I am doing a project and one of the requirement is to measure millivolt voltage signal, need 16-bit ADC. The actual input range is below 100milivolt. If I choose a ADC with input range to 4V,
1LSB=0.06mV. Shall I just measure the input without any voltage amplifier added in front of the ADC input pin?
Another requirement is another analog measurement, whose input impedance is over 20MOhm. Can I just use a normal ADC chip connect to that input without any other circuit? Or I need a very low bias current OPamp to amplify the input signal, and have the voltage amplifier's output connected to the ADC input.
Them thar homework problems are getting difficult ;-)
...Jim Thompson
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| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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Controlling 15kV to within 20 Volts meant the capacity to read microvolt changes in the feedback loop. We had no problem achieving this, despite the huge fields present in HV supply proximities.
Except that our success with it was typically to within 5 volts, and the HV feedback resistor is in the same area as the HV multiplier stage, and you don't know what the f*ck you are talking about in this HV realm.
I regularly make unusual push-the-envelope designs to 20kV and beyond, what HV realm are you talking about? It's generally the magnetic fields I struggle with.
Well, I regularly make unusual push-the-envelope designs to 20kV and beyond; what HV realm are you talking about?
For example, a recently-finished instrument, a Rydberg- atom expeller, creates two 10 to 15kV voltages, which are applied to two electrodes in a vacuum chamber.
The voltages are independently controllable, but in a precision-dump mode one electrode must be remotely set to a small voltage away from the other, ranging from 0 to 200 volts, within 0.2 volts (the experiment requires a sweep where the effect is proportional to V^2, so the values below 5 volts are especially significant). This is all measured and controlled from ground potential, so that's 0.2/10kV = 0.002% or 20ppm precision. I was able to easily create the appropriate electric-field shields.
It's generally the magnetic fields I struggle with. This case was no exception, given my (perhaps poor) choice to design a square-wave PWM with custom HV transformer to create the precision 0 to 200 volts (in retrospect, a sine wave transformer drive would have been better).
The rejected, but perhaps better choice, would have been to precisely control the 0 to 200V right at the HV end, using an optical digital link. I balked at that choice when I worried that reporting on the resulting voltage from the HV side with another optical link didn't seem fully honest, so at least one analog link should be used.
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