Sample and Hold

Sample and Hold was the way to get more than one analog output from a single DAC.
We can analog multiplex on reading analog inputs, this lets us multiplex outputs too.

How it works

There is a single DAC which is connected to an analog multiplexer chip such as the 4051, this is an 8 channel. The output of the multiplexer has a small capacitor before a voltage follower buffer. The size of the capacitor is dependent on the refresh rate, i.e how often the CPU loops round and updates the output values. 100nF works quite well if the outputs are updated every loop.

Operation

Set DAC value
Set output address
Sample
Next address
Loop

The ‘sample’ part above uses the inhibit input of the 4051 to do just that, inhibit the output while the DAC and address are being written, then the output can be enabled allowing the voltage to be stored in the hold capactitor.

The image above is simplified, single channel, the AI is Analog In. The capacitor before the second buffer is our hold capacitor, when the C input is enabled, the voltage from AI is stored and transferred to AO. The 4051 multiplexer replaces this switch, giving us 8 outputs.

Most 80s synthesizer schematics use sample and hold for all the control voltage parameters, I don’t use this method anymore because DACs are easier and cheaper now. But however there may be occasions where this could be still used, if I design any 80s based system then it is worth using to make the circuit more authentic!.

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