High Resistances for High Input Impedances Opamp - Page 1
Forum section: electronics / projects

author: mercurial
author_rank: Regular Contributor
timestamp: January 25, 2024, 04:08:45 pm
content: Was reading about high impedance opamps in this articlehttps://www.ti.com/lit/pdf/snoa664titled "Working with High Impedance Op Amps" from TIquoting from itQuoteI was wonderinga. Why are high value resistors required with high impedance op-amps.?b. Are they required for certain topologies of op-amp circuits.c. The article also shows certain ways of achieving high resistance values by using standard resistors, i didn't understand the principle of those though. Have attached an image of one such circuit.

author: mercurial
author_rank: Regular Contributor
timestamp: January 25, 2024, 04:12:08 pm
content: Hi,If any moderators are around please move this topic to beginners section. Wrongly posted it over here

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2024, 05:26:47 pm
content: Not all circuit with high impedance OP-amps need the very high resistor values. It really depends on the circuit and transimpedance amplifiers or very low current current source are the main circuit that really need the very high resisstor values. A normal amplifier can get away without it.The circuit shown is a transimpedance amplifier with divider in the feedback to avoid an even larger resistor. This type of circuit is usually not considered that good because it also amplifies the resistor noise. The circuit adds gain as if there is a larger resistor, but the noise (especiall noise current) is still similar to the case with the small resistor. So a single large resistor will give better performance (e.g. about 10 x less resistor noise compared to the x100 resistance multiplier). With modern OP-amps the resistor noise may well be the main noise source.

author: TimFox
author_rank: Super Contributor
timestamp: January 25, 2024, 05:34:44 pm
content: In that circuit, using a tee connection for the feedback resistance to avoid very large resistance discrete units, the noise current from the resistor network will be that of the actual resistor at the input (100 megohm in that example), not the equivalent value (10 gigohm), and the noise gain for the amplifier noise will be larger, as described above.

author: mercurial
author_rank: Regular Contributor
timestamp: January 25, 2024, 06:46:01 pm
content: Quote from: Kleinstein on January 25, 2024, 05:26:47 pmI see that most high value resistors (more than 1Gohm) on online sites are mostly thick film resistors. And else where I had read that thick film ones are the worst for noise, i mean noisiest of the lot with the quietest being wire wound ones and next being thin film.I guess by what you say, 1 thick film is still worth it than 3 thin film. due to that 100x noise amplification.BTW Do you know how these high value resistors are manufactured. I mean what materials are they made up off and what is the technique to build them.I see that these are quite expensive.Is there a way to mitigate resistor noise?

author: mercurial
author_rank: Regular Contributor
timestamp: January 25, 2024, 06:53:51 pm
content: Quote from: TimFox on January 25, 2024, 05:34:44 pmOk, so if we use a low noise thin film at the input we can say its noise voltage is E=4⋅R⋅k⋅T⋅ΔFSo will this be amplified by 100x at the output?I see that different resistors (thick film, thin film, wire wound) have different noise, can the same equation be applied for the different type of resistors.​

author: Kleinstein
author_rank: Super Contributor
timestamp: January 25, 2024, 07:02:43 pm
content: For the very high resistance values it is hard to get wire wound resistors. Wire wound gets pretty rare and expensive about some 10 Mohms.Thick film restors can have additional excess noise / current noise, but the high value resistors are noisy anyway, even without excess noise.The thermal resistor noise equation is universal and applies to all resistors. The excess noise with applied voltage is material / construction specific.For the really high values from some 1 Gohm on the low noise options are less practical.There are still different variants as thick film, carbon film and metal oxide film resistors. Much of the high price is from the low quantety those resistors are used in and the cleanness that is needed to avoid extra leakage currents.In some applications there is a way around the resistor noise, by integrating the current in a capacitor and look at the rate of voltage change or compensate with a photocurrent. It depends on the application (e.g. frequency / time range) if it is feasible and better.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2024, 07:08:02 pm
content: Quite often, much of the noise comes from the op-amp's noisy bias currents, although FET input op-amps should be used with high impedance circuits, to minimise this.Note how R1 is bypassed by C1, so the op-amps input is effectively connected directly to 0V at signal frequencies, which mitigates the noise generated by R1.

author: David Hess
author_rank: Super Contributor
timestamp: January 25, 2024, 08:34:24 pm
content: Quote from: mercurial on January 25, 2024, 04:08:45 pmIt is really the opposite. High impedance operational amplifiers are required to take advantage of high valued resistors. Normal operational amplifiers have high input bias currents and high input current noise which limits the use of high resistances.QuoteIt is not so much topologies as applications. Some applications require high values of resistance, which in tern requires "high impedance" operational amplifiers.QuoteThe idea here is that the output of the operational amplifier is divided by 102, which is R4/(R3+R4), before being applied to the high value R2, multiplying R2's effective resistance at the expense of also multiplying the input offset voltage and noise of the operational amplifier. I do not know why they chose a factor of 102 instead of 100.I really liked the LM11 when it was in production. I have a tube of them somewhere. The closest modern replacement is the LT1012 or one of the more recent precision JFET parts like the OPA140.

author: TimFox
author_rank: Super Contributor
timestamp: January 25, 2024, 10:48:19 pm
content: Quote from: mercurial on January 25, 2024, 06:53:51 pmIn this context, “low noise” means low excess or 1/f noise, which is above that given by your equation for thermal (Johnson) noise.Excess noise exists only when there is a voltage across (or current through) the resistor; fluctuations in the current give extra noise at low frequencies.All resistors show thermal noise at finite temperature, even with no DC voltage.Resistors with tortuous current paths (like carbon composite or thick film) show more excess noise than uniform paths in wirewound or thin film parts, but the latter are limited in maximum resistance attainable.For the transimpedance amplifier shown, it is often more relevant to calculate the noise current, which decreases with increasing resistance, since the input signal for the amplifier is a current.Spice can be a good way to combine the noise contributions from the op amp voltage and current noise along with the resistors (but it is complicated to include excess noise).

author: mercurial
author_rank: Regular Contributor
timestamp: January 26, 2024, 02:02:58 am
content: Quote from: Kleinstein on January 25, 2024, 07:02:43 pmHow does one do that?

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2024, 10:38:36 am
content: Hello use the Johnson-Nyquist thermal noise formula of 1920s to calculate the voltage and current noise in a resistance at a particular temperature.https://en.wikipedia.org/wiki/Johnson–Nyquist_noiseMost megohm resistors from Vishay, etc are not intended for low noise or precision use but as HV bleeders etc.Suggest Henry OTT's fine books like Noise Reduction Techniques in Electronic Systems 1976https://www.amazon.com/Reduction-Techniques-Electronic-19-May-1976-Hardcover/dp/B013PRO75I/Bon chanceJon

author: mercurial
author_rank: Regular Contributor
timestamp: January 26, 2024, 03:40:28 pm
content: I was thinking about the whole thing and couldn't understand why3 low noise (thin film) standard resistors in the T network, would be more noisier than a single high value (more than 1 Gohm) thick film resistor.

author: magic
author_rank: Super Contributor
timestamp: January 26, 2024, 05:56:18 pm
content: Because increasing the feedback resistor's value 100x times increases noise 10x only.Because amplifying the signal 100x times increases the feedback resistors noise 100x.

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2024, 04:32:17 pm
content: check the std opamp books like from Burr brown on feedback, BW limit caps and T network for replacement of very high R FB r.Basic stuff you should research a bitj