You can't insert an optical isolator input into the -21V output? You'll 
need to get a fairly linear output; if I recall the H11D1 is fairly 
linear. Then calibrate the H11D1 and the A/D converter, and attach the 
H11D1 to a pullup resistor and GND.

Pretty cheap design.

--Bob

Kenneth Lumia wrote:

> This is OK, but it assumes that the power supply voltages ramp up/down 
> in such a way that the +5V rail is valid before and after the -21V 
> supply is turned on/off.  If not, you will have a negative voltage on 
> the pin, that will cause the protection diodes to turn on and conduct 
> into an unknown
> impedance.  Depending on the design, you could even put a negative 
> bias on the 5V rail.  Be careful and think about the transition 
> states, especially concern yourself with loading on the -21V line.
>
> In a previous design, I used a LM358 op amp running off of  +5 and 
> ground to squeeze a +/-15v input into a 0 to 5V range output.  It 
> added complexity, however the input circuits of the LM358 can take the 
> -15V and not do anything bad, even if 5V isn't present (bipolar input 
> transistors were reverse biased).
>
> Ken
> klumia@adelphia.net
>
>> Tie the top end of your voltage divider to +5V instead of to ground. 
>> When
>> your supply is at -21V, you'll have 26V across the voltage divider.
>> Calculate the resistor between +5V and the PIC A/D when you have -21V on
>> the supply to give you 5V. When the supply goes to 0V, you'll have 5V -
>> 5/26 V across the resistor between +5V and the A/D input. The full range
>> on the input to the voltage divider would be:
>>
>> +5V = ADC Full Scale
>> -21V = ADC Zero
>>
>> Do the rest in software!
>>
>> Harold
>
>
>


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