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AD1938 CODEC Full-Scale Input Voltage (Differential)

  • The Full-Scale Input Voltage (Differential) is mentioned as 1.9Vrms (5.35Vp-p) in datasheet. But the power AVDD supply voltage is 3.3V and common mode voltage is 1.5V.How this CODEC will handle this entire input voltage range?

  • In AD1938 Eval Board (UG-045) , the ADC and DAC front end OP-AMPs are supplied with +12V and -12V power supply. But the Eval Board (UG-087), the front end OP-AMPS are supplied with 0 and +5V. Why different supplies are used for OP-AMPS in these boards?

Please guide.

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  • Hello Mahendrakumar,

    Note that the specification is differential. 1.9Vrms. So this is 5.37Vp-p. But this is between two pins.

    The pins sit at a common mode voltage of roughly 1.5V. So one pin needs to swing a total of 2.69 V for full scale input.

    So this is 1.34V above and below the 1.5V. So the max will be 1.5V + 1.34V = 2.84V and 1.5V - 1.34V= 0.16V So this is the full swing of each pin. 

    So since one pin will be full positive and the other pin full negative you get the full swing of 5.37V difference between the pins. Each pin individually is within the 3.3V and ground but the difference of the two will be greater. 

    I know it is a bit tricky to see. Look at it on a scope and you can see this in action. 

    You question about the power supplies. The AD1938 eval board was designed to have a lot more headroom and to be able to have more gain if desired. One was designed more for pro audio applications and the other for a lower cost solution suitable for most consumer applications. No huge secret here, just personal preference and applications needs.

     

    Dave T

  • Hi Dave

    I am replicating the AD1938 CODEC to my PCB. I am referring to the circuits from the UG-045 AD1938 eval board document. But I am not sure what these circuits are for. please help me understand what the circuit below does?

    1) what this opamp circuit does before the audio input from the 3.5mm aux connector is fed to the CODEC ADC inputs?

    2) why is the +12V and -12V power supply needed for the opamp(OP275)? can I just have a single 5V supply? If I just have a single positive supply what are the limitations I will have on the audio quality compared to  +12V to -12V opamp supply?

    Appreciate your support!

    Thanks

    Santosh

Reply
  • Hi Dave

    I am replicating the AD1938 CODEC to my PCB. I am referring to the circuits from the UG-045 AD1938 eval board document. But I am not sure what these circuits are for. please help me understand what the circuit below does?

    1) what this opamp circuit does before the audio input from the 3.5mm aux connector is fed to the CODEC ADC inputs?

    2) why is the +12V and -12V power supply needed for the opamp(OP275)? can I just have a single 5V supply? If I just have a single positive supply what are the limitations I will have on the audio quality compared to  +12V to -12V opamp supply?

    Appreciate your support!

    Thanks

    Santosh

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  • Hello Santosh,

    Question1) The input on the evaluation board is a single ended signal. The ADC input is differential. The circuit takes the single ended signal and makes it differential. The other thing it does is to change the reference of the signal from ground referenced to instead be referenced to 1.5V common mode voltage that is on the ADC input pins. This circuit actually gives you a choice. You can ground reference but then you require the large electrolytic cap to block the common mode voltage from being pulled down. If you do not want the cap then you need to use the CM voltage as a reference for the circuit. The CM voltage from the CM pin should always be buffered by a voltage follower if you decide to use it for the Vref. 

    Question 2) The OP275 operates well with a +-12V supply. The specification for a full scale differential signal is 1.9Vrms between the two pins. 1.9Vrms = 5.37Vpp. Since this datasheet has been written there have been a number of OpAmps designed to operate on a single ended 5V supply. It really needs to be able to go rail to rail. Here is why:

    The max input of a single pin of the ADC is around +2.84V on the positive swing and around +0.16V on the negative swing of an input signal. So if you can invert the signal to one Opamp and not the other, ( Which the circuit you included in your post does do ) then to reach full scale one OpAmp drives one pin up to +2.84V and the other OpAmp drives the other pin down to +0.16 and that gives you the 5.37V differential between the two pins. 

    Therefore, the Opamps need to be able to swing between 0.16V and 2.84V when operated using a single 5V supply. ) 0.16V is very close to the negative rail of the Opamp, in this case that is ground. +2.84 is not all that close to the positive rail but not all Opamps designed to use single ended supplies can do this. I found that out once in a design I did. The output of the Opamp would not go over 2.5V. I pulled my hair out trying to see what I did wrong in the circuit until I reread the datasheet and noticed that the positive swing of the Opamp was Vcc-2.5V. So with a 5V supply the max was 2.5v!! 

    So you can use a single 5V supply. You have to choose good Opamps that can go rail to rail and you have to reference the Opamps to either 2.5V or you can use the CM voltage of 1.5V.  Keep in mind that the ADC pins are riding up at the 1.5V CM voltage and you must not pull this up or down. 

    The other factor is that some Opamps do not perform as well when they are close to the rails. So now the "sounds good" subjective factor comes into play. Most of the really good sounding Opamps operate on bi-polar supplies. But getting into that discussion is like opening up a Pandora's Box!

    If you need help choosing an OpAmp then post a question on the linear amplifiers section of the forum. 

    One other thought on the choice to use +-12V. For many pro-audio designs the signal levels can be as high as +21dBu. For that level +-12V is still not high enough. Usually pro circuits use +-18V or greater and the low ones use +-16V but those designs use transformers to get the +6dB more on outputs. So if you are interfacing with pro audio equipment you will need to amplify the DAC outputs and be able to take in a high level signal and attenuate it for the ADC inputs. Hence using a higher bi-polar power supply for the Opams. It is not so much for the codec but for the rest of the design. 

    Dave T

  • Hi Dave 

    Thanks for the detailed explanation. 

    I understand the differential full-scale voltage of ADC input is 5.37Vpp but I am confused with the below paragraph

    "The max input of a single pin of the ADC is around +2.84V on the positive swing and around +0.16V on the negative swing of an input signal. So if you can invert the signal to one Opamp and not the other, ( Which the circuit you included in your post does do ) then to reach full scale one OpAmp drives one pin-up to +2.84V, and the other OpAmp drives the other pin down to +0.16 and that gives you the 5.37V differential between the two pins"

    1) What do you mean by inverting the signal to one opamp? 

    2) one OpAmp drives one pin-up to +2.84V, and the other OpAmp drives the other pin down to +0.16 and that gives you the 5.37V differential between the two pins.....How is this? it is confusing to me? is my signal drawing in the image below correct?

    Thanks

    Santosh

  • Hello Santosh,

    I drew some lines on your screenshot.

    This is the signal path. The signal is inverted by U12-A and sent to the ADC,

    Then this inverted signal is send through U12-B to be inverted again so it is back to not be inverted. That is sent to the other input of the ADC. So this is what I meant when I said one of the op amps input signal is being inverted. There are several ways to do this but the result will always be that one output is inverted and the other is not. 

    So when one is going negative the other is going positive. 

    The range of each ADC input is from +2.84V to +0.16V. The total range is the difference of these two numbers. So the voltage can swing a total of 2.68V. If one pin is fully positive and the other fully negative it is a differential swing of 5.36V. 

    All I can say is that it works. Look at it on a scope and see the digital output. 

    So I drew some more lines on your drawing as best as I could with a mouse.

    Imagine the two lines I circled being drawn together as one line. Pull the graphics together. The overall swing will be 5.36V. 

    I hope this helps. 

    Dave T

  • Hi Dave

    Thank you. I did a ltspice simulation and the confusion about signal waveform is clear now.

    1) Should I have two GND planes Analog and Digital Ground for this CODEC on my PCB?

    or a single GND plane is ok?

    2) What should be the trace impedance for the ADC differential inputs? 50ohms single-ended or 100ohm differential?

  • Hi Dave 

    any update on the above queries?

    1) Should I have two GND planes Analog and Digital Ground for this CODEC on my PCB?

    or a single GND plane ok?

    2) What should be the trace impedance for the ADC differential inputs? 50ohms single-ended or 100ohm differential?

    3) what to do with unused ADC inputs and DAC outputs? how to terminate them?

    Thanks

    Santosh