LT1468 Linear Technology, LT1468 Datasheet - Page 10

• Manufacturer Part Number: LT1468
• Description: 90MHz/ 22V/us 16-Bit Accurate Operational Amplifier
• Manufacturer: Linear Technology

APPLICATIONS
LT1468
current can be greater than either input current, the use of
balanced source resistance is NOT recommended as it
actually degrades DC accuracy and also increases noise.
The input bias currents vary with common mode voltage
as shown in the Typical Performance Characteristics. The
cancellation circuitry was not designed to track this com-
mon mode voltage because the settling time would have
been adversely affected.
The LT1468 inputs can be driven to the negative supply
and to within 0.5V of the positive supply without phase
reversal. As the input moves closer than 0.5V to the
positive supply, the output reverses phase.
Total Input Noise
The curve of Total Noise vs Unmatched Source Resistance
in the Typical Performance Characteristics shows that with
source resistance below 1k, the voltage noise of the am-
plifier dominates. In the 1k to 20k region the increase in
noise is due to the source resistance. Above 20k the input
current noise component is larger than the resistor noise.
Capacitive Loading
The LT1468 drives capacitive loads of up to 100pF in unity
gain and 300pF in a gain of –1. When there is a need to
drive a larger capacitive load, a small series resistor should
be inserted between the output and the load. In addition,
a capacitor should be added between the output and the
inverting input as shown in Driving Capacitive Loads.
Settling Time
The LT1468 is a single stage amplifier with an optimal
thermal layout that leads to outstanding settling
performance. Measuring settling, even at the 12-bit level
is very challenging, and at the 16-bit level requires a great
deal of subtlety and expertise. Fortunately, there are two
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R1
Input Stage Protection
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INFORMATION
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excellent Linear Technology reference sources for settling
measurements, Application Notes 47 and 74. Appendix B
of AN47 is a vital primer on 12-bit settling measurements,
and AN74 extends the state of the art while concentrating
on settling time with a 16-bit current output DAC input.
The 150 V settling curve in the Typical Performance
Characteristics is measured using the Differential Ampli-
fier method of AN74 followed by a clamped, nonsaturating
gain of 100. The total gain of 500 allows a resolution of
100 V/DIV with an oscilloscope setting of 0.05V/DIV
The settling of the DAC I-to-V converter on the front page
was measured using the exact methods of AN74. The
optimum nulling of the DAC output capacitance requires
20pF across the 6k feedback resistor. The theoretical limit
for 16-bit settling is 11.1 times this RC time constant or
1.33 s. The actual settling time is 1.7 s at the output of
the LT1468. The LT1468 is the fastest Linear Technology
amplifier in this application.
The optional noise filter adds a slight delay of 100ns, but
reduces the noise bandwidth to 1.6MHz which increases
the output resolution for 16-bit accuracy.
Distortion
The LT1468 has outstanding distortion performance as
shown in the Typical Performance curves of Total Harmonic
Distortion + Noise vs Frequency and Amplitude. The high
open-loop gain and inherently balanced architecture reduce
errors to yield 16-bit accuracy to frequencies as high as
100kHz. An example of this performance is the Typical
Application titled 100kHz Low Distortion Bandpass Filter.
This circuit is useful for cleaning up the output of a high
performance signal generator such as the B & K type 1051
or HP3326A.
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LT1468
Driving Capacitive Loads
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1468 AI04
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C
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= (2R
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10R
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)/(2 C
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5MHz)