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. Bobsnipped-for-privacy@gmail.com schrieb:
Hello,
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 useful gain. The PIC should know what gain is used for the sensor, but it is better that the PIC selects the gain.
Bye
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