Author Topic: EEVblog #572 - Cascading Opamps For Increased Bandwidth  (Read 15652 times)

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Online Dr. Frank

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #25 on: January 26, 2014, 01:00:37 pm »
Dave,
That's been your best Fundamental Friday:
Storytelling is very straightforward, every aspect very well explained.

A minute critique:
You did not mention, that by cascading those OpAmps, your initial/superior goal of high precision DC gain will deteriorate.
One stage only will give 2 x 0,05% DC max. error, 2 stages will double that worst case value by 2 because you will have 4 instead of 2 uncertain resistors.
The probable error might follow a SQRT law, therefore 2 stages will increase uncertainty by a factor of 1,41 at least.

Anyhow, a fundamental error analysis, plus data from your series production / testing would make up a nice next-next-next fundamental Friday, I suppose.

Frank
« Last Edit: January 28, 2014, 01:44:41 am by Dr. Frank »
 

Offline Echo20-127

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #26 on: January 27, 2014, 12:36:26 am »
Dave,
That's been your best Fundamental Friday:
Storytelling is very straightforward, every aspect very well explained.

A minute critique:
You did not mention, that by cascading those OpAmps, your initial/superior goal of high precision DC gain will deteriorate.
One stage only will give 2 x 0,05% DC max. error, 2 stages will double that worst case value by 2 because you will have 4 instead of 2 uncertain resistors.
The probable error might follow a SQRT law, therefore 2 stages will decrease uncertainty by a factor of 1,41 at least.

Anyhow, a fundamental error analysis, plus data from your series production / testing would make up a nice next-next-next fundamental Friday, I suppose.

Frank

This gain accuracy might actually be a problem, but I expect Dave to know what he is doing with this...


On the bandwith problem, I guess this is a rather clever solution. I'm not very experienced in this domain but I would expect the improvement in the bandwidth domain to come at the detriment of an increased phase delay as the signal propagates through one more OpAmp. (tell me if I'm wrong  :-// )
It would be an improvement when you are trying to read the amplitude of some short transient current, but the delay would be something to keep in mind.
 

Offline casper.bang

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #27 on: January 27, 2014, 02:22:01 am »
Great video... entirely too few fridays in Dave's calendar.  ;D
 

Offline qno

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #28 on: January 27, 2014, 06:37:07 am »
I normally use slew rate.
The GBW is nice but says nothing about the amplitude of the output.
Before the amplitude of the opamps drops to -3 dB the output sinus waveform has been
changed to a triangle due to the slew rate limit.

See wiki

https://en.wikipedia.org/wiki/Slew_rate
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Offline electrocat

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #29 on: January 28, 2014, 01:28:28 am »
Hi
A method I used to get a 10:1 divider is a 8 resistor network.

 ----R---R---R---R---R---R-
    !     !     !
    --R---R--
          !   

2R in parallel is the 1R in the divider
2R in parallel is series with 4 R's is the 9R

Use a 1% network with 8 individual resistors
Generally the match between individual resisters is much better then 1%, for a radiometric divider the exact value does not matter
The resistors are all from the same mix of resistive material therefore they temperature stability tracks very well.
I have used the method over the years with great success, if space is not a problem
Generally get better then 0.1% ratios with high temperature stability, only have to by 1 part plus not very expensive.

Regards


 
 

Online BravoV

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #30 on: January 28, 2014, 02:04:12 am »
A method I used to get a 10:1 divider is a 8 resistor network.

 ----R---R---R---R---R---R-
    !     !     !
    --R---R--
          !   

2R in parallel is the 1R in the divider
2R in parallel is series with 4 R's is the 9R

Interesting & noted down, thanks.  :-+

Offline German_EE

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #31 on: January 28, 2014, 04:25:14 am »
Sorry but I'm missing something here, I cannot see why the bandwidth of an op amp will change depending on what gain you set it at. All that is in the feedback path is two resistors so what causes a device that works at over 2 MHz at unity gain to only work at a few KHz?

Peter
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Offline Bored@Work

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #32 on: January 28, 2014, 05:29:57 am »
Sorry but I'm missing something here, I cannot see why the bandwidth of an op amp will change depending on what gain you set it at. All that is in the feedback path is two resistors so what causes a device that works at over 2 MHz at unity gain to only work at a few KHz?

Peter

In simplified terms:

To get a stable opamp it is internally frequency compensated with what is called a dominant pole. The result (again simplified) is that an opamp behaves like a first order low-pass filter with -20 dB/decade frequency response. I.e. the low-pass behavior limits amplification at higher frequencies.

Actually, as an EE you should know that from your introduction course to opamps.
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Offline German_EE

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #33 on: January 28, 2014, 06:49:37 am »
EE, Electrical Engineer, normally I work on bigger stuff such as 10Kw motor drivers but I do the smaller stuff like this for fun.

Peter
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

Warren Buffett
 

Offline vigsgb

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #34 on: January 28, 2014, 11:13:52 am »
I think you should create a video on bandwidth as it applies to electronics as I don't even know why you needed more bandwidth in your micro current..  was it for response time of the input?  Make tests quicker.   When I search online and YouTube I find so many variants but nothing a newbie like myself can understand.
 

Offline mexakin

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #35 on: January 29, 2014, 10:44:06 pm »
You need more bandwitdh in opamp circuits to see fast changing frequencies on your test signals,
for example try to measure a rectangle signal with a too small bandwitdh, you will see a sinus of it, so you must always know what to expect from your test and measurment equipments, otherwise there is no need to measure at all :)
 

Offline The Chump

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #36 on: January 30, 2014, 09:07:01 am »
For 9K, surely 2x18K in parallel is better than 2K2+6K8 as it reduces your BOM.
For a two resistor ratio of 9:1, what could be better than 1k8 and 200R?
 

Offline vigsgb

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #37 on: January 30, 2014, 11:11:32 am »
You need more bandwitdh in opamp circuits to see fast changing frequencies on your test signals,
for example try to measure a rectangle signal with a too small bandwitdh, you will see a sinus of it, so you must always know what to expect from your test and measurment equipments, otherwise there is no need to measure at all :)

Thanks!  That answers my question.   Tomorrow I will be playing with some op amps.   Need to get a scope soon so I can see what the wave form looks like and such.   
 

Offline VanitarNordic

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Re: EEVblog #572 - Cascading Opamps For Increased Bandwidth
« Reply #38 on: January 25, 2019, 06:30:28 am »
in the video, Dave mentions that it is better that we apply the whole gain in one opamp for a couple of reasons, what are the reasons behind this?
 


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