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A project log for Precision Voltage and Current Reference. A little something to check your digital multimeter accuracy occasionally. I was careful to solder all of the surface mounted parts except the REF, which I hand soldered to the PCB using only the 5 pins that were connected to the circuit. I made sure to remove all of the flux with alcohol and a toothbrush.
The through-hole parts were then hand soldered and the flux cleaned up as well. The LT references were inserted into gold plated sockets. When power was applied all of the circuitry functioned as designed at first glance.
When I first applied power to the board the two LTC references measured within a few mV of the target value. The REF was sitting at about 3. My first thought was that I had a defective IC. I was able to trim the part in the range of 2. I Googled "REF trim error" and the search brought forth many references to the data sheet -- even a data sheet with a Burr-Brown logo, which must have been published back in the last millennium. Nobody was complaining about the trim circuit.
I used to play the viola the trim became much more manageable. I could now trim the 2. When I tested the heater circuitry I used an open cell foam which probably does not have a very good R-factor. I used a styrofoam packing material when I assembled the two PCBs into the x68x50mm plastic box that I intended to use for this fixture. The component values that worked for the bench testing did not work for the new box configuration.
The improved insulation and enclosed box allowed for much lower power dissipation in the heater to reach the same temperature for the set point. The first cycle with the breadboard component values required about two hours to settle to the set point temperature, but the step response indicated that the system was pretty stable with a phase margin of better than 45 degrees. The result was a shorter settling time, but less phase margin maybe less than 25 degrees. This is what the response to a perturbation looks like:.
Pretty ringy-dingy, but it still settles to the correct set point in about 30 minutes. This data is produced by the Keysight A DMM, which doesn't attach a timestamp to the data -- go figure -- I had to assume a sample rate for the above plot to get the time information for the x-axis.
If this was a design meant to be for production I would never go for this, but for a one-off it might just work. It's pretty clear that my attempt to model the physical and electrical system was way off, but it's not worth the time to figure it out for just one instance. I let it settle for about 1 hour and took some measurements. The high voltage references were spot on after trimming, but the mV reference was too high -- measuring I was expecting it to be less than 0.
In addition, the 1V reference was a bit high as well, measuring 1. Both of these reference voltages were derived from resistor dividers. I removed the top cover and proceeded to probe various nodes in the circuit to get an idea of what could be wrong.
My initial suspicion was that there was a voltage drop caused by the PCB trace resistance. But the thermocouple errors should cancel if the temperature differences are small. Apparently, the heater and reference PCBs are close enough that the heater can manage the temperature of the reference board even without a cover applied. The power requirements will be higher, but it was doable.
After I closed the box and screwed the lid down I let it soak for about 30 minutes. These readings are well within the specification tolerances of the A DMM. Create an account to leave a comment. Already have an account? Log In. A few years ago, I also had the trimming problem on the reference. Are you sure? Yes, delete it Cancel. About Us Contact Hackaday.
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A project log for Precision Voltage and Current Reference A little something to check your digital multimeter accuracy occasionally. Assembly: I was careful to solder all of the surface mounted parts except the REF, which I hand soldered to the PCB using only the 5 pins that were connected to the circuit.
Two Problems: I spent a couple of days chasing down bad assumptions. The 2. This is what the response to a perturbation looks like: Pretty ringy-dingy, but it still settles to the correct set point in about 30 minutes. Previous Log Results! Become a Hackaday. Sign up. Ok, I agree.
REF5025 G. Datasheet pdf. Equivalent
10PCS REF5025I REF5025IDR REF5025 REF5025AIDR SOP-8
Low Noise, Very Low Drift, Precision VOLTAGE REFERENCE