At 23.42 2012.01.08, you wrote:
>Electron wrote:
>> say you have a single supply (+3.3V) and a bipolar AC input, which doesn=
't
>> have to be referenced to the main circuit's ground (i.e. it is isolated)=
..
>> I'd like to know if this input goes above +2V or below -2V or is inbetwe=
en.
>> After some work I came with this scheme in the attached GIF.
>>=20
>> Which works. The signal source output swing is within the base breakdown
>> limits of the transistor, so no need to protect the base-emitter junctio=
n
>> with diodes.=20
>>=20
>> I could omit a couple of resistors, but I wanted to keep the symmetry fo=
r
>> now.
>
>Actually, the only resistor you could eliminate without affecting the
>operation of the circuit would be to combine the two 1K resistors in serie=
s
>with the source into a single 2K resistor.
>
>> If I understand correctly, I almost reinvented the differential amplifie=
r,
>> but although I then studied it, there are differences (mostly the emitte=
r
>> resistances) that I cannot interprete.
>
>No, this is definitely NOT a differential amplifier.
>
>When one transistor's base is reverse-biased, the other one is being drive=
n
>by the source through a 4:1 divider -- the three 1K resistors in series wi=
th
>the source vs. the 1K base-emitter resistor. When the source begins to
>exceeed somewhere between 2.4 - 2.8V, driving the base above 0.6-0.7V, the
>transistor begins to turn on.
>
>Because of the way the resistances are distributed, the two ends of the
>signal source swing +/- 1.4V or so with respect to the circuit ground.
>
>> However, the reason I'm writing is that I hoped that with this circuit I
>> could also improve noise immunity vs a "one leg of the source is grounde=
d"
>> input circuit.
>>=20
>> But my simulations, where I add common mode voltage to both bases, do no=
t
>> show wonderful things. Perhaps its common mode rejection is very poor?
>
>Depending on exactly where you're adding a voltage source in the circuit,
>you're going to completely disrupt its operation in various ways.
>
>> Will this circuit have any advantages vs one where one of the two signal
>> source's leads is grounded? I have already built it so I just wonder if =
it
>> would be wasted work to make it differential instead, as I'd have to mod=
ify
>> several existing boards.
>
>I'm not sure I understand the question.
>
>> Last but not least.. using a single +3.3V supply (a constraint which I
>> cannot relax), and having this isolated signal source, would it be possi=
ble
>> to use a off the shelf comparator IC (again the goal is to get 2 digital
>> flags, one to say if the signal source is >=3D+2V and the other if it is
>> <=3D-2V), or it will not be possible unless I use a dual power supply (w=
hich
>> I cannot)?=20
>
>It's hard to say. Obviuosly, it would have to be a dual comparator. How
>accurate do the +2V and -2V thresholds actually need to be?

Not much!

I'd like to reject common mode noise, as I cannot use a twisted pair cable
(I cannot change the cable already used) and both legs are capacitively cou=
pled
with pulsed a electrical noise source.

If I use a single-ended input circuit, with one leg of the source grounded,
I am pretty sure I will get all the noise/interference as well. I thought t=
hat
by doing it differential, I would reject most of the interference.

Thanks,
Mario



>
>-- Dave Tweed
>--=20
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