Trying to sort out circuit simulation artifact from something that may be real. Does the noise come up [because of the inability of the OpAmp to reduce] when the OpAmp is driven too fast at too large a signal level? Circuit is a simple LT1028 in an 'inverting' configuration with power of +/-12Vdc. GNDed non-inverting input. Rinput = 1k. Rfdbk = 1k. Cfdbk = 20pF. Load is a simple 10k in parallel with 10pF. VERY simple inverter. The drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with, making the output a 'triangular' signal around 2 Vpk. Now I wish to consider noise: Using LTspice I get an 'expected' noise density function over the range of ?? to 1MHz that seems reasonable for the LT1028. With almost no input voltage and doing what I call .tranoise analyses, where .tran and .noise analyses are combined, I get approximately the same noise density function. HOWEVER, when the above sinewave is applied which obviously overdrives the LT1028, the noise density function increases around 10dB. Thus, my question. In real-life should I expect the noise density function from an overdriven OpAmp to increase?

Good question... I don't know... you would expect, since the output stage is, in essence, disconnected from the input during slew-limited operation, that the "gain" no longer exists, and noise should go down ?? I'll see if I can find an OpAmp model that is all device-level. If I can, I'll break it apart and see what I get. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at | 1962 | I love to cook with wine. Sometimes I even put it in the food.

Does the noise density function increase if an OpAmp goes into slew rate limit?

May 28, 2014 61 Replies

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Jim Thompson 12 years ago

[snip]

Can you provide a .asc file that I can run that demonstrates this noise enhancement?

The .asc files so far provided do nothing unusual. ...Jim Thompson

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RobertMacy 12 years ago

Those are 'reference' files that anybody can use to verify the 'normal' LTspice results are the same.

For anybody interested in viewing the comparison plots, I sent to Helmut, moderator for LTspice Group, the whole file set. He will post the whole package, including the .asc files, into the /temp folders at LTspice.

Will see what I can send you.

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