Allow me to offer some insight on this.
Originally Posted by
HankLloydRight
Hello folks.. I've got a question about how to handle the X/Y differential signals.
From everything I (think I) understand, the X-/Y- signals are just the X+/Y+ signals inverted. And X+/Y+ signals are 0v to +5v and X-/Y- are 0v to -5v.
Say I'm building an ADC for ILDA signals (input), I know I can run the X+/Y+signals into two ADC channels, and then invert the X-/Y- signals and run them into tow more ADC channels -- but that uses up four ADC channels. But is that necessary?
Consider these two other options instead:
1. Invert the incoming X-/Y- signals with op-amps, and then average them with the X+/Y+ signals (in theory they should be the same, right?) -- and then feed that average X/Y signal into the ADC using two ADC channels
2. Only sample the X+ and Y+ lines (using two ADC channels) and just ignore the X-/Y- signals, and just mimic the X-/Y- signals in software by inverting the X+/Y+ data stream before writing out the digital data stream.
Any flaws in these approaches? Thanks.
Sorry for this late response but I only just saw this post.
This relates to processing only X,Y digital signals and assumes binary values from 0 to whatever the upper bit range is (256, 1024, 4096, 65536, etc.).
1. Use a dual, 4 quadrant (aka bi-polar), multiplying DAC whether 8-bit, 10-bit, 12-bit, 16-bit or whatever. Dual means "2 Channel", one for the X signals and one for the Y signals.
2. Multiplying DACs have a Reference input for each channel that controls the maximum peak-to-peak analog voltage swing above and below analog zero volts, the Analog Devices AD7528 is a good example of an 8-bit, dual, multiplying DAC.
3. For DACS that have unipolar analog outputs of say, 0-5v, their outputs can be shifted by using a simple unity-gain op-amp summing amplifier with two inverting inputs. One input connects to the o-5v analog output from the DAC and the second input connects to a stable -2.5vdc reference on the second input. The result is that the summing amp shifts the DAC output by -2.5v making it have a bi-polar voltage swing of 2.5v. If the summing amp has a gain of 2 then the final summing output would have a + and - 5v signal swing. We refer to this type of bi-polar output signal as an unbalanced output since the active signal is measured with respect to analog ground.
4. Once the unipolar DAC output conversion to bi-polar analog signal is achieved then the bipolar output is feed into an op-amp circuit that converts an unbalance output to a balanced output that the standard ILDA projector input connector uses for XY signals.
An example circuit might look like this (this is a correction to my earlier schematic post):
The 50K potentiometer should be a 10-turn, precision trim pot and set to be a negative voltage equal to half of the peak value of the DAC analog output voltage, or about -2.5v. The final differential output voltage level signal swings should be close to +10V and -10V.
Let me know if this helps or if you have questions.
Last edited by lasermaster1977; 10-29-2022 at 16:12.
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Everything depends on everything else