Hi, all,
As part of the aforementioned cathodolumiescence system, I'm doing a bias supply for multipixel photon counters (MPPCs).
MPPCs are extremely voltage sensitive--the gain of this one goes from ~0 to 2E6 between 52 and 55 volts' bias. (See .)
Sooo, there's a bit of pressure to keep the bias very stable, but it doesn't have to change very often. Accordingly, I'm tentatively planning to use a 12 to 16-bit PWM with good filtering. It'll obviously have to be buffered with a tinylogic gate running from a stiff reference supply, to prevent voltage sags inside the LPC845 MCU from spoiling the accuracy.
The '845 can run its PWM clocks at 7.5 MHz, so that would be an output frequency of 114 Hz to 1.8 kHz. I'd need about 100 dB of filtering to get the output ripple on the MPPC supply down to a millivolt or so, so that's 3 RC sections with 33 dB attenuation each, i.e. corner frequencies of 2.5 Hz to 40 Hz (TCs of 4 to 60 ms). Not horrible--100k*0.68uF at 16 bits, 40k and 0.1 uF at 12 bits.
Anything else besides buffering and filtering that I haven't thought of that might limit the accuracy? How good can you make a PWM, anyway?
Thanks
Phil Hobbs