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Original question:

DRV135: Gain problem

DRV135: Gain problem

Curtis Figley
Prodigy 60 points
Part Number: DRV135
Other Parts Discussed in Thread: TINA-TI

We are seeing a similar problem in a professional audio product we are developing.
Our layout is essentially the same as Figure 29 in the data sheet, with the exception of on each output we have added a 10nF capacitor to ground and clamping diodes pointing appropriately to the supply rails that are included to act as an additional layer of transient/ESD protection.
In our prior generation of this design, the layout was essentially the same and we typically saw a stage gain of 2 within about 1% or so when the output was lightly loaded (10kohm typical) and this gain dropped very slightly, maybe by 1% or so when the loading was changed to 600ohms.
Drive levels for this testing are in the 4dBu area with the supply rails running at +/-18V, so there should be lots of headroom.
The tops of the older parts have 715EJ codes.

In the latest generation units (same schematic, updated PCB layout), the same portion of the circuit has a gain that is larger than a factor of 2 by roughly 5% or 6% at light loading, which is quite a bit larger than the gain trim tolerance specified on the datasheet.
When the load is reduced to 600ohms the gain drops more than in the previous product generation, landing somewhere below the desired factor of 2 by about 5% for a total of 10% shift from the unloaded to loaded conditions.
The latest parts are marked with 685LF codes.

So my first question is why is the nominal gain of 2 out by more than the data sheet max/min of 2%?

Also a bit confusing for me is that the Sense inputs are the feedback to the opposite phase's output driver, so the loading effect should be strongly suppressed by the magic of feedback.
Why is the feedback unable to force the drivers to maintain the appearance of a constant gain up until the device runs into its compliance margins on voltage and current?

Any thoughts or comments on this issue would be much appreciated.

  • 0
    Guru 140200 points

    Hi Curtis,

    the DRV135 is not a simple driver with differential outputs but is a balanced audio driver with a cross coupled differential output stage. It provides a low differential-mode output impedance (50 Ω) and high common-mode output impedance (1.6 kΩ) and by this imitates the behaviour of an audio transformer. But it's not the idea of the cross coupling to suppress the gain effects when driving different loads.

    The gain is optimized for a 600R load, because that's the professional standard for balanced signalling. The TINA-TI reference design gives me a gain of 2.012, which differs from a gain of 2 by only 0.6%.

    With a 10k load the gain increases to 2.164, which differs from a gain of 2 by 8.2%. This is simply the result of the 50R source resistances of each output driver in combination with the cross coupled feedback loops. For stability reasons the 50R source resistances cannot be omitted. So, this sort of cross coupling will always result in a gain which depends a bit on the load. But here the difference of gain for a 600R load and a 10k load is only 0.6dB and this is totally irrelevant for a typical audio application.

    Kai

  • 0
    TI__Expert 6290 points
    Hi Curtis,
    Kai's response is exactly correct - the cross-coupled outputs help maintain high common-mode output impedance, but doesn't compensate for the loading effect from an external load.

    To add to this, the datasheet specification of 6dB gain with an error of ±2% applies with a 600Ω load. Variation in gain with different loads is to be expected.