Theo Verelst Synth Related Subjects page

Some circuits and ideas fit in the synthesizer area, but may equally well be applied elsewhere.
The ideas on using a curve lenght feedback or a PLL framed loop based exponential conversion and derived sets of 2pow(1/n) voltages and related simulataneous (cheap) multiple multiplying da/ad convertion units are my own, and are not known to me from literature, although it is perfectly possible they exist without my knowledge. I consider them worth implementing and engineering and researching a bit furhter on, a voltage based exponatial converter can be used both for synth control voltages and lots of other applications such as analog multiplication (with a exp conv in a feedback for log conv). Please note that contrary to my other electronics pages, these ideas are as yet untested, simply because I currently don't have the facilities.
Repeated Copyright Notice: Evidently presenting these materials on the net allows you to try and built them for yourself, but any right to present, publish, or sell the results is reserved. Contact me.

Offset Compensation

In a dynamical system, opamp's offset voltage my be compensated for, by measuring the current (output) offset, and applying a proportional voltage with the right polarity to one of its inputs.

A time locked voltage based exponential decay circuit idea



                                          Offset compensated
                                        |--------------------|
                                        |Hi-Zin              |
                                        | |\      |-------|  |
 Vref >---------------------------------|-| >-----|       |----->
        |  |     |          |        |  | |/      |-------|  |
        |  |     |          |        |  |Buffer      | S&H   |
        | ---   | |        | |       |  |------------|-------|
        | ---   | | Rd     | | 10Rd  |               |
        |  | Ce | |        | |       |               |
        |  |     |          |        |     |----------
        |  |     --|        --|      |     |
        |  |       |--o       |--o   |     |
        |  |     --|  |     --|  |   |     |
        |  |     |    |     |    |   |     |
        |  -----------|------    |   |     |
        |     |       |          |   |     |
        |    ---      | ---------|---|-----|
        |             | |        |   |     
        |  -----------|-|--------|---|---------------------
        |  |  |-----------|     / \  |                    |
        |  |  | Sequencer |    /+ -\ | Comp               |
        |  |  |-----------|   /-----\|                    |
        |  |     |              | |  |                    |      
        |  |  |-----------|     | |  |                    |
        |  |  |Clock  /\/\|-----| |-------------------    |
        |  |  |-----------|          | |\    |----|  |    |
        |  |     |                   |-|+\   |    |  |    |
        |  |  |-----------|            |  >--|S&H |--|    |
        ------| V/2 (RC)  |------------|-/   |    |       |
           |  |-----------|            |/    |----|       |
           ------------------------------------------------
               (switched)       (instrumentation)



Use a dynamically mirrorred version for disturbance compensation, by switching the capacitor upside down dynamically. Outine time diagram:

|          ----\
|         /     -
|        /       \
|       /         --
|      /            \
|     /              ----                 /
|    |                   \               |
|    |                    ----------     |
|===/-------------------------------\===/
    |<-------->|<------------------>|<->|
      Tcharge        Tdecay          Tvar
    |<--------------------------------->|
        Tperiod

Application: multiplying da converter

Use the progression of voltages and a switchable integrator to (MSB first) contstrue an n bit word based analog voltage. The reference voltage acts as the voltage multiply input, with the zero point adjusted exactly at ground level. S&H for multiple outs of course, in various ways.

A PLL based alternative for accurate lin-log convertion

The switched (small scale) variated ramp is made variable through frequency based fitting.

Combination with integrating sample and holds for multichannel AD

Only an intgrator and comparator per channel.