Electronics > Metrology

Analog frontends for DMMs approaching 8.5 digits - Discussions

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Echo88:
This thread continues the analog front end discussions from the HPM7177-Thread, so it wont get even more offtopic.
Basically this thread should focus on equipping very good ADCs like Kleinsteins Multislope-ADCs, the AD7177 or AD4030-24/4630-24 with suitable analog front ends (high impedance, protected...) that dont degrade the performance that can be achieved with said ADCs.
So far we discussed my approach to build a +-10V high impedance AFE for the AD7177.

To learn how to do it correctly im now studying the 3458A schematics and copied the DCV-path (simplified/omitted control ICs) up to the ADC in Kicad, to make it easier to understand/follow. Maybe its also of interest to other users.
Im thinking wether i should also include the current source/ohms measurement (ACV is another can of worms...) or if that´ll make it too crowded...

Do you think we should first go through the 3458A-DC to then derive a suitable schematic for the AD7177 or maybe also for your Multislope-ADC Kleinstein? I know that youre interested in applying ACAL with your ADC.
Im still having a lot of questions regarding the 3458A-schematic, maybe the more experienced 3458A-scholars know the answers? :)

Questions:

Is the 1V Signal at Q21 used to do the 1V-range ACAL?
Is the 1:100 HV divider RP7 used for the 0.1V-range ACAL?
Why are the switchable signals at the JFETs divided by Q12 into two signal-"trees":
-maybe one for positive "hot" signals and the other for "gnd"-like signals?
+CurrentShuntOut on one tree, -CurrentShuntOut on the other?
What do Q23/24 measure exactly with their resistors to gnd1?
How exactly are the jumpers JM201-207 on the bottom right page of the "Sentry Current Shunts" page compensating the
TC of the shunts (so ive heard)?
   
Regarding the HPM7177-frontend we discusses beforehand Kleinstein: Does a completely symmetrical AFE (till the ADC level-shifter/driver) like attempted for the HPM7177 have any advantages compared to the approach to just set one voltage input to ground directly?

Schematics for the HPM7177-approach and said 3458A-DCV-schematic are attached.

https://xdevs.com/doc/HP_Agilent_Keysight/3458A/doc/3458A%20CLIP.pdf

dietert1:
For analog font ends (with their ADS125 24-bit ADC) TI recommend a different kind of multiplexer, with higher channel resistance and much less leakage than the ADG1408. Their recommended MUX36D08 is better than the MAX328 that i used to replace the JFET input MUX of the R6581T. Both of them are specified with leakage around 1 pA at 25 °C, except the TI part has 20x less channel resistance.

Regards, Dieter

Kleinstein:
The R6581 input is not a good example: it lacks some kind of precharge phase and thus has quite some charge pulses from the auto zero cycle.  The way the JFET gate drive signals are generated is also not ideal.

It makes some sense to inlcude the additional input path(s) for the amps range in the concept. The other point with the current ranges is that they are also linked to the low side / COM terminal and they may effect the protection, if there is more than low side instead of 1 universal COM.

The AD7177 and other ADC chips are usually differential. This can make different front ends more practical. Instead of switching between the input an zero one can do the AZ cycle of a differential ADC by swapping the inputs. Another point can be 4 wire ohms, that could use a real differential input.

The 3458 like many old DMM use JFET switches. In many aspects the modern CMOS switches can make the life a lot easier, especially when it comes to multiplexers with many inputs and if there is no good guard signal from the input to drive the JFET gates.

A big decision to make is whether one wants to use zero drift amplifiers (often as OP-amps) or wants the auto zero switching directly at the input. Both ways are possible an have there pros and cons. The auto zero switching does produce switching spikes even with precharge. These are less frequent than with an AZ OP-amp, but with the variable voltage the spikes tend to be larger and more tricky to suppress.

David Hess:

--- Quote from: Echo88 on September 17, 2022, 11:26:48 pm ---What do Q23/24 measure exactly with their resistors to gnd1?
--- End quote ---

My guess is that they are for measuring leakage current.

Kleinstein:

--- Quote from: Echo88 on September 17, 2022, 11:26:48 pm ---Questions:
...
Is the 1:100 HV divider RP7 used for the 0.1V-range ACAL?

Why are the switchable signals at the JFETs divided by Q12 into two signal-"trees":
-maybe one for positive "hot" signals and the other for "gnd"-like signals?
+CurrentShuntOut on one tree, -CurrentShuntOut on the other?


--- End quote ---
The HV divider gives a 0.1 V signal that can be used for the 0.1 V ACAL step, though with a little extra noise and high impedance that would need some extra waiting time.
An alternative 0.1 V signal could be obtained from the current shunts, but this has also additional noise. The HV divider is likely the better signal.
Ideally one would have an extra 0.1 V signal from a lower impedance divider. For an improved accuracy and as an extra self test it even makes sense to have both positive and negative signals for ACAL.

Yes the separtion in 2 parts is for the Az cycle switching: one for the signal side and one for the more GND like.
The compensation for the switching spike is quite some effort (the extra DAC and OP-amps around this and in parts also the gurad amplifier for the boot signal). With the extra switch Q12 this is only needed once and not for multiple inputs separate.

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