Neil Bernard said:
Trying to understand what you mean by balanced impedance?
The application is going to be a Micro controlled sine osc using a digital
pot for amplitude control. this is for ID and testing of levels on MIC and
LINE level circuits. I need the output to be balanced but alos need the
unit to balanced
Okay. Reading some of the whitepapers on Jensen Transformers' and Rane's
web sites will help you understand how balanced audio works. But here's the
short version.
The point of a balanced input on a device (e.g., on a mic preamp) is to be
able to detect a differential signal in the presence of common-mode noise.
You assume that there is some external noise source that is coupling to your
signal lines, and you try to set things up so that the signal will couple
equally into both lines, and then you subtract one from the other to get the
difference.
There are two key issues here: first, getting the interference signal to
couple equally to both lines; second, doing the subtraction.
Doing the subtraction entails having good common-mode rejection in the
receiver. Ideally you can measure a 1mV difference between two lines even
though both of them have got a 120V common-mode signal on them, for
instance. Note that this has NOTHING TO DO WITH THE TRANSMITTER - it's all
about the design of the receiver. However, it's easier to achieve if you
can assume identical source impedances on both signal lines, so the
transmitter does affect it some.
Getting the interference signal to couple equally is the big one. You've
basically got a voltage divider: whatever capacitive or inductive factors
are coupling the signal from the interference source to your signal lines on
one side, and then whatever impedance from the signal line back to ground.
The source impedance is a lot lower than the load impedance, and they're in
parallel electrically, so the source dominates. Imagine that one of your
signal lines has a 10 ohm impedance to ground, and the other has 20 ohms to
ground. Then the interference signal is going to couple twice as strongly
to the 20 ohm line! Obviously this is going to mean that no matter how good
your common-mode rejection is at the receiver, there will still be
differential signal and thus interference.
So, it is very important that your transmitter have equal impedance to
ground from both signal legs, at all frequencies (not just frequencies of
signal, but frequencies of interference). Keep in mind that opamp circuits
may have low output impedance at frequencies where the opamp has gain, but
at frequencies past their gain bandwidth, they may have fairly high
impedance, several hundred ohms or more.
Now, what about balancing the output voltage? Well, it helps for efficient
power transmission. But you don't care about power transmission, because
you're not the phone company. And it helps to reduce crosstalk. But your
application isn't sensitive to crosstalk, as far as I could tell. And, it
does help a bit in the event that one or the other signal line shorts to
ground, in a real-world signal transmission application. But it does NOT
HELP with regard to noise, and it is NOT REQUIRED as long as the receiver is
balanced. You do end up with less signal (because half of it looks like
common-mode to the receiver), but then, you only took half as many
amplifiers to get there.
So, in summary: use an opamp for your + output, and play some tricks (see
Jensen web page) to get the output impedance constant and low at frequencies
above where the opamp has gain. Then, simply connect your - output to
ground, THROUGH AN IMPEDANCE MATCHED NETWORK that gives it the same source
impedance as the + output at all frequencies.
-walter
