Patrick wrote:
> jeff millar wrote:
>
>
>> The conducted emissions probably create more problems than radiated
>> because the HV probe line, Plate and Grid current lines, etc. have a lot
>> of RF voltage on them and they get connected to the PIC. Imagine the
>> currents flowing into the PIC board on one pin and out on some other
>> pins...this can definitely mess up the A/D's and even logic on the PIC.
>>
>
> Just a quick thought on this part. Could you not knock the voltage
> down before sending it to the PIC and then calibrate it to the higher
> voltage for which you are testing? You would then have a lower voltage
> to measure which might mitigate some other problems. You would lose a
> little accuracy but probably still be within specifications.
>
Absolutely. In fact, the PIC probably doesn't really like 3 KV on it's
A/D inputs. ;-) The usual approach calls for a 1000:1 resistive divider
that puts out a voltage of about 1 Volt for each KV (depending the A/D
spec).
The real requirement for getting rid of the RF falls on the capacitors
bypassing the RF to ground and the residual voltage on the ground plane
where the capacitors connect. The best idea would have the resistive
divider in a box with a bypass to the box ground at the wall, then route
the wire to the box containing the PIC and bypass it again to PIC's box.
On the accuracy subject, an 8 bit A/D has 256 steps with a peak error of
+/- 1/2 step...corresponding to 0.2%. This means that the resistors need
1% or even 0.1% accuracy to keep up with even a simple A/D.
jeff, wa1hco
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