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Old 21st April 2013, 07:16 PM   #2431
davada is offline davada  Canada
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I need a rail to rail op amp that can operate from a single supply. The negative supply
must be connected to ground and positive +15. It must have at least 16 bit settling, no latch up or sticking under these condition. It must be capable of being driven hard into saturation on the negative rail and recover gracefully from this. It must be internally compensated and require no FB capacitor. It must be wide band at a gain of -4 with no phase shift up to 200kHz. It must be capable of driving capacitive loads up to 100pF and must be low noise, < 5nV/root Hz.

Any suggestions.
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Old 21st April 2013, 07:22 PM   #2432
davada is offline davada  Canada
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Originally Posted by 1audio View Post
Your computer is dumping a lot of noise back into the AC ground. The scope ground creates a loop to push it back in. Laptop on batteries is the best fix, except for the other radiated noises.
Yes I know the comp is driving noise into the AC ground.
I don't usually have the scope connected while doing low disto measurements.
Even floating it drives the noise up a bit.

At these level things are so sensitive I can see the cat farting.
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Old 21st April 2013, 07:29 PM   #2433
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
I need a rail to rail op amp that can operate from a single supply. The negative supply
must be connected to ground and positive +15. It must have at least 16 bit settling, no latch up or sticking under these condition. It must be capable of being driven hard into saturation on the negative rail and recover gracefully from this. It must be internally compensated and require no FB capacitor. It must be wide band at a gain of -4 with no phase shift up to 200kHz. It must be capable of driving capacitive loads up to 100pF and must be low noise, < 5nV/root Hz.

Any suggestions.
LT1468 ??
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Old 21st April 2013, 08:19 PM   #2434
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
LT1468 ??
The LT1468 is what I have in there now but it can't be operated rail to rail and it doesn't like single supply operation with it's non inverting input tied to ground.
It also requires a FB cap to balance input capacitance otherwise it will oscillate at 7 to 10MHz.

This op amp is the signal conditioner for the input of the ADC in the AGC.
Although the ADC I'm using will withstand a +/- 25Vpp input without damage, driving the ADC into the negative increased the SVO's distortion 3 orders of magnitude. Yuk.
Therefore the op amp must be clamped with a FB diode to confine the swing to positive only. The combined capacitance of the schotkey clamping diode and FB capacitor introduces enough phase shift to cause a level difference from the input signal to output signal of the conditioning amplifier. This causes a 100mVrms or more level error of the SVO output over the frequency span of the highest range and than 10mVrms over the frequency span of the lower ranges.

I new about this before designing the input conditioner but I thought I could operate the ADC full swing and with minimal FB capacitance around the 1468 it would work.

The input conditioner needs to be redesigned. Without a clamping diode and no FB capacitor this problem is solved. I new this before as well. A problem with the rail to rail op amp I tested was sticking from being driven to hard saturation into the negative supply which was ground. Otherwise it worked without amplitude error. With the negative supply connected to ground the output is limited to ground. No clamping diode and no FB cap was needed.

I need an op amp that will perform well under these conditions and has sufficient settling time for a 16 bit conversion. If the amplifier doesn't settle fast enough then we're back to amplitude errors.
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Last edited by davada; 21st April 2013 at 08:23 PM.
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Old 21st April 2013, 09:36 PM   #2435
1audio is offline 1audio  United States
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How much of the sine wave are you digitizing? Would a clamping amp like used for settling time measurements work?
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Old 21st April 2013, 09:49 PM   #2436
davada is offline davada  Canada
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How much of the sine wave are you digitizing? Would a clamping amp like used for settling time measurements work?

A range of 0V to +5Vp.

A clamping could possibly work.

Part number?
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Old 21st April 2013, 10:36 PM   #2437
RNMarsh is offline RNMarsh  United States
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List the opamps you have tried and do not work.

-RNM
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Old 21st April 2013, 11:38 PM   #2438
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
List the opamps you have tried and do not work.

-RNM
Anything not rail to rail won't work.

What works okay is an opa1641. But this is an audio op amp and doesn't give information on setting time. It also sticks a bit coming out of saturation.
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Old 22nd April 2013, 12:31 AM   #2439
dimitri is offline dimitri  United States
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opa1641=opa140
large signal (10V step) settling time < 1us (+-1/2LSB 16bit)
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Old 22nd April 2013, 01:06 AM   #2440
davada is offline davada  Canada
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opa1641=opa140
large signal (10V step) settling time < 1us (+-1/2LSB 16bit)
Thanks Dimitri.

That will do.

I'm out of stock. Have to order more.

There is some filtering on the unused input pins. I wonder if the filter is causing attenuation. This is an LTC1609. Take a look at the data sheet under additional input filtering section. If the higher frequencies are attenuated the AGC response is to raise the level.
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