Ethan

How is this an active attenuator?
« on: June 12, 2013, 03:09:26 PM »
When looking at the attached schematic, I see a diff-amp with an inverting stage to produce diff-out at near unity gain.  Where's the "active attenuation" that That Corp is referring to?
Quote from: That Corp Datasheet for THAT5171, pg 10
For better distortion performance with high- quality A/D converters, and to improve common-mode rejection, consider circuits like the one in Figure 6. The active (buffered) attenuator provides differential drive to the ADC, which improves performance.

Here's the link to the datasheet (page 10, first paragraph): http://www.thatcorp.com/datashts/THAT_5171_Datasheet.pdf

Am I missing something? ???
I am just the Web Geek here.


EmRR

Re: How is this an active attenuator?
« Reply #1 on: June 12, 2013, 03:17:23 PM »
are they using attenuator in reference to the reduction of common mode pickup? 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Ethan

Re: How is this an active attenuator?
« Reply #2 on: June 12, 2013, 03:21:53 PM »
I suppose that's possible but confusing in context.  Before that paragraph is a discussion of "Figure 5" which simply has a passive U-Pad attenuator, without the diff-amp stage.
Perhaps Figure 6 was supposed to have both the U-pad, followed by the diff-amp stage (but they forgot)?
I am just the Web Geek here.

Kingston

Re: How is this an active attenuator?
« Reply #3 on: June 12, 2013, 04:54:41 PM »
Note that one drawback of the circuit shown in Figure 5 is that it offers no common-mode rejection. The 1570 has unity common-mode gain regardless of its differential gain, as does the passive attenuator shown in Figure 5. This circuit 5 relies entirely upon the A/D converter’s common-mode rejection.

The image you cropped from figure 6 does the same as the u-pad (R9-R11), but with common-mode rejection. Note the heavy differential feedback resistors. It's not unity gain. (I didn't calculate, just what it reads at a glance).

One of the better app notes I've seen in a good while. Seems like quite the professional input stage. Thanks for posting.
« Last Edit: June 12, 2013, 04:59:23 PM by Kingston »

moamps

Re: How is this an active attenuator?
« Reply #4 on: June 12, 2013, 05:16:54 PM »
When looking at the attached schematic, I see a diff-amp with an inverting stage to produce diff-out at near unity gain.  Where's the "active attenuation" that That Corp is referring to?

The schematic you posted is "just" the buffer at output using for providing isolation of active feedback U circuitry from the output and for better CMRR. The active (buffered) attenuator  is the complete design with the buffer inserted.
Regards,
Milan     

JohnRoberts

Re: How is this an active attenuator?
« Reply #5 on: June 12, 2013, 05:29:36 PM »
301/2k49 is roughly -18dB

JR
Cancel the "cancel culture", do not participate in mob hatred.

Ethan

Re: How is this an active attenuator?
« Reply #6 on: June 12, 2013, 05:36:30 PM »
Agh!!  Thats what I didn't get.  Thanks for pointing that out Kingston and John!!!
I am just the Web Geek here.


 

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