Datasheet LT1222 (Analog Devices) - 7
| Fabricante | Analog Devices |
| Descripción | 500MHz, 3nV/√Hz, AV ≥ 10 Operational Amplifier |
| Páginas / Página | 12 / 7 — APPLICATIONS INFORMATION. Output Clamping. Table 1. CC (pF). f –3dB … |
| Formato / tamaño de archivo | PDF / 262 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION. Output Clamping. Table 1. CC (pF). f –3dB (MHz). Max Peaking (dB). Overshoot (%). Capacitive Loading
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LT1222
U U W U APPLICATIONS INFORMATION Output Clamping
may be used to reduce overshoot, to allow the amplifier to be used in lower noise gains, or simply to reduce band- Access to the internal compensation node at Pin 5 allows width. Table 1 shows gain and compensation capacitor the output swing of the LT1222 to be clamped. An example vresus – 3dB bandwidth, maximum frequency peaking and is shown on the first page of this data sheet. The compen- small-signal overshoot. sation node is approximately one diode drop above the output and can source or sink 1.2mA. Back-to-back Schot-
Table 1
tky diodes clamp Pin 5 to a diode drop above ground so the
AV CC (pF) f –3dB (MHz) Max Peaking (dB) Overshoot (%)
output is clamped to ±0.5V (the drop of the Schottkys at – 1 30 99 4.2 36 1.2mA). The diode reference is bypassed for good AC – 1 50 70 0.9 13 response. This circuit is useful for amplifying the voltage at – 1 82 32 0 0 false sum nodes used in settling time measurements. – 1 150 13 0 0 5 10 140 3.8 35
Capacitive Loading
5 20 100 0 5 The LT1222 is stable with capacitive loads. This is accom- 5 30 34 0 1 plished by sensing the load induced output pole and adding 5 50 15 0 0 compensation at the amplifier gain node. As the capacitive 10 0 150 9.5 45 load increases, both the bandwidth and phase margin 10 5 111 0.2 10 decrease. There will be peaking in the frequency domain as 10 10 40 0 2 shown in the curve of Frequency Response vs Capacitive 10 20 17 0 0 Load. The small-signal transient response will have more 20 0 82 0.1 10 overshoot as shown in the photo of the small-signal 20 5 24 0 0 response with 1000pF load. The large-signal response with 20 10 14 0 0 a 10,000pF load shows the output slew rate being limited to 4V/μs by the short-circuit current. The LT1222 can drive For frequencies < 10MHz the frequency response of the coaxial cable directly, but for best pulse fidelity a resistor of amplifier is approximately: value equal to the characteristic impedance of the cable (i.e., 75Ω) should be placed in series with the output. The f = 1/[2π • 53Ω • (CC + 6pF) • (Noise Gain)] other end of the cable should be terminated with the same The slew rate is affected as follows: value resistor to ground. SR = 1.2mA /(CC + 6pF)
Compensation
An example would be a gain of –10 (noise gain of 11) and The LT1222 has a typical gain-bandwidth product of CC = 20pF which has 10.5MHz bandwidth and 46V/μs slew 500MHz which allows it to have wide bandwidth in high rate. It should be noted that the LT1222 is not stable in gain configurations (i.e., in a gain of 100, it will have a AV = 1 unless CC = 50pF and a 1k resistor is used as the bandwidth of about 5MHz). For added flexibility the ampli- feedback resistor. The 1k and input capacitance increase fier frequency response may be adjusted by adding capaci- the noise gain at frequency to aid stability. tance from Pin 5 to ground. The compensation capacitor 1222fc 7
U U W U APPLICATIONS INFORMATION Output Clamping
may be used to reduce overshoot, to allow the amplifier to be used in lower noise gains, or simply to reduce band- Access to the internal compensation node at Pin 5 allows width. Table 1 shows gain and compensation capacitor the output swing of the LT1222 to be clamped. An example vresus – 3dB bandwidth, maximum frequency peaking and is shown on the first page of this data sheet. The compen- small-signal overshoot. sation node is approximately one diode drop above the output and can source or sink 1.2mA. Back-to-back Schot-
Table 1
tky diodes clamp Pin 5 to a diode drop above ground so the
AV CC (pF) f –3dB (MHz) Max Peaking (dB) Overshoot (%)
output is clamped to ±0.5V (the drop of the Schottkys at – 1 30 99 4.2 36 1.2mA). The diode reference is bypassed for good AC – 1 50 70 0.9 13 response. This circuit is useful for amplifying the voltage at – 1 82 32 0 0 false sum nodes used in settling time measurements. – 1 150 13 0 0 5 10 140 3.8 35
Capacitive Loading
5 20 100 0 5 The LT1222 is stable with capacitive loads. This is accom- 5 30 34 0 1 plished by sensing the load induced output pole and adding 5 50 15 0 0 compensation at the amplifier gain node. As the capacitive 10 0 150 9.5 45 load increases, both the bandwidth and phase margin 10 5 111 0.2 10 decrease. There will be peaking in the frequency domain as 10 10 40 0 2 shown in the curve of Frequency Response vs Capacitive 10 20 17 0 0 Load. The small-signal transient response will have more 20 0 82 0.1 10 overshoot as shown in the photo of the small-signal 20 5 24 0 0 response with 1000pF load. The large-signal response with 20 10 14 0 0 a 10,000pF load shows the output slew rate being limited to 4V/μs by the short-circuit current. The LT1222 can drive For frequencies < 10MHz the frequency response of the coaxial cable directly, but for best pulse fidelity a resistor of amplifier is approximately: value equal to the characteristic impedance of the cable (i.e., 75Ω) should be placed in series with the output. The f = 1/[2π • 53Ω • (CC + 6pF) • (Noise Gain)] other end of the cable should be terminated with the same The slew rate is affected as follows: value resistor to ground. SR = 1.2mA /(CC + 6pF)
Compensation
An example would be a gain of –10 (noise gain of 11) and The LT1222 has a typical gain-bandwidth product of CC = 20pF which has 10.5MHz bandwidth and 46V/μs slew 500MHz which allows it to have wide bandwidth in high rate. It should be noted that the LT1222 is not stable in gain configurations (i.e., in a gain of 100, it will have a AV = 1 unless CC = 50pF and a 1k resistor is used as the bandwidth of about 5MHz). For added flexibility the ampli- feedback resistor. The 1k and input capacitance increase fier frequency response may be adjusted by adding capaci- the noise gain at frequency to aid stability. tance from Pin 5 to ground. The compensation capacitor 1222fc 7