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| SR560 Low noise amp -- Noise Calculation |
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| Kleinstein:
The AD797 is not a good replacement for the LF411. The LF411 works with a rather high impedance signal and one thus need an OP-amp with low current noise. So more like an OPA134 (could be available in DIP), OPA1641 or OPA205. With well matched FETs there may still be little effect from the LF411. |
| sky2city:
--- Quote from: Kleinstein on April 06, 2024, 11:51:34 am ---The AD797 is not a good replacement for the LF411. The LF411 works with a rather high impedance signal and one thus need an OP-amp with low current noise. So more like an OPA134 (could be available in DIP), OPA1641 or OPA205. With well matched FETs there may still be little effect from the LF411. --- End quote --- Yes, I notice that, which JFET amp is the best to replacing LF411, that’s question I didn’t have idea to consider now. I am RF designer, not much experience on amp selection in low frequency application. Thanks FYI. :-+ I have SMD to DIP adaptive board on hand, JFET amps of sop8 can be work too. I will have glance on those amps and try it later on. Stay tune. |
| Kleinstein:
The OPA145, OPA1641, OPA140 or OPA141 would be my choice, with not much difference between the last 3. They would need an adapter. |
| egonotto:
Hello, I would be interested to know what the noise looks like in the time domain. Is this preamplifier still useful when using a modern oscilloscope? Could you please short-circuit your input (or with extern 50 Ohm termination) and show what it looks like at gain 100 with an oscilloscope at 1 ms/div? Best regards egonotto |
| sky2city:
--- Quote from: egonotto on April 06, 2024, 12:20:53 pm ---Hello, I would be interested to know what the noise looks like in the time domain. Is this preamplifier still useful when using a modern oscilloscope? Could you please short-circuit your input (or with extern 50 Ohm termination) and show what it looks like at gain 100 with an oscilloscope at 1 ms/div? Best regards egonotto --- End quote --- Post the measured data here. Common configuration: 1. input JFET = JFE2140 2. OP37 change to AD797 3. LF411 change to AD797 4. SR560 in low noise mode, first stage amp Gain = x110,~ 40.8dB, Total gain = ~x1100 5. SR560 in high dyn. mode, first stage amp Gain = ~x10, Total gain = ~x1000 6. For square wave test, amplitude of square wave of SG ( Agilent 1102G ) =~2mV. =========================================== Following Test is base on configuration: SR560 in Low noise mode 1. short input 2. Square wave test. 3. bandwidth test ============================================= As comparation data. Following Test is base on configuration: SR560 in dyn. mode. 1. Square wave test. 2. bandwidth test :-) |
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