Below is a schematic of the components involved, what I found, and the interesting solution. What this schematic is showing is three resistor divider circuits that divide down the +15, +5v and the -15 volt supplies to feed the A/D converter inputs (there is some additional filtering and protection not shown).
There is a problem though with the -15v divider circuit. Do you see it? The -15v circuit is dependent on the +5v voltage as well. So for instance if the +5v rail is 8% too high (within spec), and the -15v signal is 8% too low (also within spec), the resulting divided/translated voltage may be more than 10% out of whack (this is a highly technical term). The unit might then erroneously fault. I found a simple software change that put this issue to rest. We need to first read the +5 rail voltage. Then the -15v rail voltage is read and we subtract out the component of +5v rail voltage from the -15v reading. To do this requires some math and Kirchoff's voltage law.
VAD3 = V+5v - ( V+5v - V-15v) R5 /( R5 + R6)
One note on the expression parameters, V+5v is the actual measured voltage of the +5V supply. This is obtained by reading the VAD2 voltage first and calculating the +5V actual voltage. Next we have to solve the equation for the unknown voltage, V-15 :
V-15v = (V+5v - VAD3)(R5 + R6)/R5 + V+5v
Everything on the right hand side is known. I implemented this equation in the firmware of the computer and it worked like a charm! I implemented it, but did not consider this solution to be optimal. In subsequent designs I made sure we did not use the +5v supply to translate the -15v supply voltage, but for this product this software solution had several benefits:
- Easy fix for units already in field. No recall.
- No waste for already manufactured inventory. We can use existing stuffed pcb's.
- We did not need to delay an already delayed product.
- No additional cost for the pcb NRE.