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Signal Chain Basics #148: Understanding the impact of offsets in fully differential amplifier circuits
5/29/2019

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Figure 3
Offset voltage model for an FDA

Offset voltage model for an FDA

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steve.taranovich
steve.taranovich
6/3/2019 4:13:58 PM
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Re: More complete output DC offset calculations
Here is the link to Michael Steffes' article https://www.planetanalog.com/author.asp?section_id=3404&doc_id=565037& 

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jfreet
jfreet
6/3/2019 1:20:40 PM
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Re: More complete output DC offset calculations
Hi Michael, 

 

Yes, excellent point. For this article I chose to focus mainly on the amplifier's intrisic offsets and the source offset, but we definitely shouldn't forget to consider resistor matching as well. Thanks for pointing that out!

 

 

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Tucson_Mike
Tucson_Mike
5/31/2019 8:25:02 AM
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More complete output DC offset calculations
Hey Jacob, 

 

Good stuff but you left a few error terms out of the discussion. In assembling the THS4551 data sheet, I got into these issues way back in section 8.3.4 where the dominant static (not drift) output DC offset actually arises from the desired output common mode voltage converting to a differential error through the resistor ratio mismatch on the two feedback side. Table 3 summarizes this where assuming worst case 1% resistor divider mismatch gives by far the highest nominal part of the output offset total. However, the next table 4 shows it is a very small part of the drift. The lesson there is if you care about output offset error, put some effort into matching those feedback dividers using better precision on the R's and exactly matching the Rg on each side. There was actually an error in this section in how I treated the conversion of avg input bias current through resistor mismatches - corrected that in a recent FDA DC precision article over on Planet Analog 'DC Precision considerations for high speed current and fully differential amplifiers, Insight #4"

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