While I was slowly progressing with my Arduino based precision peltier controller (update soon), I stumbled over other things that took some time to sort out.
First, my valuable ultra precision 8-1/2 digit Datron 1281 DMM stopped working. I spent some time checking voltages, ripples, ROM images, but this seemed to be a more subtle problem and threatended to tie me up indefinitely. So I sold in on ebay, and got in return as much such as to afford a brand new Rigol3068. This has only 6-1/2 digits but this is enough for my various applications (temperature and laser current control).
I am very happy with the Rigol3068, it sports also an Ethernet connection which helps me to go beyond GPIB control which gives problems all the time. An advantage is also that it has 240000 counts, which means that I can measure 10K Thermistors (at around 13KOhms or so) with full 6 1/2 resolution in the 20K range, rather than having to use a more typical 100K range which cuts down by a factor of 10 in resolution. So I can safely resolve temperature differences of 1/10000 degrees.
When tinkering around I remembered I had also a faulty Solartron7081 8-1/2 digit voltmeter around and looked again into it. It had intermittent thermal problems and I had repaired a few of them by replacing ILQ74 optocouplers a few years ago. Still, some mysterious 80uV jumps remained without explication, plus an elevated noise level of about an order of magnitude larger than expected. Such things are notoriously difficult to get a handle on.
While at it, I stumbled over the eevblog which adressed these questions in this thread (me calling myself bertik for silly reasons). So I managed to relate these jumps to a faulty auto zero mechanism, which I circumvent by disabling it.
While investigating there was a serious setback: when checking the current setting for the reference Zener, I inadvertently shorted the test pin 301 with the – input of IC 304 (refers to service manual). There was a sense of smoke, which was surprising, since this test pin is supposedly at ground. I checked and alas, there was 17V AC voltage at it! With source impedance of like 2 Ohm… how could this possibly be? Was it the effect of the short? How can 17V AC go straight into the reference voltage section?
After disassembly it turned out that the test pin was wrongly placed by the manufacturer, namely on a via. And indeed there is 17V AC voltage there, fed across half of the board to a few mm near the reference section (!). It enters IC303 which is close by. So that explained it. As a consequence, IC 301 and IC 303 where blown and possibly more.
I decided to by-pass this problem by substituting an (aged!)
+10V/-10V precision reference
VRE102CA of which I had salvaged a few from trash. It can be made fit perfectly into the reference section of the 7081, by attaching the outputs to the reference terminals TP 302/TP 303 and opening the base and collector connections of TR 301 and TR 302. This works as good as before, the reference has like 1ppm/C temperature stability and like 3uV pp noise 0.1-10Hz.
For the time being I’ll leave the reference section like that, since the excessive noise problem is the next pressing one. I will report on any progress in the future.
How do I know the noise level? This has to do with another thread on eevblog, namely this thread. There a DIY low frequency noise meter was described, and this is what I wanted anyway, for various reasons. First, for checking out various voltage references that I am going to use for laser drivers. Then, primarily, to measure low intensity noise of the laser driver outputs. I like to get below like 1uA pp noise at about 200mA. So this calls for proper measurement hardware.
Given the valuable tips of very competent people in ths thread, I quickly built my own version. It works fine with a base noise level of 200nV pp at 0.1-10 Hz. I use only half of the dual opamp since I shot the other half ;-(, otherwise it would be in the order of 140nV pp. With this in hand, I characterized differend voltage sources, which is important also to judge the disease of the Solartron7081. Here a few results — more to follow:
VRE102CA (10V): 2.7 – 5uVpp, depending on specimen
VRE102CA (10V), two parallel: 1.9uVpp, 0.33uVrms
MAX6250 (5V): 1.6uVpp 0.32uVrms
Data Precision 8200 Calibrator (10V): 12 uVpp, 1.9 uVrms
Base noise, 200nV pp:
(100mV corresponds to 1uV).
Dual VRE102CA noise, 1.9uV pp:
The tips in the DIY low frequency noise meter thread were extremely valuable, in particular the issue of leakage current of the input capacitor. I never thought that this could be handled with hobbyist means. But indeed checking out a few caps I had around, there were some with <1 nA leakage current, so this was fine! Still this is a slightly tricky business.
Here are some extra comments:
1) Shielding: an aluminum case with slots on the sides didn't work at all due to overwhelming 50Hz noise. One really needs a kind of watertight shielding, I used a classic Teko case intended for RF applications. Actually two nested ones, the outer one holds battery and supply voltage splitter. I also use switcheable bandwidth control because sometimes I may be interested to go beyond 10Hz.
2) Ground loops. When measuring references in existing other setups, one is more or less guaranteed to get ground loops, at least if the scope input is grounded. This shows itself in an elevated noise which does not necessarily looks suspicious. I noticed it when connecting the input to a function generator to track the frequency response (via a 1M/10 Ohm divider).
The solution was to put between amp and scope an extra isolation amp. I use an AD210AN which I had at hand, and I also added an extra gain factor of 10, so the total gain is 10^5. This means 1uV AC input will come to 100mV output.
Now the story continues to go on, in a related direction, since in this thread it was discussed that ultra-cheap Chinese Zener diodes may have noise levels which are lower than the best industrial references. This created a group buy frenzy which reminded me of the one of a few years back about the blue 1W laser diodes – a gold rush type excitement where people buy dozens if not hundreds. Obviously I couldn’t resist to order a dozen or so as well, for potential application to ultra low noise laser drivers.
When these little gadgets arrive, I will conduct tests which are quite similar to what I was doing for laser diodes since years: automated high-precision scans over temperature and current. Let’s see what will come out, stay tuned.