Electronics > Metrology

Analog frontends for DMMs approaching 8.5 digits - Discussions

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julian1:
I looked at a few GDT datasheets. 
Most parts seem to be +-20% tolerance.
To support a 1kV input, a GDT with rated spark-over of 1.2kV (or a bit more) would be needed.
And to avoid avalanche mode, the fets would need a VDSS of 1.4kV/1.5kV, to withstand GDT part tolerance on the positive side.
But these are a bit rare in DPAK, and a package change may be needed.

Furthermore, some GDT datasheets give more information for faster HV discharge events, for 100V/us, as well as more commonly quoted 100V/s figure,
The faster voltage event has a higher spark-over voltage. eg.

                                 100V/s  100V/us.
GTCA28-122M-R03 1200V    1900V

So I think this is also a nudge toward using larger fets.

I am somewhat confident the optocoupler gate drive can drive larger fet gates.
The most recent test showed a <=1us engagement with only 20V OVC, achieved by removing some (silly ) RC left before the zener clamps,
and by directly probing the isolated fet gate to source.

The zeners themselves will clamp the voltage Ok to protect the inputs on the fast time-scales.
But the possibility of damage to the fets exists if voltages exceed datasheet maxes. 

Kleinstein:
Getting protection to 1 kV and ideally also higher transients is hard when one also wants very low leakage and very low noise (not too much resistance). One may have to compromise a little between protection and noise / precision.

One point I find odd is the inductor in series to the GDT - the inductor makes more sense in the input path, possibly behind the GDT, to slow down the very fast transients and reduce EMI. Extra impedance in the path with the FETs limits the peak current, at least for the time until the GDT fires. Most MOSFETs can withstand some transient break through with limited energy.
As FETs may fail short it would be a good idea to have some secondary protection as backup, something like PTC or fusible resistor (or both). Still more elements in series add potential thermal EMF. So it is a compromise between protection and precision.
The Keithley 2001 uses 2 FET pairs in series for the protection to get better protection. The K2002 and 2000 use only 1 pair.

The PV optocoupler should also be OK with a larger FET gate. They can usually deliver a few µA and the gate current is more in the nA range, possibly more from the protection zener than the actual gate.
The FET case is anyway a bit tricky, as the pins are often too close togehter to really reliably isolate the high voltage.

I used the small DPAK for convenience and in my circuit accepted a lower maximum voltage.  A reduced design voltage make things a bit easier with the protection and also the relays.

David Hess:

--- Quote from: Kleinstein on July 09, 2023, 05:02:36 am ---Getting protection to 1 kV and ideally also higher transients is hard when one also wants very low leakage and very low noise (not too much resistance). One may have to compromise a little between protection and noise / precision.
--- End quote ---

Oscilloscopes use like a 470k series resistor before the shunt protection and then control noise by bypassing it with about 1000 picofarads of capacitance.  This sort of thing is also common on multimeters, but of course the noise is still a problem at low frequencies.

For moderate voltages, a small high voltage (120VAC) incandescent lamp can operate as both a series PTC thermister and fuse having the advantage of low resistance when there is no overload.  Very sensitive oscilloscope inputs may also use this method.

The least common method I have seen is a pair of anti-series high voltage depletion mode MOSFETs to limit the current before the shunt network.  Like the lamp this has the advantage of low series resistance until overload occurs.

Electrometer designs have high supply voltages available for bootstrapping their input buffer, and this can also be used to bootstrap the input protection to remove leakage through the shunt protection.

julian1:
I did some reading around fets and over-voltages, and the avalanche figure and 'avalanche rating' are the interesting figures/parameters.  It is a topic of interest to smps design. 
The series inductors to the fets, indicated with 100u (may need 1000u) will limit current rates.

Perhaps the GDT series inductors (1u) were intended to ensure that once the GDT goes active, they stay on for a period.
They appear dropped from later HP designs.

From a practical standpoint, I realized DPAK will fit in DPAK2 footprints. So part choice can be defered to assembly, rather than be baked into the pcb.
Although the smaller DPAK footprints part tidier.

There are some quite small and PTC devices, but thew are only low-voltages.

iMo:

--- Quote from: julian1 on July 09, 2023, 09:23:20 am ---....,,
..Perhaps the GDT series inductors (1u) were intended to ensure that once the GDT goes active, they stay on for a period.
They appear dropped from later HP designs..

--- End quote ---
I think putting any L in series with a protection device wired in parallel with the input wire will simply cause the rising edge of the transient will propagate into the input and not fire up the protection device (as the L will block the rising edge from entering the protection device)..

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