Autoranging

[DSP newbie]

Hi,

how is autoranging implemented in the medium-to-high precision digital
voltage meters? Specifically, how do they maintain high input impedance
(that is, low input current), high dynamic range and high accuracy, all
at the same time? My Sanwa PC7000 proves it is doable. What's the trick?

Finally I have found some time to get back to my 1kW dummy load and
exactly this problem appeared.

Best regards, Piotr

 

It is well protected and high impedance which helps immensely.

It starts out at the lowest range and when it sees more voltage than the range can handle digitally switches in attenuation. It keeps doing this until the reading is not out of range. It switches the decimal point and on some display "mV" when it is at very low voltage.

This CAN be done analog but they are all digital.

The main thing is in the design, overvoltage must be withstood until the range goes down enough to handle it. To not damage anything.

They got some pretty cool designs but I am not into it because I don't have to be. I am busy with other things.

Know what ? It just occurred to me, is there suck a thing as an autoranging scope ?

 

[Piotr Wyderski]

jurb6006@gmail.com wrote:

> The main thing is in the design, overvoltage must be withstood until the range goes down enough to handle it. To not damage anything.

Clear. After some thinking about it I would add the second key
ingredient: DVMs don't have to be fast, so there is no point in
making a crazy 30-bit like ADC just to handle everything and then scale
the result down. As you
write, measure and bump up the attenuation. And survive in the meantime.

I can use the same technique to build my kind of floating-point ADC,
the voltage channel doesn't have to be particularly fast. The current
one is a different story, but its input range is much more limited and
I can use the standard saturating ADC trick or multiple ADCs in parallel
(each with a fixed, but different attenuation) or a multichannel ADC
cycling through different attenuators, whatever is more cost-effective.

Great, thanks!

Best regards, Piotr

 

[Jasen Betts]

On 2019-05-09, DSP newbie <nomail@gmail.com> wrote:

Hi,

how is autoranging implemented in the medium-to-high precision digital
voltage meters? Specifically, how do they maintain high input impedance
(that is, low input current), high dynamic range and high accuracy, all
at the same time? My Sanwa PC7000 proves it is doable. What's the trick?

Dave Jones (EEVBlog) gave a tour of the inside of an old-ish keithley desktop
voltmeter in one of his videos. lots of vacuum relays.

--
When I tried casting out nines I made a hash of it.