Datasheet LT1223 (Analog Devices) - 8

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