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#1
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"Maybe the universe has always been like this, but I've been toowrapped up in myself to notice..."
Gents,
As I was helping someone with a TIA design this afternoon, I had occasion to read the data sheet for the OPA211, which appears pretty similar to the LT1028, except that it's a bit more stable and doesn't come in a plastic DIP. It has a curve of THD+noise vs rms output voltage that I found very odd (see attachment). Everything goes swimmingly until the signal swing on the inputs reaches about 1V rms, and then it hits the wall. This is at a kilohertz, mind, so it isn't an ordinary slew rate issue. It's also _way_ within the CM range, because the plot specifies +- 15V supplies. Since the problem surfaces at the same input voltage swing makes it look like something in the input stage, but what in the world could be as slow as that? So what's up with this amp? Cheers, Phil Hobbs |
#2
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"Maybe the universe has always been like this, but I've been too wrapped up in myself to notice..."
On Thu, 11 Oct 2007 22:06:18 -0400, Phil Hobbs
wrote: Gents, As I was helping someone with a TIA design this afternoon, I had occasion to read the data sheet for the OPA211, which appears pretty similar to the LT1028, except that it's a bit more stable and doesn't come in a plastic DIP. It has a curve of THD+noise vs rms output voltage that I found very odd (see attachment). Everything goes swimmingly until the signal swing on the inputs reaches about 1V rms, and then it hits the wall. This is at a kilohertz, mind, so it isn't an ordinary slew rate issue. It's also _way_ within the CM range, because the plot specifies +- 15V supplies. Since the problem surfaces at the same input voltage swing makes it look like something in the input stage, but what in the world could be as slow as that? So what's up with this amp? Cheers, Phil Hobbs That graph makes no sense whatever. If the numbers were different, I'd say that some input back-to-back diodes were conducting... but not at 1 KHz. Also, fig 3 sort of contradicts fig 4. 140 dB is an interesting number, too. Is it an artifact of the test circuit, fig 41? If you figure it out, let us know. John |
#3
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"Maybe the universe has always been like this, but I've been too wrapped up in myself to notice..."
"Phil Hobbs" As I was helping someone with a TIA design this afternoon, I had occasion to read the data sheet for the OPA211, which appears pretty similar to the LT1028, except that it's a bit more stable and doesn't come in a plastic DIP. It has a curve of THD+noise vs rms output voltage that I found very odd (see attachment). Everything goes swimmingly until the signal swing on the inputs reaches about 1V rms, and then it hits the wall. This is at a kilohertz, mind, so it isn't an ordinary slew rate issue. It's also _way_ within the CM range, because the plot specifies +- 15V supplies. Since the problem surfaces at the same input voltage swing makes it look like something in the input stage, but what in the world could be as slow as that? So what's up with this amp? ** Nothing - Fig 4 is just plain wrong. Read the text where it states THD 0.0001% when G = 1 and output level = 3 volts rms. ....... Phil |
#4
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"Maybe the universe has always been like this, but I've beentoo wrapped up in myself to notice..."
Le Thu, 11 Oct 2007 22:06:18 -0400, Phil Hobbs a écrit:
Gents, As I was helping someone with a TIA design this afternoon, I had occasion to read the data sheet for the OPA211, which appears pretty similar to the LT1028, except that it's a bit more stable and doesn't come in a plastic DIP. It has a curve of THD+noise vs rms output voltage that I found very odd (see attachment). Everything goes swimmingly until the signal swing on the inputs reaches about 1V rms, and then it hits the wall. This is at a kilohertz, mind, so it isn't an ordinary slew rate issue. It's also _way_ within the CM range, because the plot specifies +- 15V supplies. Since the problem surfaces at the same input voltage swing makes it look like something in the input stage, but what in the world could be as slow as that? So what's up with this amp? It seems they nicely screwed the DS there. Not only the G=+1 curve is, ahem, a bit off, but it seems the G=+11 curve could the G=+1 one: the G=11 curve shows 0.15ppm@3V, which is the stated figure for the G=1 case. They have a documentation feedback link in the page footer and I just sent feedback. Let see what they say... -- Thanks, Fred. |
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