Whats the benefit or pourpose of an Attenuator? - Page 1
Forum section: electronics / beginners

author: Detzi
author_rank: Regular Contributor
timestamp: November 29, 2017, 07:00:46 am
content: Hi there,i asked myself what exactly is the poupose of an attenuator? What does it do better than a simple resistive voltage divider? Or formulated with the origin of my thought: Why does one not put a resistive voltage divider instead of a scopes Analog-Frontend attenuator ?Thanks in foreward!Detzi

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2017, 07:31:39 am
content: Resistive divider is an attenuator. But reduction in amplitude is not the only thing you want. You also want impedance matching and flatness of the frequency response.Typical input attenuator on mid-range scopes is implemented as a resistive divider with compensation capacitors.PS: If you are talking about some specific circuit, post or link it here, so discussion can be more targeted.

author: Detzi
author_rank: Regular Contributor
timestamp: November 29, 2017, 08:48:24 am
content: Thanks for your answer!Well what i am talking about is this specific Attenuator (the part marked as ATTENUATOR):http://www.eevblog.com/files/Rigol-DS1054Z-Schematic-FrontEnd.pdfIf one simulates a resistiv divider, one sees a constant gain/dampening over the whole spectrum (perfect flatness of freq. response) where the used attenuator is dampening linear for frequencys up to 1MHz (of course depending on the choosen values for the caps) . I cannot explain to myself why this is a wanted thing or see which problem makes this effort necessary.Are high frequencys damped less because of higher losses for high frequencys in the amplifierer stages, or something like that?And to which impedance is the thing matched? Wouldn't you want an impedance as high as possible without going too deep into the noise floor respectivly keeping a good Signal-to-noise ratio?

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2017, 08:56:11 am
content: Quote from: Detzi on November 29, 2017, 08:48:24 amThat is a restrictive divider with capacitive compensation.Quote from: Detzi on November 29, 2017, 08:48:24 amWell, if one can also find those ideal resistors used in a simulator to mount on a board, one would be all setReal resistors are also small inductors and capacitors.Quote from: Detzi on November 29, 2017, 08:48:24 amExactly. Where did you get that 1 MHz figure from?Quote from: Detzi on November 29, 2017, 08:48:24 amThat was a more general comment, in case of a scope attenuation is a primary function.

author: Detzi
author_rank: Regular Contributor
timestamp: November 29, 2017, 09:25:34 am
content: Quote from: ataradov on November 29, 2017, 08:56:11 amI played (''random'' values, ideal parts)around in LT Spice and did a plot.So if i understand correctly, you would use a attenuator to compensate for the parasitic properties of real-world resistors. The goal then would be to reach the response of an ideal resistive divider. Correct?If so, how far can i simulate this? Is there something like a database for parasitic properties of components in different packages and tolerances?

author: ataradov
author_rank: Super Contributor
timestamp: November 29, 2017, 09:29:49 am
content: Quote from: Detzi on November 29, 2017, 09:25:34 amCorrect. You also may be compensating for non-ideal response of the following gain stage, for example, so you probably want some amplification towards the end of the frequency range. In the end, you use this input stage to compensate for all non-ideal things in the whole analog chain before the ADC. And obviously some additional compensation is possible after the initial amplification.Quote from: Detzi on November 29, 2017, 09:25:34 amNope. That's the whole challenge. That's why that RC network is so complicated. You provision for a lot of components, and then experiment with your specific parts and PCB material. Not all of those part may end up on a board in the final product. But if you don't reserve the space, you will end up with infamous bodges.

author: Detzi
author_rank: Regular Contributor
timestamp: November 29, 2017, 09:32:29 am
content: Thanks man, you helped me a lot