• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

High Impedance Inputs

Status
Not open for further replies.

Mikebits

Well-Known Member
It is often hard to explain why high input impedance circuits can be trouble. Perhaps a picture is a better teaching aid. 1 meg ohm resistor to ground.

IMG_1836.png
 

spec

Well-Known Member
Most Helpful Member
Nice illustration Mike. Another illustration is to put your finger near a high power audio amplifier high impedance input with the volume turned up.:arghh:

spec
 

Willen

Well-Known Member
I am curious: Why touching the high impedance audio input (specially floating input) cause horrible output? What kind of signal out body have to cause such output?
 

spec

Well-Known Member
Most Helpful Member
I am curious: Why touching the high impedance audio input (specially floating input) cause horrible output? What kind of signal out body have to cause such output?
Hi Willen,

There are, essentially, two types of coupling that cause stray pick up: capacitive and inductive.

Capacitive coupling normally affects high impedance circuits and inductive coupling normally affects low impedance circuits.

Your body acts like a huge capacitor (just work out the surface area) and your body is also impinged by electromagnetic waves of all frequencies from DC to Xrays.

But the main signal in your body is the frequency of your mains supply, either 50Hz or 60Hz, so when you touch a sensitive input of an audio amplifier the signal in your body gets amplified just like the output from a CD player for example.

But, if you were is a Faraday cage and you wore an earth wrist band there would be very little effect of touching the input of an audio amplifier.

spec
 

Mikebits

Well-Known Member
Notice that I am not actually touching the resistor, just moving my finger nearby was enough to see a voltage. Remember ohms law, E=IR, so with R = 1Megohm does not take much I to develop a potential. Keep this in mind when making measurement of High Z circuits :)
 

dr pepper

Well-Known Member
Most Helpful Member
Willen, your body acts as kind of an antenna, and picks up noise from around you including mains hum, your body is a high impedance too, but if the impedance of the circuit you touch is similar or higher still then enough current flows to create an input voltage to the circuit your touching, and you get the Bzzzt.
If you tried it out in the desert away from power lines with a battery powered amp, you wouldnt get the Bzz, just a hiss and maybe some radio stations.
 

spec

Well-Known Member
Most Helpful Member
Notice that I am not actually touching the resistor, just moving my finger nearby was enough to see a voltage. Remember ohms law, E=IR, so with R = 1Megohm does not take much I to develop a potential. Keep this in mind when making measurement of High Z circuits :)
On a similar vein, if you connect the tip of a scope probe to the earth wire of a scope probe, you can still see a signal on the scope if you turn the gain up.

One doge to eliminate some of the noise pickup and also to improve the accuracy of the waveform displayed is to remove the probe end and fit a short piece of stiff wire (paper clip wire) around the earth ring of the probe. You then probe a local earth and signal on the board being monitored.

I think Ian Rogers mentioned this technique on another thread, but I cant find it now.

spec
 

JimB

Super Moderator
Most Helpful Member

Colin

Active Member
You have all forgotten to mention the fact that the input has to have a certain amount of wiring to act as an antenna to pick up the signals.
 

Mikebits

Well-Known Member
I only had mentioned this because I was measuring the resistance of a high ohmic value resistor, and I noticed how the lower ordered digits bobbled around. As an experiment I put the resistor in a metal box (see first post), and by doing so the reading settled out. So it was just an observation.
I was trying to think where one might use such high value resistors and at the moment I cant really think of any, but let's say you need an amp with a gain of 1 million for example. One might be quick to take an op amp and use the Av=RF/RI thus grabbing one big resistor for RF, but knowing what we know about the possible noise of a large ohmic resistor might instead go for a 2 stage op amp with each one having a gain of 1000, thus reducing the noise contribution associated with very large ohmic resistors. According to my spell check ohmic is not a word, hmm...
The noise issue associated with large resistance is discussed in the low level measurement handbook. I will see if I can round up the link.
 

schmitt trigger

Well-Known Member
:p
So if I wear an aluminum hat over my head, would it help the noise reduction or make it worse?
If you practice Yoga and are able to stand on your head, then the rest of your body would form a quarter wavelength radiator which could significantly enhance your transmitting gain. :p
 

Mikebits

Well-Known Member
If you practice Yoga and are able to stand on your head, then the rest of your body would form a quarter wavelength radiator which could significantly enhance your transmitting gain. :p
Maybe get an IoT port through braces in teeth :)
 

audioguru

Well-Known Member
Most Helpful Member
Opamps use a frequency compensation capacitor inside to cut high frequencies to prevent oscillation caused by phase shift when negative feedback is applied. A TL07x audio opamp has a frequency response to 3MHz when its gain is set to 1 with negative feedback and it cuts frequencies above 3MHz. Its frequency response is to 300kHz when its gain is 10, it is to 30kHz when its gain is 100, it is to only 3kHz when its gain is 1000 so high frequency sounds and hiss are reduced. The frequency response of an old opamp like a 741 or LM324 is much worse.
 

Mikebits

Well-Known Member
I was just attempting to make a point about being careful with high value resistors. The op amp example was just to illustrate a point, not a real use case, but thanks for the op-amp lesson :)
 

EvilGenius

Member
:p
If you practice Yoga and are able to stand on your head, then the rest of your body would form a quarter wavelength radiator which could significantly enhance your transmitting gain. :p
I am sorry for not being more specific. I was talking about the noises in my head!
Also I noticed OP used a Tin Mint for his project box. Don't you think that would contribute to the noise and if a plastic chocolate box was utilized, it would have been a more suitable medium?!
EG

Edit: I like chocolates
 

spec

Well-Known Member
Most Helpful Member
I only had mentioned this because I was measuring the resistance of a high ohmic value resistor, and I noticed how the lower ordered digits bobbled around. As an experiment I put the resistor in a metal box (see first post), and by doing so the reading settled out. So it was just an observation.
I was trying to think where one might use such high value resistors and at the moment I cant really think of any, but let's say you need an amp with a gain of 1 million for example. One might be quick to take an op amp and use the Av=RF/RI thus grabbing one big resistor for RF, but knowing what we know about the possible noise of a large ohmic resistor might instead go for a 2 stage op amp with each one having a gain of 1000, thus reducing the noise contribution associated with very large ohmic resistors. According to my spell check ohmic is not a word, hmm...
The noise issue associated with large resistance is discussed in the low level measurement handbook. I will see if I can round up the link.
Hi MB,

There are a few circuits where you may use a 1 M Ohm resistor and even higher:
(1) 555 timer and on an LMC555 the timing resistor limit is 10M
(2) To define the input impedance of a high impedance amplifier
(3) In electrometer circuits
(4) In low power circuits- battery monitoring for example- some on ETO
(5) In high voltage dividers
(6) Discharge resistor across a capacitor
(7) Multimeters use high value resistors

spec
 

Mikebits

Well-Known Member
Funny you should mention the 555 Spec, at my last job we were forbidden from using the part (At the time, did not know why only it was off-limits). Anyways I been reading a lot of old articles from Analog Jedi Bob Pease, I came across this bit where Mr. Pease gives his opinion on the ubiquitous 555.
Quoted from Source: http://electronicdesign.com/analog/what-s-all-555-timer-stuff-anyway

Pease replied in his characteristically straightforward way. He noted:

“Hi, Jeff H., I have almost never used a 555. Maybe never? I use op-amps, LM324's, LM311's, LF356's. I use 74HC04's and 74C14's but not 555's. I've used ECL fast logic, and discrete transistors. But the 555 just does not do anything precise, or even semi-precise, that I need done. So that's one thing I can "share" - my favorite circuit to use a 555, is: a blank piece of paper. Never touch the things. Go ahead and print that. / rap”
 

spec

Well-Known Member
Most Helpful Member
Funny you should mention the 555 Spec, at my last job we were forbidden from using the part (At the time, did not know why only it was off-limits). Anyways I been reading a lot of old articles from Analog Jedi Bob Pease, I came across this bit where Mr. Pease gives his opinion on the ubiquitous 555.
Quoted from Source: http://electronicdesign.com/analog/what-s-all-555-timer-stuff-anyway

Pease replied in his characteristically straightforward way. He noted:

“Hi, Jeff H., I have almost never used a 555. Maybe never? I use op-amps, LM324's, LM311's, LF356's. I use 74HC04's and 74C14's but not 555's. I've used ECL fast logic, and discrete transistors. But the 555 just does not do anything precise, or even semi-precise, that I need done. So that's one thing I can "share" - my favorite circuit to use a 555, is: a blank piece of paper. Never touch the things. Go ahead and print that. / rap”
RIP Bob Pease,

You know, although the 555 is the most popular chip- you can almost guarantee that if you post a schematic based on the 555 on ETO it will be liked- it is quite frustrating to use.

I would like to see Reset replaced by a /Q output and CV replaced by +Discharge to compliment the existing [-] Discharge.

That would make the 555 much more flexible.

Also, the 555 data sheet is deficient, misleading and fraudulent- how can a 555 be called a precision timer.:oops:

spec
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top