I am looking to design a conditional amplifier which will take a output
signal and amplify it only if it is high, above 700mV. This is to
allow for a greater voltage differential when using sensors with a PIC
microcontroller. Also I need to deal with a high output impedance from
the connection to the PIC. Thank You
These are first thoughts:
How about using two operational amplifiers, both connected
to the source but one controlling the other?
The first is a straight linear/logarithmic (depending on
what you want). This, as well as setting the required
transfer function, buffers the high impedance source.
The second is a voltage controlled amplifier. Its control
voltage is the output from the first amplifier, with an
offset set to the signal level below which you want no
amplification to take place. It can have a linear or
whatever relationship to the control voltage and the input
signal. Its output is the one you want.
Or you could, of course, feed it through an A/D and D/A, say
controlled by another PIC, and set whatever transfer
function you wanted in software...Far more stable and easier
than all this analogue stuff..
But you would have to give a lot more details (for me at
least), for more help.
I'm not to clear on what you want. I assume the PIC A/D has an input of 0 to
5 Volts corresponding to 0 to 1024 bits (ten bit A/D). Of course this
depends on the reference and how certain register bits are set. I also
assume that you want to encode voltages only higher than 700mV and that you
want some higher voltage, Vm, to represent full scale digitally.
Furthermore, I assume that digital values are linear the range of 700mV to
Vm. If that is the case, you simply set up a non-inverting amplifier with a
gain = 5/(Vm - 0.7) and you offset the amplifier so that zero output occurs
with 0.7 Volts input. You then drive the PIC input port with a resistance
say 1K between the amp and the PIC. The resistor is to protect the PIC from
out of range voltages.
it seems you want to amplify only if the signal is LOW, BELOW 700 mV.
You should look for programmable gain amplifiers, the PIC may select the
The PIC should know what gain is used for the sensor, but it is better
that the PIC selects the gain.