It always seemed like the community here was quick to respond to even the simple project, so thanks for all of the responses so far.
Why Not Digital (ADC->MCU->DAC)? - MosherIV
- I hadn't really considered it before, but I suppose its possible.
- Potentially a continuously adjustable range which may be useful in some testing/debugging situations.
- The tricky part being low voltage/low noice DAC section for +-2mV at 0.1% (+-2uV), seems reasonable to use a normal DAC with a precision 1000:1 voltage divider, just have to deal with noise.
- However, this would not likely not work with load cell amplifiers that measure excitation current to compensate for lead length and do not have dedicated sense lines.
- It may be possible to simulate the proper excitation current with a DAC driven current sink.
- Would have to watch power management closely for low excitation voltages.
- Trickier to allow AC excitation, but not impossible.
- Overall, this is a great idea. However, I personally would like to try and accomplish this with an analog solution first, then maybe try digital after that.
Next, lets narrow the scope of the project a little bit. Here is updated goals:
- Target 350ohm bridge load (Kleinstein), no need to be able to do all 3 at once. Should be easy to apply same principles to other loads
- Limit the number of steps to 5 (Kleinstein/jbb), 10 steps does seem like too many for an initial project, 5 discrete steps seems like plenty for its intended purpose.
- Target of precision is 0.1% across 5°C-45°C, not really precision but good enough to help spot issues in the field, may be able in do better in future revisions
The first post was a little short on construction details, so here's some more details
- The original plan was to use a nice (low contact resistance/low thermal emf) multi-position switch. Most likely, it would be salvaged from an old decade box/transfer standard.
- Both reed relays and other signal relays seem to have contact resistance specs (~100mOhm) that may make parts of this project more difficult.
- Most multi-position switches have 10mOhm max initial contact resistance
- Its expected that either solution would gain contact resistance over time, so it seems that starting out lower would lead to less error
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Anyone have any information on contact resistance vs cycle count for multi-position switches or relays? - For each step, the switch would be adding a different set of resistors both in parallel or series with the bridge legs in order to achieve the desired change.
- This is where the precision resistor stuff comes into play.
- It would be easy enough to get resistances close using discrete precision resistances
- Fine tuning can be accomplished with low TC trimmers
- It also seems like the better approach would be to keep adding resistors in parallel/series such that the power (and heat) are more distrusted, instead of switching in and out
- As mentioned, the resistors that are added in parallel are going to be less critical, so it should be easy enough to come up with proper combinations
Kleinstein also had a good idea worth exploring, split the bridge into a static section and a active section. For a 350 ohm half bridge to output 2 mv/V, the top and bottom resistance need to change (in opposite signs) by only 1.4 ohm. So it could be achieved using 340 ohm resistance of high precision, and then a set of higher tolerance resistors for each step. In order to remain in spec, the last 10 ohms of each leg would only need ~0.25% tolerance. It would be even better to split it at 345 ohm for the base and 5 ohm for the active, then the last 5 ohm would only need 0.5%.
![](https://i.imgur.com/dpsIDGP.png)
This seems like a much better topology for this project. It also limits the number of expensive components.
My next steps are to spend a little time with some uncertainty analysis to determine the actual tolerances needed for my components over the temperature spec range. Then after that, onto laying out the construction details.
I am thinking of cross posting to Metrology but maybe later. I think as a project it fits better here, but when it comes time to do adjustments and calibrations, Metrology seems like a better place.
Thanks for everyone's replies so far, and please keep them coming!!!