Timeline for How are differential inputs setup in a differential amplifier?
Current License: CC BY-SA 3.0
14 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 1, 2016 at 0:29 | vote | accept | user16307 | ||
| Oct 26, 2014 at 23:20 | history | edited | davidcary | CC BY-SA 3.0 | clarified (I hope) what "it" is. |
| Dec 13, 2013 at 20:10 | comment | added | Phil Frost | @user16307 "differential inputs" does not mean that the common mode voltage is 0. What is means is that the common mode voltage does not matter. U2 (and also U5) are inverters (the output voltage is equal but opposite the input voltage). | |
| Dec 9, 2013 at 9:40 | comment | added | user16307 | i think u described the effect of noise in balanced and unbalanced situations. but could u pls tell me of A and C are same? because before you talked about the effect of common voltage gain on differential amplifiers. if so C has common voltage gain where A has zero. and btw what is u2 (as an electronic component) in your figure? | |
| Dec 8, 2013 at 23:11 | comment | added | user16307 | but you told before that there is something called common mode gain in non ideal op amps. in case of single ended there will be a common mode voltage which will be multiplied with common mode gain and effect the output. i dont understand what u really mean. in differential inputs case common mode voltage will be zero. | |
| Dec 8, 2013 at 13:22 | comment | added | Phil Frost | @user16307 As it relates to the goal of measuring a signal with no noise, if A and C can be constructed with equally good impedance balance, then they are the same. You keep getting worried about common mode voltage, and I keep trying to explain: it doesn't matter. The important thing is to balance the impedances, so that noise is common mode, so that it can be rejected. If the signal also has a common mode component, that doesn't matter. | |
| Dec 8, 2013 at 12:43 | comment | added | user16307 | @Phil Frost My confusion was between A:Balanced and C:Balanced in your figures. My question is: Does the term "differential inputs" mean we should apply equal magnitude but opposite phase signals (like in A and B case in your figures)? Or any connection to a diff amplifier is also called "differential inputs". And isnt A better than C because in A common mode voltage is zero unlike in C? | |
| Dec 6, 2013 at 14:00 | comment | added | Scott Seidman | I agree, there's a place for in-amps, and a place for difference amps, both of which are differential amplifiers. Both are available as stand-alone IC's. I'm just pointing out there are differences, and we should be a bit careful about lumping them together for discussion, or people will walk away with the idea that difference amps have cruddy CMRR, when they don't. | |
| Dec 6, 2013 at 13:57 | history | edited | Phil Frost | CC BY-SA 3.0 | added 32 characters in body |
| Dec 6, 2013 at 13:50 | comment | added | Phil Frost | @ScottSeidman sure, if you can find a way to make induced noise common mode, which you can't, which is why when we want to implement a good differential amplifier from real op-amps, we use an instrumentation amplifier. | |
| Dec 6, 2013 at 13:48 | comment | added | Scott Seidman | "what we really care about is that the input impedances are balanced". They're not on what many would call a differential amplifier, which can still have a fine CMRR. | |
| Dec 6, 2013 at 13:45 | comment | added | Phil Frost | @ScottSeidman did I say something to the contrary? | |
| Dec 6, 2013 at 13:31 | comment | added | Scott Seidman | Input impedance in a "typical" one op-amp differential amplifier, even if packaged as a unit with laser trimmed resistors, are not the same for the two inputs. | |
| Dec 6, 2013 at 13:05 | history | answered | Phil Frost | CC BY-SA 3.0 |