Comments on: The Differential Amplifier Common-Mode Error – Part 1 http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-1/ Electronics Design and Modeling with Emphasis on Analog Design Wed, 21 Jul 2010 05:50:16 +0000 http://wordpress.org/?v=2.9 hourly 1 By: Adrian S. Nastase http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-1/#comment-26 Adrian S. Nastase Sun, 07 Jun 2009 16:01:03 +0000 http://masteringelectronicsdesign.com/?p=219#comment-26 Yes, indeed. In the numeric example of Solving the Differential Amplifier series there are two requirements: some gain for the voltage difference (actually sub-unity gain, which is attenuation) and a negative output voltage shift of almost one volt. This negative shift can be construed as common-mode voltage at the amplifier output. In that case, the requirements warrant the resistor calculations so that there is a significant gain for common-mode voltage. Yes, indeed. In the numeric example of Solving the Differential Amplifier series there are two requirements: some gain for the voltage difference (actually sub-unity gain, which is attenuation) and a negative output voltage shift of almost one volt. This negative shift can be construed as common-mode voltage at the amplifier output. In that case, the requirements warrant the resistor calculations so that there is a significant gain for common-mode voltage.

]]> By: electricguy99 http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-1/#comment-25 electricguy99 Sun, 07 Jun 2009 05:02:36 +0000 http://masteringelectronicsdesign.com/?p=219#comment-25 This is in contrast with the previous series, Solving the Differential Amplifier. In this article, the common-mode voltage at the amplifier output is not desired. In the previous article, although you did not mention the common-mode voltage calculation, in effect the output has common-mode voltage, right? This is in contrast with the previous series, Solving the Differential Amplifier. In this article, the common-mode voltage at the amplifier output is not desired. In the previous article, although you did not mention the common-mode voltage calculation, in effect the output has common-mode voltage, right?

]]> By: Adrian S. Nastase http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-1/#comment-23 Adrian S. Nastase Fri, 05 Jun 2009 02:33:20 +0000 http://masteringelectronicsdesign.com/?p=219#comment-23 It really depends on the application requirements. This article brings awareness about the common-mode error. Even if just one resistor has some tolerance, the error is large enough to be important in precision applications. In my article I used just one resistor tolerance for simplicity and demonstration purpose. In reality, all four resistors tolerances have to be taken into consideration so the error could be even larger. 10 mV error may be important versus a differential signal of 100 mV to 400 mV. In the next part I will discuss the case in which the differential amplifier is used to measure the current with a small resistor. The drop on the sense resistor is small but the common-mode voltage can be high so the current may be measured with some error. Therefore, to answer your question, 0.1% resistors might be good enough for some applications. For high precision ones, the design of the differential amplifier should have 0.05% resistors or instrumentation amplifiers like the Analog Devices AD621 or AD629. It really depends on the application requirements. This article brings awareness about the common-mode error. Even if just one resistor has some tolerance, the error is large enough to be important in precision applications. In my article I used just one resistor tolerance for simplicity and demonstration purpose. In reality, all four resistors tolerances have to be taken into consideration so the error could be even larger. 10 mV error may be important versus a differential signal of 100 mV to 400 mV.

In the next part I will discuss the case in which the differential amplifier is used to measure the current with a small resistor. The drop on the sense resistor is small but the common-mode voltage can be high so the current may be measured with some error.

Therefore, to answer your question, 0.1% resistors might be good enough for some applications. For high precision ones, the design of the differential amplifier should have 0.05% resistors or instrumentation amplifiers like the Analog Devices AD621 or AD629.

]]> By: MikeTr http://masteringelectronicsdesign.com/the-differential-amplifier-common-mode-error-part-1/#comment-22 MikeTr Thu, 04 Jun 2009 23:30:51 +0000 http://masteringelectronicsdesign.com/?p=219#comment-22 Your article is clear and easy to follow, but why should somebody be concerned with this error? Granted, if you have 10% resistors, the error is huge, but if you use 0.1% resistors everything should be ok, right? Is there something else one needs to be aware of, or 0.1% resistors are good enough? Your article is clear and easy to follow, but why should somebody be concerned with this error? Granted, if you have 10% resistors, the error is huge, but if you use 0.1% resistors everything should be ok, right? Is there something else one needs to be aware of, or 0.1% resistors are good enough?

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