Electronics > Projects, Designs, and Technical Stuff
Help with opamp for shaping circuit
ChristofferB:
For a multichannel analyzer 16 bit is way plenty resolution! This would theoretically let you have a conversion gain of 65K!!
Look up the manual for the Ortec 452 shaping amplifier (which seems very close to what you need), it's very 1970s but it has some excellent theory and a nice block diagram!
MattHollands:
--- Quote from: snx on August 10, 2020, 07:17:32 am ---
--- Quote from: OM222O on August 10, 2020, 06:57:34 am ---Regarding the number of bits, again my bad, was rushing to write an answer. It should be 4LSB to equal 2 actual bits, but if they provide an ENOB figure well then that's your actual resolution, which seems to be 12 bits in this case.
If you don't need fast switching then the digital pot idea works like a charm, giving you a very wide (almost infinite) control for the gain, since you can change both resistors in the amplifier path.
If you're going to such high sample rates just to detect the peak, why not use a peak detect + smaple and hold circuit for measuring the peak seperately?
--- End quote ---
Yes, this would be an idea, but given the short peak time, you would need a very fast peak-detector. I currently know no design that is that fast, but maybe some people around here know one.
--- End quote ---
ADI have a nice article on peak detectors. https://www.analog.com/en/technical-articles/ltc6244-high-speed-peak-detector.html#
Looks like the Current Boosted Improved Peak Detector is approaching the performance you require. Might be worth a SPICE sim.
OM222O:
https://www.analog.com/en/technical-articles/ltc6244-high-speed-peak-detector.html
I think you'd want to use the current boosted version that's promised to work at 200KHz.
You just need a fet to discharge the cap as it seems to be missing from the diagram.
Eidt: well someone else mentioned it too, posted at the same time sorry :palm:
Kleinstein:
Analog peak detection is also not that simple and accurate. Using a fast ADC is a real alternative today and can be lower noise. Anyway an analog peak detector would also need peak shaping, it would be even more critical here.
For the ADC one does not need to have the unadjusted error to be so low to get perfect 12 Bits. Some of the errors like offset and a fixed gain error can be compensated for. Chances are the 4 K or 8 K channels are also more like enough so that quantization error does not matter - so this is not the factor that determines the accuracy needs for the ADC. The number of channels more like compared to the nominal resolution needed. So 12/13 Bit minimum.
The µC internal OP specs do not look that bad as I have expected. It is however relatively high noise (especially 1/f noise) and has a limited drive capability. For the speed it is quite low in power.
I would still prefer an external one as it would be difficult to upgrade - with more common OP one would have the option to keep the board and just upgrade / exchange a chip. They often come in standard cases.
If size is an issue the LMV771 would be a good candidate for the OP.
It helps to have the same supply as the ADC, as this also gives amplitude limiting and thus saves on additional clamping. With Rail to rail OP one may have to take care of the input cross over - so ideally not use the full RR input, but stay on the better side that often is some 0-2.5 V with a 3.6 V supply.
Anyway the first step would be getting the general circuit and do some simulations there and than choose the final OPs for the circuit. Here there are many OPs to chose from and I don't think one would need extreme performance or exactly one special OP to start with.
The problem with a digital pot is that the parasitic capacitance depends on the setting. So the wiper changes more than just the divider.
If simple is needed, there are ready made variable gain amplifiers like MCP6S91, that may be good enough.
snx:
--- Quote from: Kleinstein on August 10, 2020, 08:33:36 am ---Analog peak detection is also not that simple and accurate. Using a fast ADC is a real alternative today and can be lower noise. Anyway an analog peak detector would also need peak shaping, it would be even more critical here.
For the ADC one does not need to have the unadjusted error to be so low to get perfect 12 Bits. Some of the errors like offset and a fixed gain error can be compensated for. Chances are the 4 K or 8 K channels are also more like enough so that quantization error does not matter - so this is not the factor that determines the accuracy needs for the ADC. The number of channels more like compared to the nominal resolution needed. So 12/13 Bit minimum.
--- End quote ---
So you think the ADC of the MCU can be used for this? This would save alot cost, since 16bit, >2MSPS ADC's seem to be very expensive.
--- Quote from: Kleinstein on August 10, 2020, 08:33:36 am ---The µC internal OP specs do not look that bad as I have expected. It is however relatively high noise (especially 1/f noise) and has a limited drive capability. For the speed it is quite low in power.
I would still prefer an external one as it would be difficult to upgrade - with more common OP one would have the option to keep the board and just upgrade / exchange a chip. They often come in standard cases.
If size is an issue the LMV771 would be a good candidate for the OP.
It helps to have the same supply as the ADC, as this also gives amplitude limiting and thus saves on additional clamping. With Rail to rail OP one may have to take care of the input cross over - so ideally not use the full RR input, but stay on the better side that often is some 0-2.5 V with a 3.6 V supply.
--- End quote ---
The 3.3V supply from the MCU is generated by generic SMPS wich supplies VDD and VDDA of the MCU. The 2.5V is the reference voltage for the ADC/DAC Part. Usually the references cannot supply enough Power to drive OPAMPS and stuff. So you suggest to use higher reference? For example 3V? If the Analog Input goes beyong 2.5V, the ADC will not damage, it will just display max value.
Do you know if LTC6268-10, or LTC6088 or ADA4622-2 would work too? These seem to have similar parameters an i have them in stock to do tests
--- Quote from: Kleinstein on August 10, 2020, 08:33:36 am ---If simple is needed, there are ready made variable gain amplifiers like MCP6S91, that may be good enough.
--- End quote ---
What about LTC6910? It has simple binary configuration
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