20V Input Voltmeter Using ICL7106

[d1983]

I am trying to design a 3 1/2 digit 20V voltmeter using the ICL7106 and I was wondering if anybody could help me with a few things:
1. How could I get it callibrated to a max 20V reading?
2. I will be using it as an onboard voltmeter for a variable DC power supply. If I tap the source 18V supply and convert it to +/- 5V will that interfere with my reading (Since I'm indirectly using my measured voltage as a supply to my IC)?

I know this is a bit of a specific question but any help would be greatly appreciated.
Thanks so much.

 

[berkeman]

From the ICL7106 datasheet at www.intersil.com, +/-5V supplies look like the typical setup for this DMM chip. Have you read the datasheet and application notes at the Intersil website?

 

[d1983]

The +/- 5V is standard for the ICL7107 which drives an LED display. The ICL7106 drives an LCD with a 9V battery.
I have read the application notes, and there is a way to make an autoranging voltmeter to get into the 20V range, but the schematic is a bit complex for me to reverse engineer it to a static 20V range.

 

[d1983]

I've been thinking about this today, and here's one idea:
I can put the input through a non inverting op amp in order to reduce the voltage by a factor of 10, then set up the reading on the chip to a 2V max (done in the spec sheets), and hard wire a decimal point.
Any comments on this idea? I'd like to keep error fairly low, so should I use 1% resistors? Is there a certain kind of op amp I should use?

 

[berkeman]

What is the output impedance of the voltage source that you are digitizing? If it's pretty stiff, you could just do a resistor divider with 0.1% resistors.

But it sure seems like you should be able to force the DMM chip into the 20V range. Sorry that I haven't had time to read over the datasheet more carefully.