Vref through resistor ?

Hi all,

I'm designing simple first order active low-pass filters with op-amps. I work in single supply inverting configuration. In this configuration, my op-amp positive terminal is fed with my mid-supply buffered bias (2.5V).

What confuses me when reading the litterature (datasheets, tutorials, sample circuits, ...) is that sometimes, the mid-supply is fed directly, and sometimes it is feed through a resistor.

What are the factors to consider when deciding if/when/what resistor to use ?

Thanks !

Could you show us one of each type of circuit you mean?
I suspect that what you are asking about is to do with whether the circuit is fed from a single supply or a centre tapped one.
A single supply needs 2 equal valued resistors across it to give the centre voltage but a centre tapped one already has that
centre voltage available and so needs no resistors.
hth

Here is a circuit that shows my mid-supply reference as well as two filter example. The first one connects the Vref directly to the op-amp positive terminal and the second one connects it through a resistor (R3). Which of the two approach should I use ? What are the differences ?

Thanks !

If DC offset is important for your circuit then you must use R3 that is equal to parallel resistance of R2 and R1. Othervice it is not needed.

My application is audio. VCC is 5V so Vref is 2.5V.

I am actually using two of these filters in cascade. My input is AC coupled to the first filter; then the first filter is DC coupled to the second; finally, the output of the second filter is AC coupled to the next stage.

I was expecting that the DC offset between the two filters would be 2.5V. But if I understand correctly what you said, without R3, the output of the first filter could be centered around something else than 2.5V ? Is that correct ? By how much ?

Thanks !

Yes DC offset is deviation from ideal midpoint of 2.5V. It is greatest at final second stage because of DC coupling. This error can teorethicaly lower the usable amplitude of AC signal for the amount of this offset. But this is a theory and I think you can omit R3 resistors in your audio amplifier.

Operational amplifier has in it's datasheet data for DC ofset given as input offset voltage for zero voltage output. Offset at output is calculated as Voffin*gain of amplifier. Final offset is calculated consequently in the same way.

With some op-amps, there is significant input bias current, which is the current taken by both inputs. That is usually much larger than the input offset current, which is the difference in the current taken by the two inputs.

To minimise the effect of the input bias current, the two inputs should be fed by identical resistances, so that the bias current causes both inputs to change by the same amount.

On a lot of FET op-amps the input currents are so small that it makes no difference what resistance is in series.

To summarize, there are three op amp offsets:

1) Input voltage offset -- Multiply this by the DC gain of the circuit to get the output offset due to the input voltage offset. (This is independent of the input resistances).

2) Input bias current (common to both inputs) -- Multiply this by the difference in equivalent circuit resistance between the two inputs times the circuit DC gain to get the output offset due to the input bias current

3) Input offset current (difference in bias current between the two inputs) -- If the two equivalent circuit input resistances are equal (to minimize the effects of bias current offset) then multiply this value times the circuit DC gain to get the output offset due to the input offset current.

Add the above three offsets together to get the total maximum circuit output offset.

Than you guys for improving my understanding of opamps !!

crutschow, thanks for the summary :

1) Get it.
2) Get it.
3) Hum. Not sure ! ... If it is a current, by which resistance should I multiply it before multiplying by gain ?

3) Multiply the offset current by the equivalent resistance of either input since, by definition, they have been set to be equal.

Ok. Could've guessed !!

Thanks again. I've got a much better understanding of opamps now. I have been seeing these words like "bias current" and "offset current" and overlooked them in the pass. I will now read my datasheets with new eyes !!!