Null voltmeter

[Echo88]

Full ack on the last post Kleinstein and now i get why you mentioned a MOV: the symbol that is used in front of the main OP is a MOV-symbol, i intended to use it as a symbol for a thermal jumper but forgot to add the Theta (dont know if theres a more fitting one) 
We´ll see in the end if they aid the project.

Heres the changelog for the current version, that is attached:

Low leakage diodes are now BAV199, typical leakage with <1pA is good enough, see:
https://www.eevblog.com/forum/projects/forward-leakage-of-a-diode/msg1287718/#msg1287718

Added LDO-regulators

Detailed the battery indicator section, taken from the Datron 4910. Current consumption with suitable
LEDs is marginal compared to the OP quiescent current. Only implemented for pos battery, should be good
enough as an indicator supposing nullmeter quiescent current draw is sufficiently symmetrical during operation
and recorderoutput draws current from positive supply, making it the fastest draining battery and therefore the most reasonable
to choose for "battery low"-warning.

Improved recorder-section, should now be bipolar input capable and draw absolutely neglegible current, see
https://www.elektronikpraxis.de/galvanisch-getrennte-emi-freie-messschaltungen-a-1102943/
Before i assumed the IL300 needs mA LED-drive current and V->f-converters that i found also needed at least 1.5mA

A 6V-lead acid battery enables 2.5V and 5V supplyvoltage, as the LDO-dropoutvoltage and deep discharge voltage
of the battery is fitting for the purpose. With 1.3Ah they should last 13d at expected 4mA max current draw.

What still needs to be done:

Finalizing all component values and producing a BOM
Checking the correct function of the IL-300-section

I found a few nice cases from Hammond and suitable rotary switches, gotta do the mechanical planning now.

If someone has bipolar capable very low quiescent current (<1mA) V->f and f->V-converters im also interested.
Im wary of digital isolators (capacitive and inductive) and similar stuff as a solution, since id like not to worry about the resulting EMI.

[Kleinstein]

If all amplifier parts get away with a +-2.5 V supply (or maybe a little less) one could opt. for a single battery and virtual ground. So only one 5 V regulator and than a OP-amp to buffer half of that as a virtual ground.

The IL300 part should get away with a less expensive amplifiers. The signal is +-1 V range and I don't think one would need a 5 digit dynamic for the output. Some offset can be compensated relatively easy.
For high resolution use one may want a little more current there. It is a compromise and still relative easy to change later if needed.
For the output side It may help to have only 3x1.5 V (or oven just 2x1.5) so that 5 V range amplifiers are possible.

The 10 K for the input protection are relatively small compared to the 2x22 K in the filter. For ESD spikes it would help if the 10 K can withstand higher pulses too.  So the combination looks a little odd. I would try to get away with smaller capacitors, so that the input does not get too slow with a high impedance source.

The LP filter is quite slow responding - it may help to have the option to bypass the filter, especially for the analog output or higher voltage ranges. The low speed is mainly needed with the low ranges to suppress hum and noise.

[JohanH]

What still needs to be done:

Finalizing all component values and producing a BOM
Checking the correct function of the IL-300-section

I found a few nice cases from Hammond and suitable rotary switches, gotta do the mechanical planning now.

If someone has bipolar capable very low quiescent current (<1mA) V->f and f->V-converters im also interested.
Im wary of digital isolators (capacitive and inductive) and similar stuff as a solution, since id like not to worry about the resulting EMI.

This is interesting. I like the approach. Any status update?

I'm building a KVD and already have the Hoffman Null detector adapter for a DVM. But it would be awesome with an analog display. I have to finish the KVD and some other projects first, but after that I will take a look at this. The meter and switches will be most expensive. I have some Elma switches, but they are only specified at 35V DC. There are also Chinese rotary switches specified for 150V DC. Could be a better alternative.

As Kleinstein mentioned, a single battery would be nice, too. Which op amp are you using (planning to use?) in the latest revision (Main OP, low pass and Post OP)?

[Echo88]

Hi Johan,

im not quite finished with it. But so far it works nicely apart from 3 small bugs, and needs some cosmetic changes in the mechanical pcbs and i'll exchange the knobs for more fitting ones.
Noise is about 150nVpp with the used AD8628 which are used for the complete analog path for simplicity.
It differs slightly from the schematic in the last recent post:
-only one 12V-battery is used together with a railsplitter and a ultra low quiescent current DCDC-converter for the isolated output
-with the used 7Ah battery the runtime is about a month at roughly the same case size as nullmeters like HP419A/AVM2000
-a battery charge/protection circuit was added
The panelmeter was taken from a damaged K155, but the circuit has provisions to also use normal (non +-) panelmeters.
The idea for the binding post thermal baffle was taken from the AVM2000. It can be screwed on the case-backside when not used.
 

[JohanH]

That's so awesome. I already found a null meter (with mirror scale) on ebay and ordered it. M265M 100 - 0 - 100 µA. Not as nice as yours, but will do.

What devices did you connect to the guard post? I think in null meters such as Keithley 155 only potentiometer shields are connected. Not sure about that, though. Manual states that guard should be tied to circuit gnd (-) if not used (they do it on the rear terminal). And is your inner box connected to the chassis?

[Echo88]

The guard is connected via two capacitors to outer case and negative input like in the K155, the inner case (isolated with standoffs) is also connected to guard. That should aid CMRR in my understanding, but i could be wrong.
See for reference http://hparchive.com/Application_Notes/HP-AN-123.pdf
Guard connection depends on the specific use case as lined out in the manuals of nullmeters like the K155.
Specific measurements like CMRR, NMRR, noise, etc. to completely characterize the nullmeter werent done yet though.