7
LT1220
APPLICATIONS INFORMATION
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Input Considerations
Bias current cancellation circuitry is employed on the
inputs of the LT1220 so the input bias current and input
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 LT1220 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 LT1220 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.
DAC Current-to-Voltage Amplifier
The high gain, low offset voltage, low input bias current,
and fast settling of the LT1220 make it particularly useful
as an I/V converter for current output DACs. A typical
application is shown with a 565A type, 12-bit, 2mA full-
scale output current DAC. The 5k resistor around the
LT1220 is internal to the DAC and gives a 10V full-scale
output voltage. A 5pF capacitor in parallel with the feedback
resistor compensates for the DAC output capacitance and
improves settling. The output of the LT1220 settles to
1/2LSB (1.2mV) in less than 300ns. The accuracy of this
circuit is equal to:
V
ERROR
= V
OS
+ (I
OS
• 5kΩ) + (V
OUT
/A
VOL
)
At room temperature the worst-case error is 3mV (1.2LSB).
Typically the error is 1.2mV (1/2LSB). Over the commercial
temperature range the worse-case error is 6mV (2.5LSB).
6 OUT
LT1220 • SS
BIAS 2BIAS 1
18
NULL
–IN
+IN 3
V
–
7V
+
2
4
SI PLIFIED SCHE ATIC
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