Hello staroflaw!
Welcome to The LaserBoy Forum!
Thank you for asking such a direct question and for presenting it in such a nice graphical way!
You only need to make one negative voltage reference. That same one applies to all of the individual channels of the correction amp. You need a correction amp for every channel of the sound card you plan to use for laser signals.
Ground is ground throughout the whole schemo. This is connected to the ground coming from the USB power.
As you probably know, the op-amps require positive and negative voltage with respect to ground. This is why there is a DC/DC converter. It takes the ground and +5VDC from USB and magically turns it into ground, +9VDC and -9VDC.
So, the
-Vin comes from the -9VDC from the DC/DC converter.
If you choose not to use a DC/DC converter, then you can power the op-amps with some other power supply that gives you a ground and a positive and negative voltage. It's the same deal. The negative voltage from the power supply goes to -Vin.
The 500 ohm trim pot is to adjust the negative voltage reference. You will notice that there are resisters in series with both the inverting and noninverting inputs of the op-amp. The one for the voltage offset is 15K and the one for the signal input is 10K. That is a ratio of 3:2. Actually, it is a ration of -3:2, because of the inversion between the two inputs. You need to null out the positive offset voltage that comes with the direct signals from the DAC. That voltage is about +1.25 to +1.4 VDC. Using the ration of -3:2, you can calculate that you need a negative voltage of about -1.8 to -2.1 VDC. The 500 ohm trimmer will allow you to dial that in, so you get that from the negative voltage regulator. This is like a course adjust. The fine adjustment for each channel is the 500 ohm trimmer that is part of each correction amp channel.
There is an important note here: This circuit only allows you to adjust within the range of that 500 ohm trimmer that is part of the offset voltage input to each channel of the DAC. So, what you need to do when adjusting the whole amp is find the channel that has the lowest positive offset. Once you find that channel, then you turn the 500 ohm trimmer for it all the way out CCW. Set your course adjust trimmer on the negative reference so that you get a very small positive voltage; like 10 to 50 mV. Then use the trimmer on each channel to bring that down to zero volts. The point is, if you set the negative voltage reference such that any of the channels are over compensated, you can not correct for it in the fine adjustments of each channel. Those adjustments will only allow you to add
more negative voltage.
For this reason, it is important that all of the fixed resisters are as similar as possible. When I make correction amps of this design, I measure a bunch of resisters to find groups of 6 or 8 that all measure the same.
The
IN is the direct signal from the output of each channel of the DAC.
I hope this help.
Please feel free to call me on the phone or on Skype if you have any more questions or if you just want to talk about lasers!
330 762-7137
james_laserboy
James.
