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'Help with high frequency noise in automotive appli'
1998\07\06@091351 by John Miller

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I am having some problems with an amplifier circuit in an automotive
application. Does anyone have any filter circuits to block noise from the
car 12V supply. I am currently using a couple of caps and transorbs but I
am getting some high frequency noise in an op-amp circuit that gives me
false measurements.
Thanks in advance...
John Miller

1998\07\06@103437 by tjaart

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John Miller wrote:

> I am having some problems with an amplifier circuit in an automotive
> application. Does anyone have any filter circuits to block noise from the
> car 12V supply. I am currently using a couple of caps and transorbs but I
> am getting some high frequency noise in an op-amp circuit that gives me
> false measurements.
> Thanks in advance...
> John Miller

If you are not drawing too much current, you can reducethe noise dramatically by placing a 1 - 10
ohm resistor in
series before all you protection stuff.

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1998\07\06@131743 by Peter L. Peres

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On Mon, 6 Jul 1998, John Miller wrote:

> I am having some problems with an amplifier circuit in an automotive
> application. Does anyone have any filter circuits to block noise from the
> car 12V supply. I am currently using a couple of caps and transorbs but I
> am getting some high frequency noise in an op-amp circuit that gives me
> false measurements.
> Thanks in advance...
> John Miller

HF noise in a car can come almost only from the electronic ignition (but
also wrongly installed CB radio or 110/220 V converter or fluorescent
lights with faulty circuits).  Powering any system directly from 12 V in a
car is a myth, unless it should admit being rendered temporarily useless
every now and then. FYI even car radios have the electronics around the
CPU designed not to 'forget' with input down to 6 V, and withstand 20 V
continuous and up to 50 V spikes (or more).

To remove HF noise, adopt a lower Vcc in your circuit and use a
'capacitance booster' with 1 transistor, and LC ladder filters before it
(1L and 1C and admitting 9 Vdc for the circuit is good enough for most
purposes).

For serious measurement, a DC/DC converter able to cope with the extremes
mentioned and temperature exposure (-50/+100 deg.C) followed by a usual
linear integrated regulator is a must. Canned DC/DC converters are
available, that can cover this span.

Peter

1998\07\06@175536 by ephen Rothlisberger

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>I am having some problems with an amplifier circuit in an automotive
>application. Does anyone have any filter circuits to block noise from the
>car 12V supply. I am currently using a couple of caps and transorbs but I
>am getting some high frequency noise in an op-amp circuit that gives me
>false measurements.
>Thanks in advance...
>John Miller

Put a choke in the power lines into your circuit. A big hefty ferrite ring
with the power cables wound round it several times. It may not fix the
whole problem, but it'll certainly help. This is what they do to keep
engine noise out of car stereos.

Stephen.

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