Are there any drawbacks to giving inputs high (say 1 M) input
I realise that the higher the input impedance, the more EMI you
catch but if what's at the other end of the patch cord has a low
output impedance, won't it shunt the EMI to ground ?
André Majorel http://www.teaser.fr/~amajorel/
Good question. Looking forward to the responses.
Your observation seems good so far.
The other consideration is that higher impedance requires more current gain
in your input stage.
(High impedance and high gain are characteristics of radio receiver front
The side effect is that in addition to the stray internal currents and
offset voltages on the input are a larger component of the output signals.
O'scopes tend to have very high input impedance and require special
attention to the input buffers to keep the signal to noise ratio high.
I think it is a matter of trade offs, using lower input impedance it is
easier to keep a good signal to noise ratio, higher input impedance loads
the signal source less and with additional care to preserve the signal
integrity result in a more accurate signal path.
More thermal noise and bias current offsets will feed through more
easily to the output.
V_n,RMS = sqrt( 4kTR (f_upper - f_lower) )
Any current noise will also be much more noticeable when referred
through a high-impedance node, although the current noise itself is
proportional to sqrt(Ibias).
There is an optimum impedance point for each bias current in an
amplifier, so unless you can control the bias currents or select the
amp appropriately, to high of an impedance wil just get things worse
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How do you determine the bias current?