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.
I will be happy to help you with your questions. Can you please tell me which reference design you are consulting, that has trimpots? In order to help you, I would like some other information about your system. What power supply rails do you have available? What is your maximum signal level entering this circuit (RMS or p-p)? I will be specifying the circuit with a 100 kHz low pass filter. The use of 1% resistors and 10% capacitors will yield the performance specs shown in the datasheet.
As far as capacitors are concerned: I would recommend high quality NP0 ceramics for small value filter capacitors, X7R ceramics for local power supply decoupling and high temperature electrolytics for AC coupling in the audio path. Metal film capacitors are superior for audio, but I would only recommend them in a through-hole package; I do not have good experience with this type of capacitor in a surface mount package. They do not seem to survive assembly.
I look forward to hearing more about your design
Thanks for the reply.
The design I am looking at is in the ADA4075-2 datasheet. The system will consist of either an electret microphone or a ADMP504 mems mic being preamped using the SS2167 then feeding into the ADA4075-2 and finally the AD1974. I haven't looked at the p-p etc for the different mics yet, so I can look at these details tomorrow maybe. An aside question, is it always necessary to use a preamp, like the SS2167, or is it possible to feed the mic output straight in to the ADA4075-2 amplification and increase the gains in this stage?
Regarding the electrolytic caps, what are your thoughts on aluminium vs tantalum? Aluminum seem to be more prevalent - is this just a cost factor? Also, excuse my lack of knowledge on this, but with the metal film caps - what specific types are used in audio (maybe an example part number or something would help a bit - lots of caps out there) and what sort of difficulties arise with the SMD packages? Are they used mainly for the coupling or filter portion of the audio path?
Thanks for the thorough answers to both posts!
The SSM2167 is a helpful device if you need the VCA or compression features that are included. If you need only gain with a Low-Pass Filter, I would use the ADA4075 to increase the voltage of your signal to reach the operating input level of the AD1974 (2 V RMS differential maximum). You could use a simple circuit as shown here on Page 9 of the User Guide:
Changing the value of the feedback resistor in the first stage will be the easiest way to adjust the gain of this circuit. I would change the feedback capacitor accordingly, to leave the first-order pole of this LPF around 100 kHz. Depending on your choice for a VREF (the voltage that appears at the non-inverting input of the op amps), you might not need to add AC coupling capacitors between this stage and the ADC input. And you will certainly need the inverting stage, as Harry has pointed out, in order to drive the ADC to full scale.
Aluminum electrolytics are larger, but lighter, much less expensive and much more available in varying values and voltage ratings. Aluminum electrolytics dry out over time, where tantalum do not. I would not recommend tantalum because they do not go bad slowly over time, they either short (which can be dramatic if there is enough current available. BOOM!) or open. I would use Aluminum electrolyics in the audio path and if you need a large bulk reservoir for power supply decoupling, use tantalum. Remember to give yourself plenty of headroom with voltage ratings: double the DC voltage that is biasing the cap, plus any audio which is on top of the voltage.
Thanks for the thorough responses, they address my original questions.
I have some final follow-up questions.
1) Looking at the schematics that you posted, I am unsure about a few things.
2) We have 3.3V on board but haven't yet selected the V+/V- supply for the ADA4075. The minimum for the ADA4075 is +/-4.5V, seeing as though the Vp-p to the AD1974 is around 2, is there any need/benefit to go above this miniumum? Also, what Vref can I use so I don't need the additional AC coupling caps on the output side of the opamp?
Here are responses to your follow-up questions;
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.