have wide bandwidth in high
gain configurations (i.e., in a gain of 100, it will have a
bandwidth of about 5MHz). For added flexibility the ampli-
fier frequency response may be adjusted by adding capaci-
tance from Pin 5 to ground. The compensation capacitor
may be used to reduce overshoot, to allow the amplifier to
be used in lower noise gains, or simply to reduce band-
width. Table 1 shows gain and compensation capacitor
vresus –3dB bandwidth, maximum frequency peaking and
small-signal overshoot.
Table 1
A
V
C
C
(pF) f
–3dB
(MHz) Max Peaking (dB) Overshoot (%)
– 1 30 99 4.2 36
– 1 50 70 0.9 13
– 1 82 32 0 0
– 1 150 13 0 0
5 10 140 3.8 35
5 20 100 0 5
530 34 0 1
550 15 0 0
10 0 150 9.5 45
10 5 111 0.2 10
10 10 40 0 2
10 20 17 0 0
20 0 82 0.1 10
20 5 24 0 0
20 10 14 0 0
For frequencies < 10MHz the frequency response of the
amplifier is approximately:
f = 1/[2π • 53Ω • (C
C
+ 6pF) • (Noise Gain)]
The slew rate is affected as follows:
SR = 1.2mA /(C
C
+ 6pF)
An example would be a gain of –10 (noise gain of 11) and
C
C
= 20pF which has 10.5MHz bandwidth and 46V/μs slew
rate. It should be noted that the LT1222 is not stable in
A
V
= 1 unless C
C
= 50pF and a 1k resistor is used as the
feedback resistor. The 1k and input capacitance increase
the noise gain at frequency to aid stability.
