Design validation: M101 DMM input section. Will the MUX voodoo bite? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 07:48:47 am
content: So here is the input section of my M101 DIY DMM. Is it well protected? Will the input section, dominated by the analog mux and the instrument amplifier work? Will the switching of instrument amplifier gain-setting resistor bite me? (drift of the pots can be disregarded, because I can the LM399 reference is connected to the MUX can be used to digitally calibrate the resistor drifts out.)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 22, 2015, 08:59:17 pm
content: Switchung the gain this way does not work very well. The resitance of the switches is not that small compared at least with the lower resitors.Usually most of the circuit dos not meed differential signals, but a good star GND will do.The current ranges also look strange - not working well this way. Usually there is one dedicated amplifier (e.g. x 10) for the signal from the shunts - going to the MUX only after that. Many resistors in series is not a good shunt.The MUX only work with the inputs inside the supply range. So it's not possibly to use them directly for the voltage inputs.So better have a look at existing plans first, even if this the way it was done 50 years ago with analog meters.

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 09:04:53 pm
content: Quote from: Kleinstein on August 22, 2015, 08:59:17 pmGain switching, as I said, is not an issue. The LM399, being an option at the MUX, will allow the meter to automatically calibrate out those resistances.If you paid a read to my finished project post you will see that my design requires a floating input to work. All voltages after the instrument amplifier are GND-referenced though.Also the current shunt is the 10mOhm resistor. The long string of tapped resistors actually is the input divider network and that is built with matched 0603 resistor networks.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 24, 2015, 12:33:30 pm
content: Oh - I missed that there is only one current range, but a rediculous large number of voltage ranges.Still the MUX at the input will not work for input voltages higher than the supply. Even for the inputs not selected, the voltage must be within the supply range. The 1 M resistor will limit the current so not to damage the chip at moderate voltage. However there will be still some current in the µA range flowing over the chip substrate - so accuracy can be rather bad, as you are out of the normal workung range.There usually is no use of so much division at the input: even for 1000 V at the input no more than a divide by 1000 is usefull, if the circuit can handle 1 V. Often it's even better to have the input amplifier capable to work up to 10 V and only use a single 1:100 divider at the input.Leakage from the MUX may also mess up the high impedance divider. So in short: a CMOS MUX is not a good combination with the input voltage divider.On the other side, the current shunt can be directly connected to a precission amplifier - good AZ amps are relatively cheap now, a good relay is more expensive. So no more need to use the relay at the input to switch between voltage an current reading. A relais might be an option at the voltage divider. Choosing between voltage, current and what ever readings can be done after amplification.The resitance of the CMOS switches is tempearture and voltage dependent. So it's difficult to near impossible to compensate for the poor gain switching. It's much better to do the gain switching in a way that only the OP inputs are switched - this is possible too. The other point is, that one usually only needs 2 or 3 gain ranges with a decent ADC. More ranges where good with analog meters.Many good dsktop DMMs use a rather simple voltage input setup (at least in principle):1) Divider of 1:100 or not - switchung can be by relay2) single ended input amplifier capable of -+12 or so range. Amplification either 1 or 10 or possibly 100 - choose by CMOS switch3) if ADC has +-1 V range of similar: Divider of 1 or 0.1 - choose by CMOS switch

author: technix
author_rank: Super Contributor
timestamp: August 25, 2015, 04:47:18 am
content: Quote from: Kleinstein on August 24, 2015, 12:33:30 pmThe internal stages are powered using +/-15V and including the op amp, the full scale internal voltage is +/- 10V. The reason I have so many voltage stages is that I want to make full use of my ADC's 16 bit resolution at all time.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2015, 08:25:55 am
content: If the input Amplifier can handle 10 V, no more than a divide by 100 is needed to handle 1000 V. As there likely is switchable amplification available, there is also no real need to have a divide by 10 stage, unless you want more than 6 digit readings in the 100 V range too. So unless you want extra high voltage, the input divider only needs the two settings of 1:1 and 1:100.More division (and maybe a thrid range) would be something if you need higher voltages or have an amplifier that can not handle +-10 V.The alternative would be using more divider steps, an input amplifier that only uses a smaller range (e.g. +-200 mV or less) and less (if any) steps in switchable amplification. As leakage is less critical, its slightly easier to switch amplification than choose from more input steps. Also noise can be slightly lower - though at 16 Bit this is not such an issue. So if the higher supply is available it's usually better to go for few steps at the input divider and more at the amplification.