Electronics > Beginners

Op amp not working out as planned

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Zero999:

--- Quote from: netdudeuk on June 07, 2018, 06:00:30 pm ---I have the AD818 but TBH, I'm not sure to hook it up for this application.

Here's the data sheet -

http://www.analog.com/media/en/technical-documentation/data-sheets/AD818.pdf

Also, the data sheet states (gain ≥ 2, or gain ≤ –1).  Is this really the limit of how much it can boost the signal from the AD9833 ?

Thanks

--- End quote ---
No, those are the minimum gains before it can become unstable and oscillate: ≥ means greater than or equal to and ≤ means less than or equal to.

This is very common on faster op-amps. Look at the OP37 datasheet, for example: it says gains >5.
http://www.analog.com/media/en/technical-documentation/data-sheets/OP37.pdf

jmelson:
Assuming the first scope trace is the output from the DDS chip, that is 600 mV peak to peak.  Your resistors program the op amp to have a gain of 11.  So, you are expecting to get an output of 6.6 V P-P from a 5V power supply.  Clearly, it will HAVE to clip.

Jon

netdudeuk:

--- Quote from: Hero999 on June 07, 2018, 07:19:31 pm ---
--- Quote from: netdudeuk on June 07, 2018, 06:00:30 pm ---I have the AD818 but TBH, I'm not sure to hook it up for this application.

Here's the data sheet -

http://www.analog.com/media/en/technical-documentation/data-sheets/AD818.pdf

Also, the data sheet states (gain ≥ 2, or gain ≤ –1).  Is this really the limit of how much it can boost the signal from the AD9833 ?

Thanks

--- End quote ---
No, those are the minimum gains before it can become unstable and oscillate: ≥ means greater than or equal to and ≤ means less than or equal to.

This is very common on faster op-amps. Look at the OP37 datasheet, for example: it says gains >5.
http://www.analog.com/media/en/technical-documentation/data-sheets/OP37.pdf

--- End quote ---

Thanks.  Can I just use this in an inverting amplifier configuration like I did with the previous ones, using the same two resistor calculation to get the desired gain ?

Zero999:

--- Quote from: Wimberleytech on April 05, 2018, 08:48:02 pm ---
--- Quote ---Yes you have missed something: an anti-aliasing filter will theoretically give a pure sine wave, right up to half the sampling frequency. In practise, you'll probably be limited to less than half the sample rate, but it should be possible to get a much higher frequency, than 1MHz and still have a decent sine wave.


--- End quote ---

Did you mean "smoothing filter," and more specifically a "brick wall smoothing filter" would be the only way to get a pure sine up to half the sampling frequency?

--- End quote ---
Yes, you're right, I was talking about a brick wall filter, which is the only way to get a sine wave, up to half the sample rate. Unfortunately, it's useless for other wave shapes.


--- Quote from: netdudeuk on June 08, 2018, 09:27:08 am ---
--- Quote from: Hero999 on June 07, 2018, 07:19:31 pm ---
--- Quote from: netdudeuk on June 07, 2018, 06:00:30 pm ---I have the AD818 but TBH, I'm not sure to hook it up for this application.

Here's the data sheet -

http://www.analog.com/media/en/technical-documentation/data-sheets/AD818.pdf

Also, the data sheet states (gain ≥ 2, or gain ≤ –1).  Is this really the limit of how much it can boost the signal from the AD9833 ?

Thanks

--- End quote ---
No, those are the minimum gains before it can become unstable and oscillate: ≥ means greater than or equal to and ≤ means less than or equal to.

This is very common on faster op-amps. Look at the OP37 datasheet, for example: it says gains >5.
http://www.analog.com/media/en/technical-documentation/data-sheets/OP37.pdf

--- End quote ---

Thanks.  Can I just use this in an inverting amplifier configuration like I did with the previous ones, using the same two resistor calculation to get the desired gain ?

--- End quote ---
Remind me which resistor values are you planning to use? A few schematics have been posted and I don't know which one you're referring to.

That op-amp should be fine for gains greater than 2, as long as the resistor values aren't too high. The bias current of that op-amp is fairly high, so high resistor values will cause it output DC point to drift significantly. High resistor values will also slow it down, as there it will take longer to charge/discharge the parasitic capacitances.

netdudeuk:

--- Quote from: Hero999 on June 08, 2018, 11:14:04 am ---
--- Quote from: Wimberleytech on April 05, 2018, 08:48:02 pm ---
--- Quote ---Yes you have missed something: an anti-aliasing filter will theoretically give a pure sine wave, right up to half the sampling frequency. In practise, you'll probably be limited to less than half the sample rate, but it should be possible to get a much higher frequency, than 1MHz and still have a decent sine wave.


--- End quote ---

Did you mean "smoothing filter," and more specifically a "brick wall smoothing filter" would be the only way to get a pure sine up to half the sampling frequency?

--- End quote ---
Yes, you're right, I was talking about a brick wall filter, which is the only way to get a sine wave, up to half the sample rate. Unfortunately, it's useless for other wave shapes.


--- Quote from: netdudeuk on June 08, 2018, 09:27:08 am ---
--- Quote from: Hero999 on June 07, 2018, 07:19:31 pm ---
--- Quote from: netdudeuk on June 07, 2018, 06:00:30 pm ---I have the AD818 but TBH, I'm not sure to hook it up for this application.

Here's the data sheet -

http://www.analog.com/media/en/technical-documentation/data-sheets/AD818.pdf

Also, the data sheet states (gain ≥ 2, or gain ≤ –1).  Is this really the limit of how much it can boost the signal from the AD9833 ?

Thanks

--- End quote ---
No, those are the minimum gains before it can become unstable and oscillate: ≥ means greater than or equal to and ≤ means less than or equal to.

This is very common on faster op-amps. Look at the OP37 datasheet, for example: it says gains >5.
http://www.analog.com/media/en/technical-documentation/data-sheets/OP37.pdf

--- End quote ---

Thanks.  Can I just use this in an inverting amplifier configuration like I did with the previous ones, using the same two resistor calculation to get the desired gain ?

--- End quote ---
Remind me which resistor values are you planning to use? A few schematics have been posted and I don't know which one you're referring to.

That op-amp should be fine for gains greater than 2, as long as the resistor values aren't too high. The bias current of that op-amp is fairly high, so high resistor values will cause it output DC point to drift significantly. High resistor values will also slow it down, as there it will take longer to charge/discharge the parasitic capacitances.

--- End quote ---

The very first one right at the top of the post.

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