That's right out of the chip they use. Mostly the 71x6 (LCD) or 71x7 (LED) chips orginally from Intersil but second sourced by a half a dozen outfits. (Most cheap DVMs use the 7126, the low power version that needs a floating power supply/battery). I think most panel meters might use other versions like the 7106 (Cheap), 7107 (LED) or MAX138 (a version similar to the 7126 but has an onboard negative supply and has a real ground connection so it can run on gnd/+5). These are 3 1/2 digit (2000 count) units, if you want a 4 1/2 digit (20000 count), that's some different chip sets and cost more, and then for more resolution than that, you're in the big bucks territory.
The floating power supply reqirement for the 7106/7126/7136 comes from an internal shunt regulator that sets a virtual ground used internally a couple of volts below the plus side of the chip's supply. It can be burned out if you connect it up to the wrong polarity of the chip's power supply. That's why you can't measure the meter's own battery. (I did a more detailed post on this several years ago, but memory fades).
It might be possible to use one of these cheap meters with the negative supply at the ground for the rest of the circuit (ie be able to measure it s own power) , but as I remember it involves cutting the connection between the Common? pin and the output of the virtual ground regulator and then using a three resistor voltage divider to put the Input and Common within the common mode range of the chip. (I never got a round to testing this, and details are now vague. I think it might add more noise to the lowest digit).
Already suggested: Marlin P Jones, All Electronics Also: Jameco
Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)