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previous: Ed Rembold O.K. whats the rule of thum... -- 8/25/1999 10:19 PM View Thread

Re: 4.5V-multi Op amp question

8/27/1999 12:57 AM
Mark HammerRe: 4.5V-multi Op amp question
I'm not an EE either, but the simple version of the correct reply runs something like this....(assming I've got it right)  
 
 
 
The "4.5V bias" you speak of is often indicated on schematics as VRef, meaning that it is a reference voltage; a fake "middle" that any signal changes are in "reference" to. It is not 0 volts, but IS the midpoint of the power supply voltage that is available, the same way that ground is the midpoint of a bipolar power supply.  
 
 
 
Any time an op-amp stage is about to do something relative to a reference point, you generally need to take out the DC from the previous stage to, in effect, "reset" the bias. The reference voltage is then introduced to the next stage all over again.  
 
 
 
Op-amps vary in terms of offset errors introduced. I'm sure there is a precise technical term for it, but when looking through more detailed catalogues or manufacturers applications books, what you'll see is that the really pricey op-amps are often those that introduce very little offset error. If the offset is a half a millivolt, and the op-amp stage has a gain of 100, the next stage is not seeing something that hovers above and below exactly 4.5V. Unless you block DC with a cap, this error is likely to be compounded by the next stage, hence the RC coupling and reintroduction of a "true" VRef for each relevant stage. Alternatively, you can stick in a trimpot for each op-amp and compensate for offset error. Most folks prefer to just use a DC-blocking cap and rebias because it requires no adjustments, and is simply much more expedient if you lack a scope or good meter.  
 
 
 
An anecdote: About 14 years ago, I was helping a prof buddy build a precision electrophysiological recording pre-amp for some work he was doing. He was using DC coupling and a bipolar supply, but he was also using cheesy 741's (I take it from his behaviour that Horowitz & Hill is a terrific book to learn about design, but lacks much info on op-amp selection). Needless to say "precision" was not the word that popped into his head when he used it. I suggested popping in some CA3140 MosFet op-amps (because I was fond of using them at the time, rather than because I knew about the more exotic properties, such as offset error). The difference was like night and day. He was so thrilled by the change in performance that he ran out and bought me the very soldering iron I use to this day, just to say thank you.  
 
 
 
DC couple at your own risk, my friend.

 
Replies:
paul perry Mark Hammer said "He was using DC c... -- 8/27/1999 2:38 PM