Yes roughly speaking. For 0V DAC output (ignoring offset current), 0V current to opamps and 0V out. 4.1V DAC output, delivers a current to that opamp inverting virtual earth = 4.1V/total input string R, while there is a slight scale issue related to bias resistor to negative voltage forming a divider and stealing some current from the inverter input.juniorhifikit wrote:Let me see if I've got this right: It seems that the scale of the VCA is determined by the maximum DAC output (4.1v) times the buffer's Rfb (1K in my case) divided by the total series resistance of the CV path (aprox 10K2 in my case) which lands around 400mV for each CV port - roughly half of the single port design shown in the data sheet. So far so good.
The Ec ports are both driven from opamp outputs so <50 ohms. The value of the inverter input and feedback resistors are kept relatively low Z to manage noise (self noise of resistors) and keep divider and offset impedances sensible, but exact values are arbitrary and can be changed as long as both are equal for EC drive symmetry.If I were to raise the series resistance between the DAC and the buffer, and consequently the buffer's Rfb (to keep my scale correct) I feared I would be driving the CV port with too high an impedance, as it wants to see less than 50 Ohms. I was kind of under the impression that this was not a negotiable item.
R7 is absolutely necessary to generate the non-inverted polarity of the control voltage.In regards to my drawing on page 2, would I gain anything by removing C8 & R7 WRT this problem?
C8 is sort of working at cross purposes. It us useful to provide a pole of filtering to reduce noise in raw CV, and slow the rate of change. It somewhat defeats my first order cancellation of the inverter stage noise that is also present at the top of R7. The relative benefit of C8 being large and exactly there depends on relative contribution of noise sources in your actual working unit. If raw CV noise dominates C8 is good, If inverting opamp noise dominates, not so good. My guess is c8 helps more than it hurts.
Note: c8 is in addition to pole already in DAC output, but you do need some filtering in the negative bias string especially if you remove c8. I would be tempted to add pole in negative bias string, and experiment on one channel to see if noise floor is better with/without c8 . You should be able to tell by comparing two similar channels noise floor at boost or unity gain.
Lighten load= higher impedance.. sorry if my verbiage is imprecise.I thought I wanted to increase the impedance between the DAC and the rest of the world, to keep stray start-up current flowing elsewhere. Now I am confusedIf you're going to invest any effort into this approach a simple tracking supply can probably be made from one opamp... but this may not be necessary if we lighten up the impedances the DAC is seeing.
If you can design an inverting opamp you can do this yourself. 0 to +5V input V through say a 5k input resistor for round numbers, with a 12k feedback resistor for equally round numbers will result in a 0 to -12V output tracking 0 to +5V input. If the opamp is too wimpy for the cuirrent required a pnp transistor can be added in series with the opamps output (base to OA output, collector to - unregulated, and emitter the new higher current output. Connect the 12k feedback resistor to the emitter not the opamp output (a 1k resistor from opamp output to emitter will also provide a DC path for opposite polarity drive very close to 0V if there are small DC errors. Note: there is a temptation to filter and slow this down that works against the quick tracking response. It will still be best, to impedances around the DAC higher and less harmful.As to a tracking supply, I assume you mean the bias supply tracking the 5v DAC supply? I just started hunting for a design...
You can do this simple design yourself, and analysis of the VCA for endpoints, scale, and offset are derived using ohms law, and making assumptions, like unity gain is 0V at top of c8 so only DAC and offset resistors are involved in that calculation. I don't derive fancy complex equations to do this, I just use trial and error and ohms law to converge on good values.
JR
