Datasheet LT1217 (Analog Devices) - 5

LT1217
W U TYPICAL PERFOR A CE CHAR C A TERISTICS Settling Time to 10mV vs Settling Time to 1mV vs Output Step Output Step Supply Current vs Supply Voltage
10 10 1.4 8 V 8 VS = ±15V T = 125°C S = ±15V R R F = RG = 3k 1.2 F = RG = 3k 6 6 INVERTING 4 4 1.0 NON-INVERTING T = 25°C INVERTING 2 2 NON-INVERTING 0.8 0 0 T = –55°C NON-INVERTING 0.6 –2 –2 T = –55°C OUTPUT STEP (V) OUTPUT STEP (V) NON-INVERTING –4 –4 0.4 T = 25°C, 125°C SUPPLY CURRENT (mA) –6 –6 0.2 SHUTDOWN –8 INVERTING –8 INVERTING PIN 8 AT GND –10 –10 0.0 0 50 100 150 200 250 300 0 100 200 300 400 500 0 2 4 6 8 10 12 14 16 18 SETTLING TIME (ns) SETTLING TIME (ns) SUPPLY VOLTAGE (±V) LT1217 • TPC19 LT1217 • TPC20 LT1217 • TPC21
O U U W U APPLICATI S I FOR ATIO Current Feedback Basics Feedback Resistor Selection
The small signal bandwidth of the LT1217, like all current The small signal bandwidth of the LT1217 is set by the feedback amplifiers, isn’t a straight inverse function of the external feedback resistors and the internal junction ca- closed loop gain. This is because the feedback resistors pacitors. As a result, the bandwidth is a function of the determine the amount of current driving the amplifier’s supply voltage, the value of the feedback resistor, the internal compensation capacitor. In fact, the amplifier’s closed loop gain and load resistor. The characteristic feedback resistor (RF) from output to inverting input curves of bandwidth versus supply voltage are done with works with internal junction capacitances of the LT1217 to a heavy load (100Ω) and a light load (1kΩ) to show the set the closed loop bandwidth. effect of loading. These graphs also show the family of curves that result from various values of the feedback Even though the gain set resistor (RG) from inverting input resistor. These curves use a solid line when the response to ground works with RF to set the voltage gain just like it has less than 0.5dB of peaking and a dashed line when the does in a voltage feedback op amp, the closed loop response has 0.5dB to 5dB of peaking. The curves stop bandwidth does not change. This is because the equivalent where the response has more than 5dB of peaking. gain bandwidth product of the current feedback amplifier is set by the Thevenin equivalent resistance at the inverting At a gain of two, on ±15V supplies with a 3kΩ feedback input and the internal compensation capacitor. By keeping resistor, the bandwidth into a light load is 13.5MHz with a RF constant and changing the gain with RG, the Thevenin little peaking, but into a heavy load the bandwidth is resistance changes by the same amount as the change in 10MHz with no peaking. At very high closed loop gains, the gain. As a result, the net closed loop bandwidth of the bandwidth is limited by the gain bandwidth product of LT1217 remains the same for various closed loop gains. about 100MHz. The curves show that the bandwidth at a closed loop gain of 100 is about 1MHz. The curve on the first page shows the LT1217 voltage gain versus frequency while driving 100Ω, for five gain settings
Capacitance on the Inverting Input
from 1 to 100. The feedback resistor is a constant 3k and the gain resistor is varied from infinity to 30Ω. Second Current feedback amplifiers want resistive feedback from order effects reduce the bandwidth somewhat at the the output to the inverting input for stable operation. Take higher gain settings. 5