Datasheet LT1251, LT1256 (Analog Devices) - 10

LT1251/LT1256
U U W U APPLICATIONS INFORMATION Supply Voltage
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 The LT1251/LT1256 are high speed amplifiers. To prevent than 6V, the protection circuits will cause the 1mV settling problems, use a ground plane with point-to-point wiring time to become hundreds of microseconds. and small bypass capacitors (0.01µF to 0.1µF) at each supply pin. For good settling characteristics, especially
Feedback Resistor Selection
driving heavy loads, a 4.7µF tantalum within an inch or two of each supply pin is recommended. The feedback resistor value determines the bandwidth of the LT1251/LT1256 as in other current feedback amplifi- The LT1251/LT1256 can be operated on single or split ers. The curves in the Typical Performance Characteristics supplies. The minimum total supply is 4V (Pins 7 to 9). show the effect of the feedback resistor on small-signal However, the input common mode range is only guaran- bandwidth for various loads, gains and supply voltages. teed to within 2V of each supply. On a 4V supply the parts The bandwidth is limited at high gains by the 500MHz to must be operated in the inverting mode with the noninvert- 800MHz gain-bandwidth product as shown in the curves. ing input biased half way between Pin 7 and Pin 9. See the Capacitance on the inverting input will cause peaking and Typical Applications section for the proper biasing for increase the bandwidth. Take care to minimize the stray single supply operation. capacitance on Pins 2 and 13 during printed circuit board The op amps in the control section operate from V– layout for flat response. (Pin 7) to within 2V of V + (Pin 9). For this reason the If the two input stages are not operating with equal gain, positive supply should be 4.5V or greater in order to use the gain versus control voltage characteristic will be 2.5V control and full-scale voltages. nonlinear. This is true even if RF1 equals RF2. This is because the open-loop characteristic of a current feed-
Inputs
back amplifier is dependent on the Thevenin impedance at The noninverting inputs (Pins 1 and 14) are easy to drive the inverting input. For linear control of the gain, the loop since they look like a 17M resistor in parallel with a 1.5pF gain of the two stages must be equal. For an extreme capacitor at most frequencies. However, the input stage example, let’s take a gain of 101 on input 1, RF1 = 1.5k and can oscillate at very high frequencies (100MHz to 200MHz) RG1 = 15Ω, and unity-gain on input 2, RF2 = 1.5k. The curve if the source impedance is inductive (like an unterminated in Figure 1 shows about 25% error at midscale. To cable). Several inches of wire look inductive at these high eliminate this nonlinearity we must change the value of frequencies and can cause oscillations. Check for oscilla- RF2. The correct value is the Thevenin impedance at tions at the inverting inputs (Pins 2 and 13) with a 10× inverting input 1 (including the internal resistance of 27Ω) probe and a 200MHz oscilloscope. A small capacitor times the gain set at input 1. For a linear gain versus (10pF to 50pF) from the input to ground or a small resistor control voltage characteristic when input 2 is operating at (100Ω to 300Ω) in series with the input will stop these unity-gain, the formula is: parasitic oscillations, even when the source is inductive. R These components must be within an inch of the IC in F2 = (AV1)(RF1RG1 + 27) order to be effective. RF2 = (101)(14.85 + 27) = 4227 All of the inputs to the LT1251/LT1256 have ESD protec- Because the feedback resistor of the unity-gain input is tion circuits. During normal operation these circuits have increased, the bandwidth will be lower and the output no effect. If the voltage between the noninverting and noise will be higher. We can improve this situation by inverting inputs exceeds 6V, the protection circuits will reducing the values of RF1 and RG1, but at high gains the trigger and attempt to short the inputs together. This internal 27Ω dominates. condition will continue until the voltage drops to less than 10