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My plan in the below circuit was to use a voltage divider from a Max6225 2.5Vref, to get 2V and 1V from it in order to use the full 16bit bipolar scale of the ADS1115s, but I'm getting strange resistance readings from a DMM across the resistors - can I use a voltage divider with the Vref like this?

EDIT: Apologies for the drawing - not sure how to do the buried zener vref part, so I've marked the connections instead OK.

EDIT 2: Please ignore the test resistor in the bottom picture - this was just to test if the same thing happened however I connected a resistance from Vref out back to ground, rather than a short or something on the board

enter image description here

Using Vref with voltage divider

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  • \$\begingroup\$ How did you measure strange R's ? The Max IC can supply 20mA \$\endgroup\$ – Tony Stewart EE75 Aug 30 '18 at 22:14
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    \$\begingroup\$ Preferably not, why would you use a precise voltage ref to spoil it with resistors? At lest you should buffer and divide with some opamp. The best is to use separate voltage refs. \$\endgroup\$ – Marko Buršič Aug 30 '18 at 22:16
  • \$\begingroup\$ @TonyEErocketscientist Without a power supply connected, I connected the DMM across the resistors. The bottom (in the picture) blue resistor is 3.09K - but came out 2.54 for example. \$\endgroup\$ – Oliver Walters Aug 30 '18 at 22:25
  • \$\begingroup\$ @MarkoBuršič For one, I was going to use it for some thermistors in a voltage divider format. I'd rather not have to use seperate vrefs - if Opamps will solve this problem I will do that then, cheers \$\endgroup\$ – Oliver Walters Aug 30 '18 at 22:31
  • \$\begingroup\$ It looks like you have two resistors in parallel connected to one in series. You can't make voltage divider this way. All three resistors should be in series. However it is impossible to tell for sure from your photo. If you want definite answer you need to post a schematics, or at least the part of it in question. \$\endgroup\$ – Maple Aug 30 '18 at 22:49
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IMO trying to tweak dividers and multiple op-amp buffers is a bit passé.
Since you already have a MAX5541 16 bit DAC on your board, which I assume is using your MAX6225 as its reference, you could simply generate the 2V and 1V references from that and store it in a Maxim DS4303.

The DS4303 will provide a 12bit resolution of your reference voltage, but you can tweak the expected output if you have an accurate enough 4-6 digit multimeter.

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