SCOTTIE wrote:
> I have found this one: <br>http://www.allaboutcircuits.com/vol_3/chpt_8/7.html
> If I replace that 250 ohm resistor with a 1M ohm resistor, then we can convert
> 0.1~10 Volt input to 0.1u~10u Amp current; -0.1 - -10 Volt to -0.1u ~-10u Amp
> current. <br><br>Can any expert provide your opinion?
In theory this will work. In practice however you will get all kinds of
effects but rarely the actual signal. Noise will be a significant
problem too at those levels. But the first and foremost problem is the
fact that the sample you want to apply some current to will have a
resistance too. This resistance will be in series to your current
limiting resistor and therefore the actual current flowing will be some
more or less significant amount smaller than what you think it should be.
With careful selection of the OpAmps to reduce their input offset
voltages the schematic on the TI application note seems the most simple
solution to me. In addition you may have to consider a fully galvanic
isolated amplifier to reduce noise problems. This would involve an
isolation amplifier before the VtoI converter and also a DC-DC converter
for the power supply of the output stage of that isolation amplifier as
well as the op-amps of the VtoI converter.
Rolf Kalbermatter