>Would it be silly of me to suggest a current mirror?
>A pair of transistors, and a couple resistors.

Well I thought of a current source and resistor, which at the most negative 
voltage gives an output voltage close to 0V, and at minimum required 
negative voltage gave close to 5V - IIRC he was looking to measure -15 
to -10V. Only problem with this arrangement is that it gives an upside down 
scaling.

A current mirror would be nicer if the measurement range did not scale 1:1 
to the ADC range, and also has the advantage that minimum negative 
corresponds to minimum  positive input to the ADC. 

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