Cover from Derek Gavey (Flickr)
Hi everyone , hope you’ve enjoyed summer , because the start of Autumn brings a lot of work to do 😉
Perfect time to share some concept.
Recently I wondered myself about polyvalent input in ADC systems (or any low impedance inputs). Even if my electronics skills are growing up I searched a way to build an single-supply input who can allow single-ended and differential signals.
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A single-ended signal is an Continuous (DC) or an Alternative (AC) signal set on one wire and measured between this wire and a Ground wire. It is probably the most common signal mode.
A differential signal is a combination of Symmetric or Asymmetric signals on two wire . The measurement is done between the two signals.
So what is the interest to “build” an input to those signals ?
It’s a question of impedance. (and it’s probably the most annoying thing in electronics for me ಠ_ಠ)
If the impedance (Z) of an input is too low , or if an output Z is to high , the signal could be lost or too weak (or altered).
If my signal is : 200mV and 1µA and my input impedance is 200 Ohm.
U = RI => 200*0.000001 = 0.2mV max of amplitude in the measurement device.
My signal cannot be fully processed , so I need at least 100 000 Ohm of impedance.
Furthermore, a good output Z involve a good “Current supply”from the system to the signal for the same reason (Higher the I is the lower R must be).
A solution ?
Most of people use Op-Amp (operational amplifier) because their input have at least 1 M Ohm of impedance and their output can deliver a bunch of milliamperes (it lower the output Z).
The best way to receive a signal, without modifying it, is the Op-Amp in “follower/buffer mode”.
But as you can see, it could only accept a single-ended input.
The differential variant exists but it involves two signal output.
The cheapest/easiest solution is a classic differential amplifier.
When R1=Rf=R2=Rg the Signal-Gain is 1 => Lucky day , we don’t want any Gain (Follower equivalent).
With this configuration, if the input is differential , the output will be the difference between the two inputs and if it’s single-ended, the V1 is ground-connected and it’s a classic follower (out = V2).
However this schematic needs a symmetric supply [and we have no money (╯°□°）╯ ], so we can offset the Reference voltage to make the signal only positive (in a +/- (Vdd/2) range).
The Op-Amp supply will be only positive. Again, don’t forget to choose resistances with a good value (Cf. Impedance).
Finally we have a Polyvalent Single-Supply input with a single-ended output.
But we can improve it again.
To make sure your current-output-capacity is at maximum you can put a single-ended follower after the first stage (with offset). It allows you to put frequency filters before (high Z value) and after (low Z value) witch is advisable with ADC (Analog to Digital converter).
Even better, if your Op-Amp have a good Output Current Supply you can replace the follower by a Non-inverter Amplifier with variable gains.
I’ve done that before (for example) :
Hope that it lightened up your mind on Impedance , Signals and Op-Amp Offsets.
Again, not rocket science, but good to know 😉