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LT1256CN Datasheet(PDF) 11 Page - Linear Technology |
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LT1256CN Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  11 LT1251/LT1256 APPLICATIONS INFORMATION Capacitive Loads Increasing the value of the feedback resistor reduces the bandwidth and open-loop gain of the LT1251/LT1256; therefore, the pole introduced by a capacitive load can be overcome. If there is little or no resistive load in parallel with the load capacitance, the output stage will resonate, peak and possibly oscillate. With a resistive load of 150 Ω, any capacitive load can be accommodated by increasing the feedback resistor. If the capacitive load cannot be paralleled with a DC load of 150 Ω, a network of 200pF in series with 100 Ω should be placed from the output to ground. Then the feedback resistor should be selected for best response. The Null Pin Pin 6 can be used to adjust the gain of an internal current mirror to change the output offset. The open circuit voltage at Pin 6 is set by the full scale current IFS flowing through 200 Ω to the negative supply. Therefore, the NULL pin sits 100mV above the negative supply with VFS equal to 2.5V. Any op amp whose output swings within a few millivolts of the negative supply can drive the NULL pin. The AM modulator application shows an LT1077 driving the NULL pin to eliminate the output DC offset voltage. Crosstalk The amount of signal from the off input that appears at the output is a function of frequency and the circuit topology. The nature of a current feedback input stage is to force the voltage at the inverting input to be equal to the voltage at the noninverting input. This is independent of feedback and forced by a buffer amplifier between the inputs. When the LT1251/LT1256 are operating noninverting, the off input signal is present at the inverting input. Since one end of the feedback resistor is connected to this input, the off signal is only a feedback resistor away from the output. The amount of unwanted signal at the output is deter- mined by the size of the feedback resistor and the output impedance of the LT1251/LT1256. The output impedance rises with increasing frequency resulting in more crosstalk at higher frequencies. Additionally, the current that flows in the inverting input is diverted to the supplies within the chip and some of this signal will also show up at the output. With a 1.5k feedback resistor, the crosstalk is down about 86dB at low frequencies and rises to – 78dB at 1MHz and on to – 60dB at 6MHz. The curves show the details. Distortion When only one input is contributing to the output (VC = 0% or 100%) the LT1251/LT1256 have very low distortion. As the control reduces the output, the distortion will increase. The amount of increase is a function of the current that flows in the inverting input. Larger input signals generate more distortion. Using a larger feedback resistor will reduce the distortion at the expense of higher output noise. CONTROL VOLTAGE (V) 0 100 50 0 2.0 1251/56 F01 0.5 1.0 1.5 2.5 RF2 = 4.3k RF2 = 1.5k VFS = 2.5V Figure 1. Linear Gain Control from 0 to 101 |
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