Electronics > Projects, Designs, and Technical Stuff
Laboratory Amplifier
MaxFrister:
Thank you all for the great suggestions.
Regarding the design, I have no illusion that I can match the specs of the extraordinarily well designed SRS and Tek gear. My goals are much more modest:
1. Learn something, and
2. Build a useful bit of lab gear.
Regarding adjusting the offset of an LF411 using V+, I tried it last night. Nope, it doesn't work. I don't think I would have tried that experiment with a $25 op amp.
I'll spend a bit more time comparing op amps but then settle on my choices. I hate choosing op amps; there are a billion of them and the specs are always incomparable. For the lowest noise and highest bandwidth, I suspect the front end should be discrete anyway.
I didn't know about the am502. I thought about putting my design in a 500 module, but didn't like idea of roar of the TM500 mainframe and the difficulty of working on that physical format.
I like the idea of matching scope probe impedance and optional AC coupling.
In what situations is the infinite impedance input useful? Does it occur often enough that the switch should be brought to the front panel? I am trying to avoid feature creep....
I have also considered including a calibrated attenuator that would make it easier to attach high impedance sources to the spectrum analyzer.
Conrad Hoffman:
As far as using offset null pins, do what the data sheet says and nothing else. Using those pins can often degrade other properties, so if you don't need to, don't. I find infinite impedance more useful on DVMs for metrology work, and in 44 years of using scopes, have never wished I had it. Remember that at higher frequencies the input capacitance will load things more than the DCR anyway. IMO, the most useful thing you can learn from the Tek schematics is how they did the input attenuators and compensated the probes. Don't underestimate the value of those bandwidth filters, as when you apply high gain you'll see nothing useful without them. That also tells you that trying to build a very high bandwidth circuit for high gain applications is probably a waste of time.
macboy:
If you are looking for only 100 kHz, you might want to have a good hard look at some of the audio power amplifiers offered by TI (originally NS), such as LM3886, LM3875, LM1875. These are essentially power opamps. The LM3886 can be powered from over 80 V (+/- 40) and can be loaded with as little as 8 Ohms at that voltage. The LM1875 is lower power, but is a simple small 5 pin TO-220. All of these devices have SOA and thermal protections built in. Another interesting device is TI (Burr Brown) OPA549, which has a current limit input pin to program the output current limit from 0 to 10 A using a resistor or a voltage. That would be a great feature for a bench amplifier used as a four-quadrant power supply, don't you think? Its GBWP is quite a bit lower than the National devices but still good enough for 100 kHz.
David Hess:
--- Quote from: MaxFrister on November 28, 2018, 03:06:32 pm ---Regarding adjusting the offset of an LF411 using V+, I tried it last night. Nope, it doesn't work. I don't think I would have tried that experiment with a $25 op amp.
--- End quote ---
The LF411 datasheet is old enough that it includes a full schematic which show the offset null connections which is what I used to verify how they should be used.
--- Quote ---I'll spend a bit more time comparing op amps but then settle on my choices. I hate choosing op amps; there are a billion of them and the specs are always incomparable. For the lowest noise and highest bandwidth, I suspect the front end should be discrete anyway.
--- End quote ---
It takes experience to recognize the different types of operational amplifiers. A JFET input operational amplifier would generally be the best choice for the input because of noise and input bias current but a discrete design has some advantages for input protection, controlling input capacitance, and coupling between the inputs.
The LT1102 JFET instrumentation amplifier is one of the very few suitable parts for the input amplifier but check out its common mode rejection ratio at 50kHz of less than 40dB versus 100dB for the Tektronix AM502. An AMP02 does a little better and would be another good choice for a simple design.
Getting better performance will require using JFET operational amplifiers or discrete JFETs.
--- Quote ---I didn't know about the am502. I thought about putting my design in a 500 module, but didn't like idea of roar of the TM500 mainframe and the difficulty of working on that physical format.
--- End quote ---
I do not know about a "roar" but I would not suggest building a TM500 module either. The 1, 2, and 3 bay TM500 mainframes have no fan.
--- Quote ---I like the idea of matching scope probe impedance and optional AC coupling.
--- End quote ---
The big advantage of being able to use x10 oscilloscope probes is that they provide a lower input capacitance and shielding. The problem is that their mismatch compromises common mode rejection.
With that in mind, I would make the 1 megohm input shunts trimmable so that a pair of standard x10 probes can be used without compromising common mode rejection. The way Tektronix achieved this was to make special adjustable x10 probes.
--- Quote ---In what situations is the infinite impedance input useful? Does it occur often enough that the switch should be brought to the front panel? I am trying to avoid feature creep....
--- End quote ---
The AM502 just has a jumper on the inside. Trying to switch such a high impedance point is difficult to do without adding leakage or coupling.
--- Quote from: Conrad Hoffman on November 28, 2018, 03:23:48 pm ---Don't underestimate the value of those bandwidth filters, as when you apply high gain you'll see nothing useful without them. That also tells you that trying to build a very high bandwidth circuit for high gain applications is probably a waste of time.
--- End quote ---
The need for bandwidth limiting becomes very apparent when using the 5A22/7A22/AM502 at high gain or high sensitivity with an oscilloscope. Note that these three instruments are all the same but the AM502 is intended for stand alone operation with a separate instrument. An AM502 would have been paired with an AF501 variable bandpass filter to make spot noise measurements before FFT spectrum analyzers became available.
Noise reduction can also be implemented through the averaging and high resolution functions on a DSO.
duak:
I could envisage cases where 10M (or even higher) Zin is desireable, but the attenuation of the probes is not. I have an idea for a Hi-z input circuit where the 1M resistor is selectively bootstrapped. ie., the bottom end is either tied to common to give 1M Zin or is driven by a percentage of the input signal by a buffer. If the bootstrap signal is 90%, you get a 10M input impedance. An advantage is that the actual signal line is not switched so the little PCB leaf switches Tek used aren't needed. Separate buffer amps would be needed, but then a less expensive INA could be used.
Cheers,
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