In my current line of work there is a requirement to "count" the number of high-energy positive ions implanted onto (and into) a target material. The time the ion beam is on can range from a few seconds (at low doses) to several days (at high doses). The target is mounted to an insulated aluminum carousel wheel (we can load up to 25 targets) mounted in vacuum. A rotary electrical vacuum feed-through connects the target and the wheel to the outside world.
To "count" the number of ions implanted, we measure the electron current that flows from ground to the wheel, each electron presumed to neutralize one positive ion. Now, if we could integrate that analog ion current signal (without drift!) the integrated result would be exactly proportional to the number of ions that landed on the target and were neutralized by electrons comprising the current we measure. An analog integrator, such as I described above, would not be accurate enough for our purpose when used for long intervals of time. Besides that, we need to know when a certain dose of ions has been completed, so as to turn off the ion beam and perhaps move on to the next target. If a perfect, drift-free, analog integrator existed, we could compare the integrator output against a set-point voltage, perhaps from a potentiometer connected to a stable precision voltage source, and turn off the ion beam when the integrator output voltage reached the set-point voltage.
Fortunately there is a better, more accurate way. Our accelerator uses a hybrid analog/digital device called a Model 1000C Current Integrator, manufactured by Brookhaven Instruments Corporation out of Austin TX. It is basically a selectable-gain current-to-voltage converter that drives a voltage-to-frequency converter. The latter has a full-scale frequency of 1000 Hz when the input is full scale. At less than full-scale, the frequency is a series of pulses with a repetition rate that varies from zero to 1000 Hz. The pulses are counted by now-obsolete Texas Instrument BCD counter/display ICs to a maximum of 9999 counts. The desired count is preset on four thumbwheel switches and the BIC stops the ion beam when that count is reached. The pulses can be divided down from 1000 Hz full scale to 100, 10, and 1 pulse per second to accommodate higher doses.
This was a garage-shop project a very clever engineer built about twenty years ago. I don't know how many are in existence, but not many. Their main application is to measure ion doses produced by particle accelerators, and there ain't a whole lot of them around. The instrument is almost bullet-proof. The input at the BNC is a "virtual ground" on any range where the output is less than full scale. It is important to select the correct range and not apply a voltage signal to the input. Voltage signals providing sufficient current will vaporize a small 100 Ω 1/8 watt resistor in the input path. The designer thoughtfully mounted this resistor on stand-off Teflon insulators, so it is easy to replace. I think I've smoked, or at least crisped, perhaps a half-dozen over the last eighteen years. Please don't ask how this happens; it's embarrassing.
So, if you need to integrate an analog signal, I unconditionally recommend this "V-to-F and count pulses" technique. Accuracy will depend mainly on your V-to-F circuit design, of which there are dozens of examples available elsewhere on the Web. The main thing, however, is to have FUN!
BIC 1000C Current Integrator:
