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| Hacking the Siglent SDM3055 Bench DMM |
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| Nornand:
Gainsetting seems to be 8 for the PGA. Vref/8=0.3125V |
| alexvg:
Yes the PGA is used... Shame. The DMM is not an internal 240.000 points because it's possible to have up to 312500 points. I've done a continuous decimation with a sinc function over 4096 samples captured with speed=FALSE. The result is similar to the speed=SLOW with 30sec statistics. I confirm the fact that it's possible to have a super fast (with a low resolution) bar meter and a high precision (slow) display number simultaneously. I'll try to enable a more faster tranfert between ADC, FPGA and ARM (4800SPS). I'll need to check if the current onboard Linux is compatible with critical realtime processing (or change to another Linux). I've found the LT6657 voltage reference... Better than the LT6655... Probably a stupid idea (or not) : If I'll build a LTZ1000AH voltage reference to supply multiple LT6657A-2.5. Does the final output voltage have a lower drift than a standard low-noise 5V voltage regulator powering multiple LT6657A-2.5 ? |
| Kleinstein:
A standard 5 V regulator (if not getting too much heat and variable current) is good enough to supply a LT6655 or similar reference. The next step up would be a bootstrapped supply - though it should not be needed here. In many aspects the LT6657 is not better than LT6655 (especially the LS8 version). Both are lower noise than the max6325, but the TC may be higher. The big weakness of all these reference is not so much the noise, but the drift, hysteresis and sensitivity to humidity. So the difference between the MSOP and LS8 case may be significant. Anyway for a 5.5 digit meter they should be good enough. As the ADC needs the 2.5 V reference level, there are not that many better references to choose from. Several in parallel help with noise, but only little with long term drift rate. The other limitation of the DMM design is that it is only +- 2.5 V (may be a little more, but not much) with high impedance. So there are limitations to the design, that can not be circumvented. |
| alexvg:
The LTC6655 (LTC6655BHLS8-2.5) and LTC6657 (LT6657AHMS8-2.5) have a better long term drift than MAX6425 after more than 1000Hr and the noise is lower : the ADC used does not like the noise into its reference. It's possible to use a LTZ1000A into the SDM3055 with a programmable voltage divisor (based on Vishay high precision low drift resistances). It's simple to do. I study all possibility : After some little modifications on the device, I would like to know what level of precision it is possible to reach. |
| alexvg:
--- Quote from: Kleinstein on September 02, 2019, 03:44:38 pm ---A standard 5 V regulator (if not getting too much heat and variable current) is good enough to supply a LT6655 or similar reference. The next step up would be a bootstrapped supply - though it should not be needed here. In many aspects the LT6657 is not better than LT6655 (especially the LS8 version). Both are lower noise than the max6325, but the TC may be higher. The big weakness of all these reference is not so much the noise, but the drift, hysteresis and sensitivity to humidity. So the difference between the MSOP and LS8 case may be significant. Anyway for a 5.5 digit meter they should be good enough. As the ADC needs the 2.5 V reference level, there are not that many better references to choose from. Several in parallel help with noise, but only little with long term drift rate. The other limitation of the DMM design is that it is only +- 2.5 V (may be a little more, but not much) with high impedance. So there are limitations to the design, that can not be circumvented. --- End quote --- You're right ! Using the LT6657 will provide lower performance than the LT6655 (LT6655BHLS) with the AD7190. So what to do ? 1. I could try to use up to 6x LT6655 ? (with a thermal isolation) 2. I could try to use up to 4x LM399A ? (but with a voltage conversion adding drift and low pass filter) 3. I could try to use one LTZ1000A ? (like LM399 with a similar voltage conversion adding drift and low pass filter) Don't know what to do... |
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