LT1228_12 Datasheet, PDF (11/22 Pages) Linear Technology – 100MHz Current Feedback

LT1228
APPLICATIONS INFORMATION
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 Amp Small-Signal Response
I_SET = 500µA, R1 = 50Ω
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 100µA, the bandwidth is
approximately:
–3dB bandwidth = 3 • 10¹¹ I_SET
The peak bandwidth is about 80MHz at 500µA. When a
resistor is used to convert the output current to a volt-
age, 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.
CURRENT FEEDBACK AMPLIFIER
The LT1228 current feedback amplifier has very high
noninverting input impedance and is therefore an excellent
buffer for the output of the transconductance amplifier.
The noninverting input is at Pin 1, the inverting input at
Pin 8 and the output at Pin 6. The current feedback ampli-
fier 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 cur-
rent 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 feedback
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 char-
acteristic curves of bandwidth versus supply voltage are
done with a heavy load (100Ω) and a light load (1k) to
1228fd
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