Tech Forum

Although it doesn't use a microcontroller, Intersil described a circuit for making an auto-ranging DVM in their Application Note AN046. The URL for the PDF file of this article is www.intersil.com/data/an/an046.pdf.

Their basic circuit (Figure 1 in the app note) is for a 7106 DVM chip which is meant to drive an LCD display and run off batteries. A modification of that circuit is required to use a 7107 chip which uses a dual supply instead of a single supply, and has a higher power drain due to the LED display needs. That modification is given at the end of the Intersil article, in the text and in Figure 8. This circuit design utilizes the over-range/under-range states of the DVM chip to trigger the auto-ranging function, so depending on your application it might be of benefit to use an improved DVM chip that incorporates a zero-integrate phase feature to improve the recovery from over-range. Some manufacturers have added zero-integrate phase to their modern versions of the 7107 (e.g., Maxim), but DVM chip versions that explicitly include this improvement include the 7136 chip (sub for the 7106) and the 7137 chip (sub for the 7107). I noticed a couple of minor typos in the Figure 1 circuit of this article. The CD4028 logic chip is mislabeled as CD402T, and both output lines are labeled as over-range ("OR") instead of one line being labeled over-range ("OR") and one being labeled under-range ("UR").