Datasheet LT1217 (Analog Devices) - 6

LT1217
O U U W U APPLICATI S I FOR ATIO
care to minimize the stray capacitance between the output same amount. The advantage of resistive isolation is that and the inverting input. Capacitance on the inverting input the bandwidth is only reduced when the capacitive load is to ground will cause peaking in the frequency response present. The disadvantage of resistor isolation is that (and overshoot in the transient response), but it does not resistive loading causes gain errors. Because the DC degrade the stability of the amplifier. The amount of accuracy is not degraded with resistive loading, the de- capacitance that is necessary to cause peaking is a func- sired way of driving capacitive loads, such as flash tion of the closed loop gain taken. converters, is to increase the feedback resistor. The Maxi- mum Capacitive Load versus Feedback Resistor curve The higher the gain, the more capacitance is required to shows the value of feedback resistor and capacitive load cause peaking. We can add capacitance from the inverting that gives 5dB of peaking. For less peaking, use a larger input to ground to increase the bandwidth in high gain feedback resistor. applications. For example, in this gain of 100 application, the bandwidth can be increased from 1MHz to 2MHz by
Power Supplies
adding a 2200pF capacitor. The LT1217 may be operated with single or split supplies as low as ±4.5V (9V total) to as high as ±18V (36V total). V + IN It is not necessary to use equal value split supplies, LT1217 VOUT however, the offset voltage will degrade about 350µV per – volt of mismatch. The internal compensation capacitor RF 3k decreases with increasing supply voltage. The –3dB Band- width versus Supply Voltage curves show how this affects R C G G 30Ω the bandwidth for various feedback resistors. Generally, LT1229 • TA03 the bandwidth at ±5V supplies is about half the value it is at ±15V supplies for a given feedback resistor.
Boosting Bandwidth of High Gain Amplifier with Capacitance on Inverting Input
The LT1217 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 45 44 settling time that 4.7µF tantalum capacitors be placed 43 within 0.5 inches of the supply pins. CG = 4700pF 42
Input Range
41 CG = 2200pF 40 The non-inverting input of the LT1217 looks like a 100MΩ GAIN (dB) 39 resistor in parallel with a 3pF capacitor until the common 38 CG = 0 mode range is exceeded. The input impedance drops 37 somewhat and the input current rises to about 10µA when 36 the input comes too close to the supplies. Eventually, 35 100k 1M 10M when the input exceeds the supply by one diode drop, the FREQUENCY (Hz) base collector junction of the input transistor forward LT1217 • TA04 biases and the input current rises dramatically. The input
Capacitive Loads
current should be limited to 10mA when exceeding the The LT1217 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. 6