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ADA4075-2 and SSM2167 Q's

Hi,

We are looking at using the ADA4075-2 instead of the OP275, as recommended by ADI, as a balanced line driver to the AD1974. However, the reference design uses trimpots whereas these are not shown for the OP275 reference design. As we are looking to have 12-16 inputs per PCB, it isn't feasible to tune each individual trimpot. Can these be left out or are there standard resistor values which are used which produce 'good enough' results.

Further to this, there are in some places specifications for the ADA4075-2 and SSM2167 in the datasheets as to what type of capacitors to use, however not always. I was wondering for a general idea of what types of capacitors are suitable for these devices. I have read conflicting information about how 'bad' ceramic capacitors are in audio etc and just want to get some device specific info.

Thanks

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  • Hi Daniel,

    Here are responses to your follow-up questions;

    1)

    • The RC filter at the input serves two purposes. The R keeps the cathode end of the 47 uF referenced to Ground, stopping it from 'wandering' away from 0 V; leakage in this capacitor through the anode to the cathode would result in voltage at the jack. If you were to plug a connector into the jack, the voltage would discharge resulting in a POP. In a design where signal is hardwired to this node, these resistors are not necessary. The 100 pF shunt is a standard value used to keep EMI from entering the board through the hookup cable.
    • In the case where the input buffer op amps are referenced to GND, the polarized AC coupling capacitors are required to block the Common Mode (CM=1.5VDC) voltage present at the ADC input pins. This CM voltage is supplied by a regulator inside the AD1974 and is the reference for the analog section of the AD1974. Without an AC coupling capacitor, the 'bias' voltage on the ADC pins would be dragged down close to 0 V and there would be very little signal headroom.
    • The AC coupling capacitor value was calculated using the ADC port input impedance of 15 k ohms. The Fc for this value is below 1 Hz; you may reduce the value of this capacitor as you see fit.
    • The 237R/1nF/100pF RC circuit keeps any out of band energy from coming out of the ADC port. There is not much of this energy present in this continuous-time ADC design, and this RC eliminates all of it.

    2) Let's clarify: the max signal for the ADC is 2 V RMS differential. This is a differential Vp-p of 5.6 V, so each amp will need to be able to drive 2.8 Vp-p. Keep in mind that, you will need to use rails that allow the ADA4075 to handle the incoming voltage level, if it is higher than 2 V RMS differential or 1 V RMS single ended. If you use a VREF of 1.5 V DC (the same as the CM voltage supplied by the AD1974 ADC pins), you will not need the AC coupling between the buffers and the ADC and your power rails must be able to handle your total signal level (Vp+CM) since the buffer will offset your audio signal by the CM voltage of 1.5 VDC. One other factor: the specs for the ADA4075 are given for +/-15VDC. I would expect that self-noise and THD performance will be reduced somewhat by using lower rails, but I do not have any info about this.

    Regards,

    Coleman

Reply
  • Hi Daniel,

    Here are responses to your follow-up questions;

    1)

    • The RC filter at the input serves two purposes. The R keeps the cathode end of the 47 uF referenced to Ground, stopping it from 'wandering' away from 0 V; leakage in this capacitor through the anode to the cathode would result in voltage at the jack. If you were to plug a connector into the jack, the voltage would discharge resulting in a POP. In a design where signal is hardwired to this node, these resistors are not necessary. The 100 pF shunt is a standard value used to keep EMI from entering the board through the hookup cable.
    • In the case where the input buffer op amps are referenced to GND, the polarized AC coupling capacitors are required to block the Common Mode (CM=1.5VDC) voltage present at the ADC input pins. This CM voltage is supplied by a regulator inside the AD1974 and is the reference for the analog section of the AD1974. Without an AC coupling capacitor, the 'bias' voltage on the ADC pins would be dragged down close to 0 V and there would be very little signal headroom.
    • The AC coupling capacitor value was calculated using the ADC port input impedance of 15 k ohms. The Fc for this value is below 1 Hz; you may reduce the value of this capacitor as you see fit.
    • The 237R/1nF/100pF RC circuit keeps any out of band energy from coming out of the ADC port. There is not much of this energy present in this continuous-time ADC design, and this RC eliminates all of it.

    2) Let's clarify: the max signal for the ADC is 2 V RMS differential. This is a differential Vp-p of 5.6 V, so each amp will need to be able to drive 2.8 Vp-p. Keep in mind that, you will need to use rails that allow the ADA4075 to handle the incoming voltage level, if it is higher than 2 V RMS differential or 1 V RMS single ended. If you use a VREF of 1.5 V DC (the same as the CM voltage supplied by the AD1974 ADC pins), you will not need the AC coupling between the buffers and the ADC and your power rails must be able to handle your total signal level (Vp+CM) since the buffer will offset your audio signal by the CM voltage of 1.5 VDC. One other factor: the specs for the ADA4075 are given for +/-15VDC. I would expect that self-noise and THD performance will be reduced somewhat by using lower rails, but I do not have any info about this.

    Regards,

    Coleman

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