[Concept] Single-Supply Polyvalent Input

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.

——————————  Context  ———————————–

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.

Examples :

https://i1.wp.com/zone.ni.com/images/reference/en-XX/help/373680C-01/fig3-loc_eps_unitgain_se.gif

A differential signal is a combination of Symmetric or Asymmetric signals on two wire . The measurement is done between the two signals.

Examples :

https://i1.wp.com/zone.ni.com/images/reference/en-XX/help/370524P-01/loc_eps_commonmode.gif

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).

Explanation :

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”.

Op-Amp Unity-Gain Buffer.svg

But as you can see, it could only accept a single-ended input.

The differential variant exists but it involves two signal output.

capture
A differential Follower schematic

The cheapest/easiest solution is a classic differential amplifier.

Op-Amp Differential Amplifier.svg

When R1=Rf=R2=Rg the Signal-Gain is 1 =>  V_{\text{out}} = V_2 - V_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).

sans-titre
The output will be (Vdd/2) + Vsignal

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) :

captureg

Hope that it lightened up your mind on Impedance , Signals and Op-Amp Offsets.

Again, not rocket science, but good to know 😉

 

 

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