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LT1223MJ8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1223MJ8 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LT1223 8 1223fb APPLICATIO S I FOR ATIO The curve on the first page shows the LT1223 voltage gain versus frequency while driving 100 Ω,forfivegainsettings from 1 to 100. The feedback resistor is a constant 1k and the gain resistor is varied from infinity to 10 Ω. Shown for comparison is a plot of the fixed 100MHz gain bandwidth limitation that a voltage feedback amplifier would have. It is obvious that for gains greater than one, the LT1223 provides 3 to 20 times more bandwidth. It is also evident that second order effects reduce the bandwidth somewhat at the higher gain settings. Feedback Resistor Selection Because the feedback resistor determines the compensa- tion of the LT1223, bandwidth and transient response can be optimized for almost every application. To increase the bandwidth when using higher gains, the feedback resistor (and gain resistor) can be reduced from the nominal 1k value. The Minimum Feedback Resistor versus Voltage Gain curve shows the values to use for ±15V supplies. Larger feedback resistors can also be used to slow down the LT1223 as shown in the –3dB Bandwidth versus Feedback Resistor curve. Capacitive Loads The LT1223 can be isolated from capacitive loads with a small resistor (10 Ω to 20Ω) or it can drive the capacitive load directly if the feedback resistor is increased. Both techniques lower the amplifier’s bandwidth about the same amount. The advantage of resistive isolation is that the bandwidth is only reduced when the capacitive load is present. The disadvantage of resistor isolation is that resistive loading causes gain errors. Because the DC accuracy is not degraded with resistive loading, the de- sired way of driving capacitive loads, such as flash con- verters, is to increase the feedback resistor. The Maximum Capacitive Load versus Feedback Resistor curve shows the value of feedback resistor and capacitive load that gives 5dB of peaking. For less peaking, use a larger feedback resistor. Power Supplies The LT1223 may be operated with single or split supplies as low as ±4V (8V total) to as high as ±18V (36V total). It is not necessary to use equal value split supplies, how- ever, the offset voltage will degrade about 350 µV per volt of mismatch. The internal compensation capacitor de- creases with increasing supply voltage. The –3dB Band- width versus Supply Voltage curve shows how this affects the bandwidth for various feedback resistors. Generally, the bandwidth at ±5V supplies is about half the value it is at ±15V supplies for a given feedback resistor. The LT1223 is very stable even with minimal supply bypassing, however, the transient response will suffer if the supply rings. It is recommended for good slew rate and settling time that 4.7 µF tantalum capacitors be placed within 0.5 inches of the supply pins. Input Range The noninverting input of the LT1223 looks like a 10M resistor in parallel with a 3pF capacitor until the common mode range is exceeded. The input impedance drops somewhat and the input current rises to about 10 µA when the input comes too close to the supplies. Eventually, when the input exceeds the supply by one diode drop, the base collector junction of the input transistor forward biases and the input current rises dramatically. The input current should be limited to 10mA when exceeding the supplies. The amplifier will recover quickly when the input is returned to its normal common mode range unless the input was over 500mV beyond the supplies, then it will take an extra 100ns. Offset Adjust Output offset voltage is equal to the input offset voltage times the gain plus the inverting input bias current times the feedback resistor. For low gain applications (3 or less) a 10k Ω pot connected to Pins 1 and 5 with wiper to V+ will trim the inverting input current ( ±10µA) to null the output; it does not change the offset voltage very much. If the LT1223 is used in a high gain application, where input offset voltage is the dominate error, it can be nulled by pulling approximately 100 µA from Pin 1 or 5. The easy way to do this is to use a 10k ΩpotbetweenPin1and5with a 150k resistor from the wiper to ground for 15V supply applications. Use a 47k resistor when operating on a 5V supply. |
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