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Single ended signal to differential line driver

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Misterbenn

Active Member
Hi there, I could use a hand getting my head around a design for a single ended signal to differential input&output amplifier.

I'm using an AC current sensor (LTSR-6NP, http://www.lem.com/docs/products/ltsr_6-np.pdf) which gives a single ended voltage output referenced to ground with the zero current point at mid supply rail.
current sensor output.JPG
This particular sensor also has a voltage reference input to adjust the zero current midpoint.

I want to make this signal differential so its more noise immune, to do this i found a differential line driver ADA4049-1 (http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4940-1_4940-2.pdf)
This has a fully differential input and output as well as a output common mode voltage set point, Vocm.

This is the kind of thing i'm thinking of at the moment. I'm not sure if i need the Rg and Rf on the -IN input, or if i should just connect the 2.5V mid point reference directly to the negative input?
amp.JPG

or this
amp2.JPG
 

ronsimpson

Well-Known Member
Most Helpful Member
I did not look up your current sensor but; the ones I used before have 1/2 supply=0 current.

You want two outputs. Do you want 0v=0 current or do you want 1/2 supply = 0 current? (is your supply +5V? or +/-5V)
 

Misterbenn

Active Member
Hi Ron,

yes current sensor gives 1/2 supply at zero current.

supply is just 5V, from an LDO.

I want differential outputs, such that:
input = 2.5V: outputs = 2.5V and 2.5V giving 0V differential
input = 5V: outputs = 5V and 0V giving 5V differential
input = 0V: outputs = 0V and 5V giving -5V differential

hope that make sense
 

Misterbenn

Active Member
So I've attached the schematic that I've settled on in the end,

It turns out my single end to differential signal circuit can only have a gain of <=1 so I added a second differential amplifier stage. both amps have an output common mode Vocm of 2.5V from a precision source/sink reference.
The current sensor being 2.5V +/[email protected] range.
so when the current is at + full range my sensor output is 5V and my differential signal output is Vdiff = 5V
when the current is a - full range my sensor output is 0V and my differential signal output is Vdiff = -5V

One thing I could use some expert help on is my first stage gain setting. I've accounted for the RC filter impedance of 100Ohms by using an Rg of 1k in the non-inverting leg and Rg of 1k1 in the inverting leg. Is this correct, or should I be ignoring the filter impedance as I want to measure the node voltage at the output of the RC filter?

Thanks
Capture2.JPG
 

Misterbenn

Active Member
small update,

It was pointed out that the RC filter matching I've done is only valid for DC signals not AC.
So i've added a buffer to the RC filter output to give a low impedance input to the first amplifier stage. all the Rg and Rf resistances on that first stage are now 1k to give a balanced gain of 1 on the inverting and non-inverting input over the frequency range.
 
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