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'[EE] Interesting rail splitter problem'
2007\12\12@194334 by Martin K

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I have a circuit here that uses a TLE2426
(http://focus.ti.com/lit/ds/symlink/tle2426.pdf) "rail splitter" to
provide a virtual ground for an opamp circuit. I was/am having trouble
with it because it was actually creating the ground at about 3/4*(Vin).
The overall load is close to 1mA going into the virtual ground.

So I was there trying to troubleshoot the problem. I accidentally
slipped with the osc. probe and shorted the output to -Vin for a
fraction of a second. I noticed that the output came back immediately to
where it should be (1/2*Vcc). I can repeat this over and over. Remove
power, reapply, short it to ground for a second, and it ends up where it
should be. I thought it might be oscillating but I don't see any
oscillations. The datasheet doesn't seem to say anything about such a
problem. Do you know what my issue (with the circuit) might be?

Thank you
-
Martin K

2007\12\12@195859 by David VanHorn

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> So I was there trying to troubleshoot the problem. I accidentally
> slipped with the osc. probe and shorted the output to -Vin for a
> fraction of a second. I noticed that the output came back immediately to
> where it should be (1/2*Vcc). I can repeat this over and over. Remove
> power, reapply, short it to ground for a second, and it ends up where it
> should be. I thought it might be oscillating but I don't see any
> oscillations. The datasheet doesn't seem to say anything about such a
> problem. Do you know what my issue (with the circuit) might be?

Sounds like an oscillation problem to me..
Maybe it needs a larger cap on the output?

2007\12\12@200221 by Rich

picon face
I believe that you can also do that with the LM380 which has dc output at
1/2 VCC with capacitively coupled to gnd,  The LM384 will source and sink
more current and also has idle at 1/2 VCC.  Both have sufficiently low
output impedance for good noise performance of op amp.  Both LM380 and LM384
are available.  Series zeners will also provide a rail splitter with low out
Z.




{Original Message removed}

2007\12\12@201555 by Marcel Duchamp

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Martin K wrote:
> I have a circuit here that uses a TLE2426
> (http://focus.ti.com/lit/ds/symlink/tle2426.pdf) "rail splitter" to
> provide a virtual ground for an opamp circuit. I was/am having trouble
> with it because it was actually creating the ground at about 3/4*(Vin).
> The overall load is close to 1mA going into the virtual ground.
>
> So I was there trying to troubleshoot the problem. I accidentally
> slipped with the osc. probe and shorted the output to -Vin for a
> fraction of a second. I noticed that the output came back immediately to
> where it should be (1/2*Vcc). I can repeat this over and over. Remove
> power, reapply, short it to ground for a second, and it ends up where it
> should be. I thought it might be oscillating but I don't see any
> oscillations. The datasheet doesn't seem to say anything about such a
> problem. Do you know what my issue (with the circuit) might be?
>
> Thank you
> -
> Martin K
>

Does the oscilloscope probe show the wrong voltage simultaneously with
showing no oscillation?  If so, that is a strange one and I don't have
an answer.

Maybe you have a peculiar load that causes this behavior.  I would try
one if possible out of circuit.  If it works correctly, put it in the
circuit and see what happens.  Figure 17 of the data sheet does show
that for some ranges of output capacitance, it is clearly unstable.  If
you are near that area, the result could be what you see.

2007\12\12@225851 by Spehro Pefhany

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At 07:44 PM 12/12/2007, you wrote:
>I have a circuit here that uses a TLE2426
>(http://focus.ti.com/lit/ds/symlink/tle2426.pdf) "rail splitter" to
>provide a virtual ground for an opamp circuit. I was/am having trouble
>with it because it was actually creating the ground at about 3/4*(Vin).
>The overall load is close to 1mA going into the virtual ground.
>
>So I was there trying to troubleshoot the problem. I accidentally
>slipped with the osc. probe and shorted the output to -Vin for a
>fraction of a second. I noticed that the output came back immediately to
>where it should be (1/2*Vcc). I can repeat this over and over. Remove
>power, reapply, short it to ground for a second, and it ends up where it
>should be. I thought it might be oscillating but I don't see any
>oscillations. The datasheet doesn't seem to say anything about such a
>problem. Do you know what my issue (with the circuit) might be?
>
>Thank you
>-
>Martin K

I agree that it sounds very much like an oscillation problem. The problem
might not be particularly visible at the output even with a decent 'scope.

Make sure you're not in the instability zone shown in Figure 17, and also
I would be concerned about the impact on stability of the use of larger
ceramic bypass capacitors on the output regardless of Fig. 17 because
of their very low ESR.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2007\12\13@132923 by Martin Klingensmith

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Martin K wrote:
> I have a circuit here that uses a TLE2426
> (http://focus.ti.com/lit/ds/symlink/tle2426.pdf) "rail splitter" to
> provide a virtual ground for an opamp circuit. I was/am having trouble
> with it because it was actually creating the ground at about 3/4*(Vin).
> The overall load is close to 1mA going into the virtual ground.
>
> So I was there trying to troubleshoot the problem. I accidentally
> slipped with the osc. probe and shorted the output to -Vin for a
> fraction of a second. I noticed that the output came back immediately to
> where it should be (1/2*Vcc). I can repeat this over and over. Remove
> power, reapply, short it to ground for a second, and it ends up where it
> should be. I thought it might be oscillating but I don't see any
> oscillations. The datasheet doesn't seem to say anything about such a
> problem. Do you know what my issue (with the circuit) might be?
>
> Thank you
> -
> Martin K
>
>  
I've made the situation a bit better by placing a 10uF ceramic on the
input and output (output to -Vin). I think I've compounded the problem
by running a LDO from +Vin to virtual ground to output 5v - I needed 5v
regulated from a divided 10-14v battery supply, and the ability to power
bipolar opamps. So with no load there doesn't appear to be any oscillation.
Thanks for the advice everyone.
-Martin

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