Use of Voltage follower in designing a Arduino based Voltmeter. - Page 1
Forum section: electronics / beginners

author: vishalkr123
author_rank: Contributor
timestamp: October 10, 2024, 04:46:38 am
content: I am trying to design a voltmeter using Arduino to measure the output of a tribolelectric generator (TG). A TG usually generated an alternating voltage. I followed a design suggested by Electronoobs for building Voltmeter -https://electronoobs.com/eng_arduino_tut188.php, it uses ADS115 to measure differential voltage. But I observed that the circuit was not able to reproduce the output voltage of the TG and was getting much lower voltage. I realized that this is because the current generated by TG in only few uA range, and not enough for the ADC to measure in differential mode. Therefore, I added a voltage follower circuit to boost the current and measure the output voltage, which is giving me the results. But I have confusion about voltage follower circuit, like the use of capacitor and feedback resistor. I am attached my circuit design, can you provide your valuable feedback on the design?

author: Analog Kid
author_rank: Super Contributor
timestamp: October 10, 2024, 05:13:55 am
content: I'm somewhat of a n00b when it comes to opamps, but for a true voltage follower (gain ≈ 1), I believe you just tie the inverting input directly to the output.

author: vishalkr123
author_rank: Contributor
timestamp: October 10, 2024, 05:20:51 am
content: Thanks, yes, I also thought so. Being a noob myself, I searched online and followed this posthttps://electronics.stackexchange.com/questions/185830/parallel-resistor-and-capacitor-in-non-inverting-voltage-follower. EEVblog is my go-to forum for awesome suggestions.

author: vishalkr123
author_rank: Contributor
timestamp: October 24, 2024, 05:03:52 pm
content: I tried It worked, but the voltage output was less, but it was able to capture features properly. My learning following a triboelectric generator's a typically current limiting, i.e., they supply constant current over a wide range of voltage output from few kohms to Megaohms. 2) and have very high impedance.Therefore, the current of few microamps flowing through 10kohm will generate voltage in millivolts, and so either I have to use high resistance in MegaOhms or a transimpedance amplifier like configuration.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2024, 05:16:32 pm
content: Triboelectric generator usually give more a DC output with very low current, maybe the µA or even lower range, but possibly very high voltage (kV range).One would want a really high resistance divider, like in the 100 M or 1 Gohm range and thus also good isolation on the PCB in this area as leakage paths may get comparable if not careful. In some areas one can use the buffered signal as a guard to reduce leakage.For the buffer the resistor and capacitors are likely there for protection or symmetriy with the input (which should not be needed with such a high impedance OP-amp). The relatively old OP-amp type used is not rail to rail and thus needs the extra supply charge pump. I would today more look at a RR op-amp (e.g. MCP6006 as a cheap one (still good enough for the µC internal ADC) or OPA377 for low noise / better precision) so one could use the same supply as the ADC.

author: vishalkr123
author_rank: Contributor
timestamp: October 28, 2024, 11:45:34 am
content: Thanks for the info. I can suggest some other opamps in DIP package. I checked the ones you suggested, they only come as SMD. It would be really helpful if you could give some inputs on how you choose opamps.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 28, 2024, 02:51:39 pm
content: For a really high impedance signal one should look at OP-amps with low input bias current. So something like CMOS or JFET input types. In a DIP case I would look for TLC272 / TLC271 as old relatively common ones.

author: David Hess
author_rank: Super Contributor
timestamp: October 28, 2024, 07:34:05 pm
content: Quote from: vishalkr123 on October 10, 2024, 04:46:38 amR7 is present to limit the current into the non-inverting input under overload conditions if the input went outside of the supply voltages.With a higher input bias current operational amplifier, like a uA741 or LM324, R6 is added, and made the same value as R7, so that the additional input offset voltage caused by the input bias currents through the external resistances cancel. C2 is then added to reduce additional high frequency noise from R6. R6 and C2 are *not required* with low input bias current operational amplifiers like the CA3140 except for some unusual exceptions which do not apply here. Leave R6 and C2 in or not; they do not matter.The CA3140 is a low input bias current operational amplifier and exceptionally suitable for buffering high impedance sources. Similar modern parts are almost any JFET or CMOS input operational amplifier. The CA3140 is also low power. Older suitable low input bias current and low power replacements include the TL031 (JFET), TL061 (JFET), TLE2061 (JFET), LMC6061 (CMOS), and like Kleinstein mentions, the TLC271 (CMOS), although it is lower voltage. A modern low power part that I might use is the OPA145 (JFET), OPA196 (CMOS), or OPA990/OPA991 (CMOS).

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 28, 2024, 08:02:11 pm
content: Quoted from the linked webpage:The purpose of the resistor R2 is to eliminate the DC offset caused by the op-amp input bias currents. If the bias currents are exactly matched, then the voltage drop across each 1K resistor (R1 and R2) is the same and the output voltage is equal to the input voltage.But the CA3140's datasheet indicates the input bias current to be 10 pA. This current thru a 1k resistor will give an offset of 10 nV, which is lower than trivial compared to the opamp's maximum 15mV (or 5mV for the "A" version) offset voltage.My opinion(*), is to remove the resistor and capacitor and directly connect the inverting input to the output. Leave the protection resistor at the noninverting input.(*)Being an opinion, someone may decide to sh!t on it.