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LT1228 Datasheet(PDF) 11 Page - Linear Technology |
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LT1228 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1228 1228fc S APPLICATI I FOR ATIO compliance compatible with Pin 5 of the LT1228. There- fore, the easiest way to digitally control the set current is to use a voltage output DAC and a voltage-to-current circuit. The previous voltage-to-current converter will take the output of any voltage output DAC and drive Pin 5 with a proportional current. The R, 2R CMOS multiplying DACs operating in the voltage switching mode work well on both single and split supplies with the above circuit. Logarithmic control is often easier to use than linear control. A simple circuit that doubles the set current for each additional volt of input is shown in the voltage controlled state variable filter application near the end of this data sheet. Transconductance Amplifier Frequency Response The bandwidth of the transconductance amplifier is a function of the set current as shown in the characteristic curves. At set currents below 100 µA, the bandwidth is approximately: –3dB bandwidth = 3 • 1011 ISET The peak bandwidth is about 80MHz at 500 µA. When a resistor is used to convert the output current to a voltage, the capacitance at the output forms a pole with the resistor. The best case output capacitance is about 5pF with ±15V supplies and 6pF with ±5V supplies. You must add any PC board or socket capacitance to these values to get the total output capacitance. When using a 1k resistor at the output of the transconductance amp, the output capacitance limits the bandwidth to about 25MHz. The output slew rate of the transconductance amplifier is the set current divided by the output capacitance, which is 6pF plus board and socket capacitance. For example with the set current at 1mA, the slew rate would be over 100V/ µs. Transconductance Amp Small-Signal Response ISET = 500µA, R1 = 50Ω CURRENT FEEDBACK AMPLIFIER The LT1228 current feedback amplifier has very high noninverting input impedance and is therefore an excel- lent buffer for the output of the transconductance ampli- fier. The noninverting input is at Pin 1, the inverting input at Pin 8 and the output at Pin 6. The current feedback amplifier maintains its wide bandwidth for almost all voltage gains making it easy to interface the output levels of the transconductance amplifier to other circuitry. The current feedback amplifier is designed to drive low imped- ance loads such as cables with excellent linearity at high frequencies. Feedback Resistor Selection The small-signal bandwidth of the LT1228 current feed- back amplifier is set by the external feedback resistors and the internal junction capacitors. As a result, the bandwidth is a function of the supply voltage, the value of the feedback resistor, the closed-loop gain and load resistor. The characteristic curves of bandwidth versus supply voltage are done with a heavy load (100 Ω) and a light load (1k) to show the effect of loading. These graphs also show |
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