Half power rail reference

I am mystified why you need this centre reference voltage to be so exactly half way between the two supply rails. Linear opamps work fine with a slightly asymmetric power supply provided that they don't clip.

You just have to make sure that everything is measured consistently relative to the right signal earth reference.

An instrumentation amplifier on the front end helps in extreme cases but it sounds like your sensor has already conditioned the signals somewhat.

You might want to consider a higher order low pass analogue filter in front of the ADC to attenuate any electrical noise that may be present from long probe leads and fluorescent lighting in the environment.

Search on Google groups if you can remember any keywords or authors.

No I'm blind and unable to print with a monospace font.

Your schematic is a good start but if you want advice then please spend some time improving it.

In your position I'd want to know the following in addition to the schematic you've already posted.

1. More information about the sensor characteristics is needed. How it connected to the board and by how long a cable? Is the cable shielded? What kind of cable is it? Is the grounding common? What is the source impedance?
2. What are the component values for the op-amp circuit and show the + and - inputs. It does not matter if the values are not yet finalised. What power rails is the op-amp using? What is the part number of the op-amp?
3. Is any protection needed for matters such as ESD or someone connecting the wrong sensor? Is there any risk of the sensor input going outside the range 0 to 5V? Perhaps if a fault occurs in the senssor?
4. Is filtering required above 100Hz? See Martin Brown's excellent advice.
5. Is that an individual DAC (Part number?) or part of a chip with multiple DACs (Part number?)

Expect more questions when the above is taken care of. Also expect advice from people who read this group who can trivially show you how to design the best signal conditioning circuitry for this application, but only if they have a clear picture of what you need. This doesn't mean they will just give you the complete design, but you might learn something.

If you have different sensors with very different characteristics then one schematic per sensor may be needed.

At this stage I'd be using a hand drawn schematic but ASCII art is ok.

Enjoy your ice cream.

r generating an accurate voltage at half the supply rail by switching an ou tput at a 50/50 duty cycle to feed an RC. I can't seem to find that thread . I'm wondering what the limitations might be on the accuracy. Obviously the difference in drive capability is a factor, but that can be mitigated b y the external resistor.

es within the digital device. That can be mitigated by using a slower cycl

waveform be 0.5 MHz.

integrating capacitor might be important as well. Also ceramic caps can b e microphonic, so film caps will be needed. In fact I need to specify that all through the sensor power supply.

the thread going all the way back to December. It couldn't have been that long ago could it? Seems like maybe three for four months back.

. That would add error somewhere between 0.1% and 0.2% (not thinking about it too hard). There will be other sources of error from this same effect elsewhere (dividing the 5V signal source to 3.3 volts... or maybe not. Per haps that can be mitigated by not using equal value resistors in the input and feedback paths.

think about this stuff. I guess they are mostly used to just plopping down chips. Heck, a new FPGA guy wanted to use a $20 Xilinx part instead of a $4-5 Gowin part!

or around a dime. I thought they didn't come that large. They do get larg e!

Please tell meh ow to calibrate absolute accuracy? I still don't follow wh y such a simple technique should not be used.

What does the mux do???

Sorry, that is your initial misunderstanding. I am not asking for design advice. I am asking for someone who remembers the thread where this was discussed a few months back. I'm not able to find it.

Rather than ask for fully documented design information, why don't you tell me what you are thinking and I can help you understand.

In that case could you please give me a simple means of identifying this specific ventilator (if that's what it is) so that if I ever end up in a hospital I can refuse to let them use it on me.

lol So do you have anything constructive to offer? I find it amazing that you need full specifications of everything in the design to talk about generating a half Vcc voltage using a 50/50 duty cycle digital output.

Whatever. "Forget about it Jake, it's the Internet."

I think enough has already been offered.

I'm not surprised because I haven't asked for "full specifications of everything in the design" just a reasonable description of what you're trying to achieve with your sensor to ADC circuit.

No-one except you would generate an ADC reference voltage like that. I'm not saying it won't work, and if you're trying to reduce component cost to an absolute minimum then it may help with that. But it wouldn't be my first choice for an ADC reference voltage in a hospital ventilator.

Assuming it wasn't sufficient to just use two resistors, or two resistors

I searched GG with "PWM DAC accuracy", and the first thread that came up was "Attainable PWM accuracy?" from a year ago.

Cheers

Phil Hobbs

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Thanks, that's not the thread. I did searches for splitting the voltage ra il to half voltage which is what I recall and didn't find anything. This i sn't really PWM because it's a fixed 50/50 duty cycle. I didn't see anyone in that thread discussing such a division.

Maybe I imagined it or it was in another forum entirely. I recall the guy doing it was talking about secondary impacts on the accuracy such as unequa l impedances in the high and low states of the output. Wouldn't be the fir st time an old guy's memory failed him.

I had forgotten that the resistor accuracy becomes a secondary effect on ac curacy, so I'm happy to use this to get very low error in the Vref with a s ingle resistor.

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You are asking questions about the cabling of the sensor to the board, a ci rcuit that is not even connected to the reference voltage except through th e power rails.

Ok, the sensor is mounted on the same PCB with an unspecified layout and an unspecified circuit to the ADC input, but will like be either a simple wi re, or a pair of resistors as a voltage divider. The sensor provides an ou tput voltage at a ratio to the sensor supply voltage depending on the signa l being measured.

The inverting input to the differential comparator has a capacitor to groun d, a resistor from the feedback output and either a series resistor from th e sensor or a voltage divider from the sensor acting as a series resistor a nd reducing the voltage.

None of that is in electrical contact with the non-inverting input which wi ll be connected to a capacitor to ground and a series resistor to an output switching at a high frequency up to 33.55 MHz.

I assume if you are blind you are reading this via a text reader? My 94 ye ar old friend who is blind uses an ORCAM device to read magazines. He is a military collector and is still doing research in his specialties. The de vice has a hard time with many of the terms. .22 cal reads as "twenty two California". WWII reads as "dubya dubya eleven". Sometimes it's just tota lly lost because it fails to understand punctuation. When he bought it the y said he could get updates to improve the product. They never said they w ould produce updates and they have not. Stinking Orcam! My friend is pret ty durn impressive really. He is just not giving up no matter what. Still doing the independent living thing, but he does make me think about my own mortality when I think about his.

Don't you think that if schematics were best done in words then that is how they would be done? For some reason most people don't seem to do their schematics in words, they use schematics instead. Why would that be?

Imagine what music would be like if learning to play the piano required reading a book along the lines of: First press middle C and hold it for half a second, at the same time press G a bit harder but hold it for one second and then also press E for two seconds. etc. For some reason it's not done like that. Why?

How does a blind guy who can't read the drawing I gave him read a schematic?

Clearly you are a troll. Go away!

I thought you said you didn't want design advice.