LT1256CS datasheet(10/24 Pages) LINER

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LT1251/LT1256

APPLICATIONS INFORMATION

Supply Voltage

The LT1251/LT1256 are high speed amplifiers. To prevent

problems, use a ground plane with point-to-point wiring

and small bypass capacitors (0.01

µF to 0.1µF) at each

supply pin. For good settling characteristics, especially

driving heavy loads, a 4.7

µF tantalum within an inch or two

of each supply pin is recommended.

The LT1251/LT1256 can be operated on single or split

supplies. The minimum total supply is 4V (Pins 7 to 9).

However, the input common mode range is only guaran-

teed to within 2V of each supply. On a 4V supply the parts

must be operated in the inverting mode with the noninvert-

ing input biased half way between Pin 7 and Pin 9. See the

Typical Applications section for the proper biasing for

single supply operation.

The op amps in the control section operate from V–

(Pin 7) to within 2V of V + (Pin 9). For this reason the

positive supply should be 4.5V or greater in order to use

2.5V control and full-scale voltages.

Inputs

The noninverting inputs (Pins 1 and 14) are easy to drive

since they look like a 17M resistor in parallel with a 1.5pF

capacitor at most frequencies. However, the input stage

can oscillate at very high frequencies (100MHz to 200MHz)

if the source impedance is inductive (like an unterminated

cable). Several inches of wire look inductive at these high

frequencies and can cause oscillations. Check for oscilla-

tions at the inverting inputs (Pins 2 and 13) with a 10

probe and a 200MHz oscilloscope. A small capacitor

(10pF to 50pF) from the input to ground or a small resistor

(100

Ω to 300Ω) in series with the input will stop these

parasitic oscillations, even when the source is inductive.

These components must be within an inch of the IC in

order to be effective.

All of the inputs to the LT1251/LT1256 have ESD protec-

tion circuits. During normal operation these circuits have

no effect. If the voltage between the noninverting and

inverting inputs exceeds 6V, the protection circuits will

trigger and attempt to short the inputs together. This

condition will continue until the voltage drops to less than

500mV or the current to less than 10mA. If a very fast edge

is used to measure settling time with an input step of more

than 6V, the protection circuits will cause the 1mV settling

time to become hundreds of microseconds.

Feedback Resistor Selection

The feedback resistor value determines the bandwidth of

the LT1251/LT1256 as in other current feedback amplifi-

ers. The curves in the Typical Performance Characteristics

show the effect of the feedback resistor on small-signal

bandwidth for various loads, gains and supply voltages.

The bandwidth is limited at high gains by the 500MHz to

800MHz gain-bandwidth product as shown in the curves.

Capacitance on the inverting input will cause peaking and

increase the bandwidth. Take care to minimize the stray

capacitance on Pins 2 and 13 during printed circuit board

layout for flat response.

If the two input stages are not operating with equal gain,

the gain versus control voltage characteristic will be

nonlinear. This is true even if RF1 equals RF2. This is

because the open-loop characteristic of a current feed-

back amplifier is dependent on the Thevenin impedance at

the inverting input. For linear control of the gain, the loop

gain of the two stages must be equal. For an extreme

example, let’s take a gain of 101 on input 1, RF1 = 1.5k and

RG1 = 15Ω, and unity-gain on input 2, RF2 = 1.5k. The curve

in Figure 1 shows about 25% error at midscale. To

eliminate this nonlinearity we must change the value of

RF2. The correct value is the Thevenin impedance at

inverting input 1 (including the internal resistance of 27

Ω)

times the gain set at input 1. For a linear gain versus

control voltage characteristic when input 2 is operating at

unity-gain, the formula is:

RF2 = (AV1)(RF1RG1 + 27)

RF2 = (101)(14.85 + 27) = 4227

Because the feedback resistor of the unity-gain input is

increased, the bandwidth will be lower and the output

noise will be higher. We can improve this situation by

reducing the values of RF1 and RG1, but at high gains the

internal 27

Ω dominates.

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