Shunt Reference Problems

Michael Kellett · 2020-02-16T14:51:21+00:00

I?m working on a power supply design. It has 4 independent and isolated channels which use Microchip LM4040 2.5V references. It also has opto isolated readback for each channel which uses Analog ADR530B references and a tiny ST micro. The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm max. There is a note saying ?Guaranteed by design, but not production tested ? . When I ran it though a temperature cycle, I was a bit disappointed to get the results in For the temperature tests the power supply runs off the mains, its on board processor reports results to a PC outside the test chamber. The 4 PSU outputs are also monitored by a Keysight 34972A outside the chamber. I?m not bothered by the actual output voltages ? these are dependent on the cheap and cheerful LM4040s and within spec (although I don?t like the steps on the blue 15V channel). The 15 and 5 V measurement errors plots come from comparing the measured (by 34972A) outputs with the self-reported output based on the ADR530B references and the temperature drift is much higher than 40ppm. My first thought was that external parts, probably voltage divider resistors or maybe even the tiny processor ADC was causing the problem, so I replaced the resistors with expensive ones and added another 4 x 34972A channels to monitor the ADR530Bs directly. This got me There?s still some bad stuff going on with the blue 15V channel ? but I don?t care about that yet ? concentrate on the last plot ? that?s the 4 ADR530B chips monitored directly, the temperature coefficients over the 5- 60C range work out at: 65, 56, 53, 53 ppm/C. Well, it could still be my ADC or some other thing I?ve missed in my design, so I built a little test board with 2 x Microchip LM4040 and 2 x Analog ADR530B, there?s a picture of it set up in the temperature cycler. Each ref chip is connected to a common 10V supply by a 2k2 resistor. Each ref chip is shunted with a 470nF cap. The results are shown in The...

Feb 16, 2020 40 Replies

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legg 6 years ago

As a precaution, I always isolate the the 34972A measurement lines with >=100R in series with each wire. You can add that into error, if significant. It should be constant, if you're measuring deltas.

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