| Electronics > Projects, Designs, and Technical Stuff |
| Differing Response in Transient vs AC Analysis in LTSpice |
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| TeddyPython:
I'm trying to design a HV differential probe as a project which should teach me more about op amps (so far I've only covered just simple 10's kHz amps). After some background reading, I settled on trying to achieve the following specs: +-500V input (AC+DC) 20MHz bandwidth (-3dB) +-5V output (100:1 attenuation) And I've created the following circuit as a starting attempt: With some op amps I have chosen for their high input Z, high bandwidth, and relatively low cost. When I perform an AC analysis the circuit seems to have a very flat gain with <1dB of ripple - the following says I should expect values very close to 5V up to just over 20MHz. However, when I perform a transient simulation at 20 MHz I see the output is much smaller when compared to 5 MHz. 20MHz: 5MHz: Why am I seeing this difference between simulation types in LTSpice? Or rather, how should I be interpreting these two different simulations? Presumably I'm not understanding the output of the AC analysis. I've also included the schematic file for LTSpice if anyone would like them. |
| Someone:
Depends on the opamp models, try switching them to "ideal" infinite speed devices and see if the results match. GBW != slew rate |
| Jay_Diddy_B:
TeddyPython, --- Quote from: Someone on May 31, 2020, 10:45:11 pm ---Depends on the opamp models, try switching them to "ideal" infinite speed devices and see if the results match. GBW != slew rate --- End quote --- +1 Here is a slightly different model: I placed the AD817 on the schematic and used the symbol for a generic op-amp. Doing it this way it is portable. I am stepping the Frequency so that I can both results on the same graph. If I test at 100V peak I get a result consistent with the AC analysis. At 500V peak it is amplitude limited. This indicates a slew rate limitation. eevblogprobe_JDB.asc (7.36 kB - downloaded 24 times.) Regards, Jay_Diddy_B |
| TeddyPython:
I see, thanks both for the information. Also thanks Jay for the tip making the schematic more portable. I will look for a faster slew rate op amp tomorrow and report back. |
| TeddyPython:
I've done some tests with a THS4631 and the result is much better, indeed gain is acceptable (to me) up to almost 25MHz at +-500V, or +-700V at 20 MHz. This did need some small capacitances though, but I was thinking these might occur from parasitics anyway, or could be added with some wires bodged over components. Alternatively, is there a better way to get a decent response without resorting to such small capacitances? Also included another version of the spice schematic. |
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