Floating Differential Signal

Zaaphod · 2011-09-21 12:53 am

I am trying to use a DRV134 to send a differential signal over UTP cable. On the other end, I wish to amplify the differential signal with a mono BTL amplifier. I built a prototype, and using the schematics directly from the data sheets for the DRV134 and the TPA3122D2, It is as if I have no signal at all, even though I am measuring my differential signal between +vo and -vo. I have two completely separate power supplies, one +/- 15V Split supply for the DRV134, and a 24V power supply for the TPA3122D2. If I connect the grounds together, it WORKS! but this defeats the whole purpose of sending complimentary signals down UTP cable. I want to be able to send a floating differential signal down the cable, and somehow get it to work on the other end. Does anyone have any advice on the best way to do this? I have a few ideas, but since I have no real experience in this kind of thing, I will be just guessing at the best method for referencing my signal to the amplifier. I am wondering if I just need to leave out the coupling capacitors, or perhaps just put matching load resistors from each input to ground on the amplifier end. I suppose I could put a INA137 at the end of the cable and then another DRV134 to get a new differential signal, but that seems silly since I already had a perfectly good differential signal.

Any advice on the proper way to get this to work would be appreciated.

nigelwright7557 · 2011-09-21 1:37 am

subscribe.

marce · 2011-09-21 12:36 pm

Differential reciever, possibly.

PMA · 2011-09-21 12:57 pm

You cannot use the TPA3122D2 as a differential floating input receiver.

Zaaphod · 2011-09-21 6:40 pm

PMA said:
You cannot use the TPA3122D2 as a differential floating input receiver.

How about the TPA3112D1? I was hoping to stay away from surface mount, but if I can do this with fewer components, I guess I'll do surface mount. The TPA3112D1 datasheet states:

DIFFERENTIAL INPUTS
The differential input stage of the amplifier cancels any noise that appears on both input lines of the channel. To
use the TPA3112D1 with a differential source, connect the positive lead of the audio source to the INP input and
the negative lead from the audio source to the INN input. To use the TPA3112D1 with a single-ended source, ac
ground the INP or INN input through a capacitor equal in value to the input capacitor on INN or INP and apply
the audio source to either input. In a single-ended input application, the unused input should be ac grounded at
the audio source instead of at the device input for best noise performance.

This doesn't really say anything about the differential input being floating though, but if it worked with this one, it seems like it would work leaving the caps off the TPA3122D2 as long as it was being run in BTL mode, it's very similar to the TPA3112D1, however I realize the input stage of the two chips are quite different.

I would like to know if anyone knew for sure if the TPA3112D1 would work with floating differential inputs as it is not very easy to prototype surface mount IC's If not then I guess I can use an INA137 and another DRV134 with my TPA3122D2

Frank Berry · 2011-09-21 6:50 pm

It looks like you need a third wire, common ground, between the DRV134 and the TPA3122D2.

darkfenriz · 2011-09-21 6:55 pm

Since you are using PWM power stage (class D), why don't you send a discrete PWM signal to the UTP?
Modulate it first so that it is easy to send and optocouple and then just "power buffer" the pulses!

Frank Berry · 2011-09-21 7:05 pm

You could add an audio transformer to feed the input of the TPA3122D2.
The secondary of the transformer, feeding the L and R inputs of the TPA3122D2, must have a center tap which should be connected to the analog grounds of the TPA3122D2s.

PMA · 2011-09-21 7:11 pm

CMV (common mode voltage) is a problem (in the circuit shown in the 1st post). It must not exceed certain limit.

Zaaphod · 2011-09-21 11:50 pm

darkfenriz said:
Since you are using PWM power stage (class D), why don't you send a discrete PWM signal to the UTP?
Modulate it first so that it is easy to send and optocouple and then just "power buffer" the pulses!

That sounds like a really cool idea! Could I just use my TPA3122D2 as a PWM signal generator? or does that require the load to operate correctly? What would I use for my remote power stage? could I just use some mosfets or would I need an op amp or something?

Zaaphod · 2011-09-22 12:04 am

Frank Berry said:
You could add an audio transformer to feed the input of the TPA3122D2.
The secondary of the transformer, feeding the L and R inputs of the TPA3122D2, must have a center tap which should be connected to the analog grounds of the TPA3122D2s.

I'm sure that would work, I just want to try to keep the cost down on these, because I want to make a lot of them. Since my signal is defiantly floating, could I do something like 2 resistors to ground like this schematic?

Zaaphod · 2011-09-22 12:32 am

Perhaps this would be better with a HF filter capacitor. I saw this when doing a search for differential receiver, but it was going into an op amp, and did not use the ac coupling capacitors.

Zaaphod · 2011-09-22 2:17 am

Zaaphod said:
That sounds like a really cool idea! Could I just use my TPA3122D2 as a PWM signal generator? or does that require the load to operate correctly? What would I use for my remote power stage? could I just use some mosfets or would I need an op amp or something?

This whole idea sounds like a lot of fun... perhaps something like this? perhaps there is some kind of power mosfet half bridge device I could use?

darkfenriz · 2011-09-22 6:45 pm

Sort of like that, actually one double-output comparator can be a good enough PWM modulator.
For a ready-made half bridges you can try some low side drivers, like TC4429, they are powerfull enough power stages themselves for below 20V supplies.

Zaaphod · 2011-09-23 12:19 am

darkfenriz said:
Sort of like that, actually one double-output comparator can be a good enough PWM modulator.
For a ready-made half bridges you can try some low side drivers, like TC4429, they are powerfull enough power stages themselves for below 20V supplies.

Could you please tell me how I could use a double output comparator to make a PWM modulator?

I had a look at the TC4429, that looks really cool. Does anyone happen to make a dual version of something like that? I was fixing to order some parts soon, I think I may get a few of those to play with.. this whole idea of sending the PWM signal down the twisted pair instead of differential audio really has me fascinated.

nigelwright7557 · 2011-09-23 12:27 am

Zaaphod said:
I had a look at the TC4429, that looks really cool. Does anyone happen to make a dual version of something like that? I was fixing to order some parts soon, I think I may get a few of those to play with.. this whole idea of sending the PWM signal down the twisted pair instead of differential audio really has me fascinated.

They come in 8 pin packages so no big deal using two.

darkfenriz · 2011-09-23 8:42 am

Zaaphod said:
Could you please tell me how I could use a double output comparator to make a PWM modulator?

Try a hysteresis-based relaxation oscillator.
File:OpAmpHystereticOscillator.svg - Wikipedia, the free encyclopedia

Just feed one of the "grounds" with audio signal and use differential output to drive the UTP (sinlge output should work too).
Any simulator will help you find the values for passives to get required switching frequency and input sensivity, around 10 minutes of trials and errors.

Zaaphod · 2011-09-23 4:15 pm

How about something like this using the TC4420?
That op amp oscillator looks interesting, and very simple. When I looked up PWM methods, I saw a lot of things like running a triangle wave into one input and the signal into the other one.. but then I need a triangle wave generator too.

I can't seem to find a double output comparator, any suggestion on a chip for that? I am guessing it is like a regular op-amp but with both inverted and non-inverted outputs.

Zaaphod · 2011-09-23 5:31 pm

Some information I came across... the microchip website suggests MCP1407 instead of the TC4420. I guess it's the new version of it. Looks like it is slightly faster and wider temperature range. I checked the prices on Digikey, the TC4420 is $1.72, and the MCP1407 is only $1.06, so it's a bit less expensive as well.

I also came across this application note

http://ww1.microchip.com/downloads/en/AppNotes/00763c.pdf

about latchup of the driver.. it mentions this could happen driving and inductive load, which I would be doing. It says to put reverse biased diode clamps from both the input and output to each power supply rail. Does anyone have any experience with this? Do I need these on my inputs since I am using optocouplers?

darkfenriz · 2011-09-23 7:36 pm

Zaaphod said:
I can't seem to find a double output comparator, any suggestion on a chip for that? I am guessing it is like a regular op-amp but with both inverted and non-inverted outputs.

LT1016, TL3016, AD8561, LT1711, ...

Floating Differential Signal

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