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LTC6240HS8 数据表(PDF) 18 Page - Linear Technology |
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LTC6240HS8 数据表(HTML) 18 Page - Linear Technology |
18 / 28 page LTC6240/LTC6241/LTC6242 18 624012fc APPLICATIO S I FOR ATIO Proprietary design techniques are used to obtain simulta- neous low 1/f noise and low input capacitance. Low input capacitance is important when the amplifier is used with high value source and feedback resistors. High frequency noise from the amplifier tail current source, ITAIL in Fig- ure 1, couples through the input capacitance and appears across these large source and feedback resistors. As an example, the photodiode amplifier of Figure 15 on the last page of this data sheet shows the noise results from the LTC6241 and the results of a competitive CMOS amplifier. The LTC6241 output is the ideal noise of a 1MΩ resistor at room temperature, 130nV√Hz. Half the Noise The circuit shown in Figure 3 can be used to achieve even lower noise voltage. By paralleling 4 amplifiers the noise voltage can be lowered by √4, or half as much noise. The √ comes about from an RMS summing of uncorrelated noise sources. This circuit maintains extremely high input resistance, and has a 250Ω output resistance. For lower output resistance, a buffer amplifier can be added without influencing the noise. Stability The good noise performance of these op amps can be at- tributed to large input devices in the differential pair. Above several hundred kilohertz, the input capacitance rises and can cause amplifier stability problems if left unchecked. When the feedback around the op amp is resistive (RF), a pole will be created with RF, the source resistance, source capacitance (RS, CS), and the amplifier input capacitance. In low gain configurations and with RF and RS in even the kilohm range (Figure 4), this pole can create excess phase shift and possibly oscillation. A small capacitor CF in parallel with RF eliminates this problem. Low Noise Single-Ended Input to Differential Output Amplifier The circuit on the first page of the data sheet is a low noise single-ended input to differential output amplifier, with a 200k input impedance. The very low input bias current of the LTC6241 allows for these large input and feedback resistors. The 200k resistors, R1 and R2, along with C1 and C2 set the –3dB bandwidth to 80kHz. Capacitor C3 is used to cancel effects of input capacitance, while C4 adds Figure 3. Parallel Amplifier Lowers Noise by 2x Figure 4. Compensating Input Capacitance + – CIN CS 6241 F04 RF RS OUTPUT CF 10 Ω 6241 F03 – + 1/4 LTC6242 1k 1k 10 Ω – + 1/4 LTC6242 1k 1k 10 Ω – + 1/4 LTC6242 1k 1k 10 Ω – + 1/4 LTC6242 1k 1k +2.5 –2.5 VIN VO |
类似零件编号 - LTC6240HS8 |
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类似说明 - LTC6240HS8 |
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