Electronic – how to identify IC codes

There are many many different ADC chips available. Most major IC manufacturers make them - TI, Analog Devices, Maxim, to name but a few. There's literally thousands of different ADC chips available.

To convert an analog value into a digital value you need an ADC. It is possible to build an ADC out of simpler building blocks, and there are lots of different designs.

One of the simplest is a comparator and a sweeping voltage. It's not fast, but requires very few components.

Basically you have to generate an analog voltage from your Pi. This could be a filtered PWM signal, as long as it's well filtered and stable.

You then sweep that voltage from 0V up to 3.3V a step at a time, and use a comparator to compare that voltage to your incoming voltage. When your sweep voltage reaches the incoming voltage you then know what the incoming voltage is.

For best results you want a "Rail-to-Rail Input/Output" OP-Amp.

^{simulate this circuit – Schematic created using CircuitLab}

You can also generate the sweep voltage using a DAC (Digital to Analog Converter). This can give a more reliable voltage than filtering PWM, and a simple one can be made from just resistors in the form of what is called an "R-2R Ladder" DAC. It requires one GPIO for each bit of resolution you want in your DAC:

^{simulate this circuit}

The actual resistor values don't matter that much, as long as you have one set as exactly twice the resistance as the other set, so 1K/2K, 10K/20K, 4K/8K, etc. Also keep the resistors at no more than around 10% the input impedance of the op-amp you want to use. 1K/2K is simple enough to get hold of and a good value to use (Hint: 2K resistors could be two 1K resistors in series).

I will leave the rest to your imagination.

It's not clear exactly what you need, but a few things seem clear. The first is that your 74S124 produces a digital output, so you can use a digital buffer rather than an op amp. Furthermore, your -1.5 volt output occurs as a result of bad physical layout interacting with the sharp fall time of the VCO, and does not represent a guaranteed effect.

Buffering the VCO with a high-speed buffer such as the 74FCT244 would seem to be your best bet. These are available from sources such as Digikey for a buck apiece. The circuit would look like

^{simulate this circuit – Schematic created using CircuitLab} This will not provide perfect amplitude stability, but should probably be adequate for your needs. The output current capacity of the FCT244 is great enough that you should not have problems.