On 16/04/2011 01:07, Bob Blick wrote:
>> >  But I don't understand why the high side sensing is difficult. Isn't =
it
>> >  the
>> >  same idea? Just measure the voltage drop across the resistor? I'll ju=
st
>> >  sample some nice diff amp from LT or something.

On "high" side you need the difference of two voltages, traditionally=20
one may be at supply rail level. In this application you could use a=20
separate +9V or +12V rail to power op amp.
Traditionally there are common mode issues with high side sensing. Often=20
the "norton" type amp LM3900 has been used (e.g. supply current sense of=20
regulators, Transmitters and lamps at the battery / supply side. The=20
LM3900 has essentially unlimited positive common mode above its own=20
supply rail as it current driven.

> Operating dual supply it would be no problem but single supply you will
> need a good single supply diff amp or buffer a divider to feed a regular
> diff amp reference. So choosing your diff amp will be an exercise in
> datasheet comprehension.
>
>> >  With the low side sensing, I can just use a good opamp?
> Even an LM324 would work on the low side. It just needs to have inputs
> and outputs valid to ground.

On the "low" side you have a voltage WRT to ground proportional to the=20
current. A differential amp and common mode ranges are not an issue. If=20
there is an ADC it may even read the voltage directly.
--=20
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