Declaration.
Discussion of another LCR is off-topic and please do not develop it, but the question is really useful for subj.
Let's take both LCRs, they are built on a similar scheme (there are some differences, but it is not important for now). These schemes are very similar to the previously existing QPLCR. For example, let's take the scheme on 3s (
pdf).
I don't know what your level of training is, but I, for example, do not see any attempts to protect the device from external voltage (or a charged capacitor).
If you apply voltage (on the same charged capacitor, or a DC source), it will connect between J1/2 and J3/4. Contacts J1-2 are shorted to each other, as are J3-4, these are Kelvin probes. So, external voltage is applied to two nodes in the LCR - to the output of the sine wave generator and to the voltage measurement input. The signal output is made on U1A and R9 with a virtual "0" on the "IV" of U5. If you apply an increased voltage, say 15 volts, then these 15 volts will go to R9 and then to the output of U1A. At the output of the microcircuit, the voltage cannot be greater than the V supply of the microcircuit +0.7 V due to the mandatory diode inside the microcircuit from the output to gnd and to Vdd. Due to the nominal value of R9, the current will be about 15/150 = 100 mA. This current will go through pin 8 to VddA of the processor. The processor will not be able to utilize 100 mA in its power supply and its voltage will begin to increase instantly. At 4 V, the microcircuit irreversibly burns out. If the polarity of the 15V application is different, then "-" voltage will go to R9 and negative current will go to the output of U1A. The built-in diode will send this current to gnd and if the diode withstands, then nothing will happen, but ... But, on the other hand, there will be "+" and it will go to the input-output of U5, resistors R1,2, microcircuits U3,4 and according to the same scenario, a current of 100 mA will cause this current to pass through the built-in diodes into the power supply of the microcircuits and, ultimately, into VddA. The processor will also burn out. This was about the "output", but there is also an "input" - this is U6 and its resistors R4,5. Inside the amplifier microcircuit there are also protective diodes and excess voltage will also be sent to the gnd and V power supply terminals of the microcircuit. R4,5 ratings = 100 Ohm (tears), the power supply of U6 is connected to VddA. The processor is finished again. In order to eliminate this obvious drawback, it is necessary not to (!) let the increased voltage to the input of the microcircuits. Moreover, the main problem is not with the ground, but with the "power supply" - draining excess power into the "power supply" is IMPOSSIBLE, in principle, impossible! And this is a big problem. If you just install a "USB interference suppressor", your processor will burn out even faster. The solution is not so simple, but you have to think a lot, because a simple zener diode has a very large capacity (didn't know? look at the documentation) and extremely disgusting voltage stabilization for 3-4 volts. At the same time, it is necessary to utilize currents of the order of 100 mA. In addition to general protection, it is also necessary to take into account the thermal power - supplying high voltage will cause a large current, as a result, a huge power will be released in a small box and it will burn out, literally burn out. To suppress the long-term passage of current, it is necessary to install "capacitors" in the measuring circuits of sufficient size and good quality. Ceramic capacitors change their value when the voltage changes (except for NPO), so you need film and/or electrolytic ones. This will not fit into the subj case.
Apparently, QPLINK has these problems solved from the start. This means that it is guaranteed to be protected from charged capacitors up to the permissible operating voltage of the device (I don’t know, but 20 volts for sure). What does such a device give? Firstly, a more reliable and stable input. It is unlikely that anyone in their right mind will shove capacitors charged from a 230 V network, they are not even worth touching, and the device is immune to low-voltage charging. The second is measuring the ESR of rechargeable batteries and disposable batteries. It is needed to reject old/dead cans and in general, to understand what you bought - a battery or a bucket of sand. At one time, I made an RLC with exactly this function, measuring under voltage. And you know, it came in handy many times. And check the acid batteries (not to mention lithium cans) and look at the banal batteries - their ESR increases linearly as they discharge. The function is really useful.
Of course, all that has been said applies to QPLINK only under the heading "if" - I have not seen this device and judge only by those products that I developed myself. ))