PCM6260-Q1: Attenuation value of the Input attenuator

As described in  PCM6260 Datasheet "The front-end input attenuator allows the device to accept the high-voltage input signal that is attenuated by the
input attenuator circuit before being routed to a low-noise programmable gain amplifier (PGA)".

But I found that  there was no Attenuation value of the Input attenuator described  in the datasheet.

how much is it?

Is it a fixed value？

Amy Luo：

Hi,

The PCM6240-Q1 includes an input attenuator network which allows for the high-swing mode input ranges on the single lower voltage supply.  The full-scale input range in line input mode is 10Vrms as shown in the specification table, but will be somewhat dependent on having a proper external common-mode voltage to achieve the full range.  The maximum recommended operating input pin voltages are shown in Table 7.3 and are 0V to 14.2V in line-in configurations.  The absolute maximum voltages (max voltage before damage) are shown in Table 7.1

The attenuator is not meant to be bypassed and is always in the signal-path.  In microphone input mode, the CHx_MIC_RANGE, register dictates the input swing range. 

More about the input attenuation can be learned using the following calculator tool:

PCM6xx0-Q1 AC Coupled External Resistor Calculator

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?? ?：

Hi

   I use the  DC-Coupled Microphone Differential Input  mode. so if the  DC differential common mode voltage IN1P – IN1M < 4.2V, i  should set the CHx_MIC_IN_RANGE to Low swing mode. If  set  to the High swing mode, there would   have Additional attenuation.

Am I  right? 

by the way, do you have   DC  coupled External Ressitor calculator  tools?

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Amy Luo：

Yes, you are right. It can be calculated that it is -14dB.

For DC coupled, external resistors can be selected according to the following description,  i.e. 'For a differential input, the value of the external bias resistance is recommended to be used for half of the microphone impedance, where as for a single ended input, the external bias resistor is recommended to be the same as the microphone impedance.'

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?? ?：

Hi,

The value of -14dB you have calculated is match the actual measurementsour in our application(DC-Coupled Microphone Differential Input  mode). 

Now we want to change using DC-Coupled Microphone Single-end Input mode. 

Can you calculated the attenuation value again. Also I wish you can share the formula with us, thank you.

,

?? ?：

Hi,

Because our software algorithms is base on -10.5dB, and our MIC modules sensitivity is also -10.5dB, so we don't want exist any attenuation by ADCs.

As we already know, this attenuation is an inherent property of the ADCs, so we have to calibrate the -14dB attenuation back to 0dB.

By checking datasheet, I found there are two ways to calibrate the attenuation through GAIN (CHx_GAIN[5:0]) or VOLUME (CHx_DVOL[7:0]) registers.

Am I right?

Can you give me some advice, whether to adjust  the GAIN or VOLUME registers？Which way is better and why？

Thanks！

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Amy Luo：

Sorry for missing your question.

when CHx_MIC_IN_RANGE=0，Differential input full-scale of 2-VRMS=2*1.414=2.828Vpp

when CHx_MIC_IN_RANGE=1，Differential input full-scale of 2-VRMS=14.14Vpp

The attenuation value can be calculated : 20log（2.828Vpp/14.14Vpp）=-14dB

When using single ended input, the input amplitude is half of the original, which is 1.414Vpp and 7.07Vpp.

Both methods should be possible, and it is recommended to choose the Gain register to achieve low noise performance as described in the screenshot of the datasheet. It should be noted that after ADC is powered on, the GAIN cannot be changed.

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?? ?：

Thanks for your reply.

According to your tips, when we use single-end mode, the input amplitude is half of the original, which is 1.414Vpp and 7.07Vpp.

As the same time, according the formula:20log(1.414Vpp/7.07Vpp),  the value of attenuation is still -14dB, does it right?

But the actual measurements in our system, the attenuation value is -8dB.

Can you help me to analyze this result?

Thank you~

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Amy Luo：

Under the same configuration, the gain of single-ended input and differential input is different, and the gain of single-ended input is twice that of differential input, or they have a relationship of 6dB.

I would like to know under what circumstances you measured -8dB, could you describe it in detail?

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?? ?：

Yes, in our system, we found that under the same configuration, the difference between single-ended input and differential input attenuation is 6dB.

The circumstances is DC-Coupled Microphone Single-end Input High swing mode. →the attenuation value is -8dB

The circumstances is DC-Coupled Microphone Differential Input High swing mode. →the attenuation value is -14dB

According to your tips, the formula of differential attenuation is 20log（2.828Vpp/14.14Vpp）=-14dB.Can you tell me what the formula of single-end attenuation is? 

Thank you.

,

?? ?：

Does the formula of single-end is 20log((1.414/7.07)*2)=-8dB?

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Amy Luo：

When AC-coupled single-ended Input connection, what is the attenuation gain?

The above Excel calculator tool has its input circuit, and I believe you can simulate the gain under different input configurations. When DC coupled single ended Input is used, its gain may be -8dB, but I did not simulate it. You can simulate and verify it.