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Old 20th September 2011, 11:53 PM   #1
Zaaphod is offline Zaaphod  United States
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Lightbulb Floating Differential Signal

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.
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Old 21st September 2011, 12:37 AM   #2
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Old 21st September 2011, 11:36 AM   #3
marce is online now marce  United Kingdom
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Differential reciever, possibly.
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Old 21st September 2011, 11:57 AM   #4
PMA is offline PMA  Europe
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You cannot use the TPA3122D2 as a differential floating input receiver.
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Old 21st September 2011, 05:40 PM   #5
Zaaphod is offline Zaaphod  United States
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Quote:
Originally Posted by PMA View Post
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:

Quote:
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
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Old 21st September 2011, 05:50 PM   #6
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It looks like you need a third wire, common ground, between the DRV134 and the TPA3122D2.
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Old 21st September 2011, 05:55 PM   #7
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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!
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Old 21st September 2011, 06:05 PM   #8
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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.
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Old 21st September 2011, 06:11 PM   #9
PMA is offline PMA  Europe
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CMV (common mode voltage) is a problem (in the circuit shown in the 1st post). It must not exceed certain limit.
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Last edited by PMA; 21st September 2011 at 06:13 PM.
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Old 21st September 2011, 10:50 PM   #10
Zaaphod is offline Zaaphod  United States
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Quote:
Originally Posted by darkfenriz View Post
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?
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