Input and output characteristics

Hello, agarciagon,

I'll be happy to answer your questions, but I will want to check some of them on the bench before I commit to some of them.

• Input:  This is certainly a valid use case, and other customer have done similar things.
  • The offset is automatically internally applied to the input pins when the device is enabled.  All that is required is that you use an AC coupling capacitor in series between the source and the input pin.  The value of this capacitor can affect the frequency response, so higher value capacitance will pass lower frequencies.  1µF is usually sufficient for most cases.
• Output:
  • The output voltage will depend on Vspkvdd.  Since our datasheet does not provide open load measurements, I will need to verify these values on the bench.
  • The power numbers in the datasheet are RMS, not peak.
  • The speaker output is Class D, so this may affect what kind of amplifier can accept the inputs.  A low-pass filter may be required between the speaker outputs and the amplifier inputs.  I will need to verify if there is any DC offset with an open load on the bench.
  • I don't have any data on the output impedance, but I may be able to find that data.  Please stand by for this.

Let me spend some time on the bench and I will be able to provide better answers to your questions.

Ed

Input:

• On the bench, I was able to reach full-scale speaker output voltage with up to 1.44 Vp-p on the input.  This is higher than what the datasheet reports, so I would still recommend not exceeding the 1.05Vp-p to ensure that that the input will not clip.
• The input signal that I used was single-ended referenced to ground, but it uses a 1µF AC coupling capacitor in series with the MIC1 P input.  MIC1 N was tied to ground, also with a 1µF capacitor in series.

Output:

• Without a load, the output can approach ~2x Vspkvdd without exceeding 1% THD.
  • Vspkvdd = 5.5V (EC Table Maximum)
  • MIC1 input = 1.022Vpp
  • MIC1 gain = 0dB preamp, 0dB PGA
  • Speaker gain = 6dB
  • Speaker Output = 3.855Vrms (10.9Vp-p) @ 1% THD+N

Attached is a scope shot of the speaker output with 1% THD+N.  The output is starting to clip on the positive edge as it is reaching the limits with such a large signal.  The scope shot includes the speaker outputs before and after going through a low-pass filter.

I am still researching the output impedance.  I will update this if I am able to locate this data.

Hello Ed,

First of all thank you so much for your help, it will be very useful for me.

I have some question:

• Input: regarding the input you said that it is recommended not to surpass 1.05Vp-p (0.5Vp). As you used decoupling capacitors the input signal is AC only with MICP= +0.25Vp and MICN=-0.25Vp. This allow not exceeding the -0.3 to +2.2V range indicated in the absolute maximum ratings but is really near the minimum value and quite far of the maximum value. Is it not really needed a DC offset and dc coupling at the input? (I mean, no capacitors in series).
• Output: Is it possible to see a scope shot of the headphones output with no load under the same testing conditions? With 0dB gain at the output I mean, as the headphones doesn't have the 6dB amplifier.

If you know the output impedance for speaker stage and headphones stage please let me know.

Thank you very much.

Hello, Agarciagon,

I am using the MAX98089 EVKit to make the bench measurements.  Please find the schematic at the following link: max98089evkit_wlp.pdf (analog.com)

I am applying the input signal to the MIC1P test point, which passes through the 1µF AC coupling capacitor C121.  I have JU36 inserted, which applies ground to the input side of C118 which AC couples the MIC1N pin.

The input signal that I'm using is 1Vp-p, single-ended referenced to ground, so the input is a 0V - 1V signal.

If you do not want to use coupling capacitors at the input, be aware that the signal needs to be biased at ~AVDD/2.  If the bias is not set correctly on the input signal, there will be DC components on the output signal.  It is much easier to use capacitors in series to prevent this.

Here is a screenshot of the headphone output, unloaded, with a 1Vp-p input on MIC1P and with the headphone gain set to 0dB.

I am still researching the output impedance.

Hello Ed,

Thank you very much for the information. About the filtered class D signal you showed, is it taken after the filter it is present in the evaluation board?

II mean this one:

Thank you,

Hello, agarciagon,

My filtered scope shot was taken after the signal passed through an Audio Precision AUX-0025 Class D Filter.

The filter on the EVKit is intended to reduce EMI when using long speaker cables.  I don't think that it is sufficient to remove all of the high frequency switching from the speaker outputs if that is a requirement for the amplifier that will be connected to it.

Hello Ed,

Thank you for your response. Another doubt that comes to me is regarding the mic to adc frequency response graph that is located on page 29 on the datasheet.

First of all, I suppose this is the characteristic behaviour of the codec from the input to the ADC, without taking into account digital voice filters after the ADC.

Where it is specified 'freq mode' I suppose it means that clocking mode is working on Exact Integer Mode (table 11 register 0x11), and the two indications of "MODE=1" and "MODE=0" I suppose it refers to IIR and FIR. But I'm not pretty sure because I've readen that there is a 'MODE1' parameter in register 0x18 that configurates the passband filtering mode, but this passband filter is located after the ADC. And I've not seen any "MODE" parameter in the registers. Moreover, in page 7 it says that MODE=0 is IIR voice and MODE=1 is FIR audio.

Can I get your help to clarify this?

Thank you.