I was wondering if the chopper amplifiers are same as chopper stabilized amplifiers. If no, can you explain what is the difference?
Electronic – Are chopper amplifiers same as chopper stabilized Amplifiers
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I see you are talking of the resistor from source to ground, and the total resistance on output for the gain of a common source/emitter 1 transistor amplifier.
As regards the sign, I don't think the equation takes it into account as the common source/emitter amplifier is an inverting amplifier. It is just the absolute gain.
Of course you could specify the equation as $$ -\begin{equation} \ Rdrain \over\ Rsource \end{equation} $$ to make sure you always get the right polarity.
In fact (just noticed) if you check the Wiki page on Common source the (more exact) equation is as follows:
You could use a classic "series capacitor" supply which uses the reactance of a capacitor as the main portion of the voltage dropping element.
i ~~= V/Xc = 230 x (2 x Pi x Freq x C) or C ~~= i / (230 x 2 x Pi x Freq) C per mA = 0.001 /(72256) @ 50 HZ.
Better - C = about 15 nF per mA with 230 VAC 50 Hz supply
So for 40 mA C = 40 x 15 = 600 nF = 0.6 uF
So eg a 1 uF 230 VAC X or Y rated capacitor plus the usual circuitry should work.
Above I use 230 VAC and say C ~~= as current supplied is not directly related to the RMS Voltage. The above should be close enough to start.
Note that the capacitors MUST be X or Y rated at the voltage used.
If the capacitor fails fully or partially short you will probably destroy the input circuit including the 2 x somewhat expensive HCPL-7520 amplifiers but the isolation will be maintained. Note that a capacitor based supply of this sort notionally has a "hot" side where phase/live is input and a notionally low voltage side where neutral/return is connected. However, ALWAYS assume that ALL points in such a supply are ALWAYS at full mains potential. Murphy ensures that sometimes they will be.
Another approach which is potentially slightly more accurate, lower cost and just as good long term but not quite s flexible experimentally, is to operate the microcontroller without mains isolation (so no expensive isolation amplifiers and no added errors) and the couple the digital outputs via eg opto isolators.
I am currently working on similar designs and am using the digital opto isolator approach. This has the advantages of lower cost isolation and no information losses across the isolation barrier due to signals being digital. The isolated power supply can be much lower current so the X or Y rated cap is smaller.
Worth considering is to use a PCBA from a mins to USB charger or other commercial PSU. If these are safe enough to connect to your cellphone they may be safe enough to use in your power meter live side supply* - and if they fail you still have the isolation amplifiers protecting you. You can also use such a supply to power a whole floating meter with processor and if you have optoisolated digital output you are s=till safe.
(* Pulling apart some cheap ones may make you wonder about this)
Best Answer
A chopper circuit is any circuit that commutates or "chops" during it's fundamental operation. DC/DC convertors and switched cap filters are some examples. An amplifier that chops the input and runs a reference through the circuitry in an alternating fashion so that temperature based non-linearities can be corrected on the fly are know as chopper stabilized circuits and they imply that there is some temperature /process invariant reference against which to compare.
A chopper amp can be an amplifier that is configured to be used in chopper circuits but most commonly it is a short hand for chopper stabilized amplifier. However, the vernacular has changed so there is no one true version. You should clarify or determine from context.
One thing to note is that chopper circuits that are not synchronized can have anomalous artifacts or frequency spurs added to the noise that may not always be noticeable. Until they cause problems ...
Some of these same techniques are used in ADC's to correct the on board references. In that case they are sometimes called "sub-band encoded corrections" or some other ambiguous language that you should be on the look out for.