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I'm trying to design simple multichannel ADC interface for recording live performances from audio mixing console. Many AD converter chips have differential input. Also direct output from mixing console is also differential.

Question is: Do I need to place differential ops amplifier in front of ADC chip or I can feed output from console directly to AD IC?

clabacchio
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ozzo
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    It is impossible to tell without specs about what is coming out of the mixer and what the A/D want. In general though, a buffer amp is a good idea when in doubt. – Olin Lathrop Dec 22 '11 at 00:34
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    If its a standard balanced line out then a differential opamp in front of a single ended ADC should do fine. Most (digital) mixer inputs have something like this, so they can take balanced or unbalanced signals (e.g. with unbalanced the shield shorts the ring/one side of input to ground and the tip varies relative to this) – Oli Glaser Dec 22 '11 at 01:37
  • console is mackie onyx 32-4. AD is AKM ak5385b. they both have differential output/input. – ozzo Dec 22 '11 at 02:07

3 Answers3

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Since you want to convert the output from an audio mixer, it will be at the line level, which is standardized to be an AC signal with peak values defined by the standard in use. The maximum voltage that you can expect is +-2.192 V, or 4.384 Vpp.

Your ADC takes signals in the range [-0.3 ÷ AVdd+0.3], so you'll need to both amplify your signal and traslate it from centered in 0 to centered in AVdd/2. You'll need to tune your amplifier/shifter (better to use pots) to cover as well as possible the range of the ADC without distorting (consider that amps usually distort before the saturation threshold).

If you look at the datasheet of the ADC, it suggests an input buffer that you can use to condition the signal from the XLR input:

enter image description here

Edited out:

If instead the output is single ended, you can also do it with a normal precision op-amp in level shifter configuration. You can as well use a rail-to-rail op-amp, or use a "normal" one with slightly larger voltage range. But using the same supply of the ADC may result handy. You'll need to tune your amplifier/shifter (better to use pots) to cover as well as possible the range of the ADC without distorting (consider that the op-amp usually distorts near the saturation threshold).

In this post you can find some ideas for level shifters, such as:

Olin

Clabacchio

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clabacchio
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  • Also, an op-amp is really much more then is strictly necessary. Two resistors and a series capacitor would likely work just as well, you just have to be sure the rolloff of the filter it forms is below your signal's range. – Connor Wolf May 15 '12 at 08:37
  • @FakeName The OP is asking about differential signals, but if I read the manual correctly, it's talking about line level signals, which are not, especially in the Stereo case. The op-amp is necessary because you have to both amplify and shift – clabacchio May 15 '12 at 08:46
  • In pro audio, signals *are* generally differential (as in, two wires per channel (and a common ground, too)). See [Balanced Audio](http://en.wikipedia.org/wiki/Balanced_audio) on Wikipedia. – Connor Wolf May 15 '12 at 08:52
  • @FakeName The OP said the mixer outs are differential, but this is not always the case. Often people will use the channel insert jacks as "send only" to multi-track recorders and these are not differential. –  May 15 '12 at 14:43
  • Thanks for the answers. I didn't pay attention to voltage range of AD, so I missed the need to shift voltage. Also this specific console has dedicated direct output channels which are differential. – ozzo Sep 13 '12 at 00:09
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The simplest way to level shift (e.g. inject an offset) your inputs is just two resistors and a capacitor:
enter image description here
Simply use one per channel of your differential signal.

However, this does not provide any amplitude change, if you need it. It also provides very little input protection.

Lastly, the capacitor and resistors form an RC network, so it will also act as a high-pass filter. You need to be sure that the roll-off is below the range of your signal of interest.

For stability, I would recommend tying the top of the resistor divider to your ADC voltage reference, if possible.

Connor Wolf
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  • How can it level shift and not traslate? Aren't they synonims? – clabacchio May 15 '12 at 08:48
  • @clabacchio - Good point, that's not very clear. What I mean is that it injects an offset, but does not change the signal's amplitude. – Connor Wolf May 15 '12 at 08:50
  • Fixed the answer. – Connor Wolf May 15 '12 at 08:51
  • But depending on the output peak voltage, he will probably need some amplification, to use all the resolution possible with the ADC...in audio application the precision is quite important, and ADC bits are expensive. But what's the point in shifting both channels of a differential signal? – clabacchio May 15 '12 at 08:52
  • @clabacchio - Maybe, but if you're working with purely differential signals, you can use some of TI's cool [fully-differential op-amps](http://www.ti.com/product/ths4521), or something. – Connor Wolf May 15 '12 at 08:55
  • Also, if it were me, I would have all the analog front-end running off +-5V, and do the level shifting as late as possible. That way, you're not stuck with trying to create a virtual-ground to apply filtering to the signals, etc... – Connor Wolf May 15 '12 at 08:55
  • Anyways, if the input signal is indeed 4.384 Vpp, with a *little* bit of attenuation, you would get the entire input range of an ADC with a 4.096V reference. – Connor Wolf May 15 '12 at 08:57
  • -1 'cuz you need a lot more than just a DC level shift. Some attenuation, low-pass filtering, protection, and controlling input/output impedance is also needed. –  May 15 '12 at 14:31
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Sorry any suggestions are invalid without known System Specs.

There are many unknowns, that need design specs 1st;

1) Console source impedance.. old school 600Ω balanced or new school 60Ω balanced matches most cables used in industry for flat frewuency response.

If long cables, which types? many are 32pF/ft, some 12 pF/ft and Mogami 2944A shielded pair at 6 pF/ft. 600Ω termination will give poor LPF performance and also Console current limit will have slew rate limit on full scale, so 60Ω source is recommended.

2) What is CMR spec for System? 75dB? 90dB is possible with Cap trimmed with grounded center tap transformer from Jensen Transformers and Reichenbach Engineering.

CMR is unknown in ADC.. Only 0.1dB typ 0.5dB gain mismatch is specified. Depending on Common Mode EMI signal level, and rejection ratio (CMRR) may induce hum from stray CM levels. If so balanced Instrument Amp is recommended for 100 dB . 3) Desired Termination impedance. 100kΩ? 10kΩ? differential? or single ended.

4) Headroom before ADC clipping ?

5) Desired Sampling rate and pre-filtering to prevent aliasing or IM distortion.

6) If system BW must be reduced to < 1/2 of Sampling Rate of ADC, what is desired Roll-off rate or LPF Order and LPF spec 1dB or 3dB @50kHz? 23kHz?

7) What is console Current drive limit. ADC has 10mA protection beyond supply rails with internal small signal Schottky diode.

8) What transient current protection is needed by ESD?

9) What DC supply will be used for ADC?

10) What is HPF cut-off spec? 3dB @1Hz? 10Hz?

11) What is console max output Voltage 30dBu? 10dBu? 12) Decimation Filter spec for ADC

The answers of these specs questions will detemine if you can direct connect to ADC or need simple poor unity gain buffer to 0.5dB mismatched gain error for ADC inputs or need better solution. Filter needs to be defined as well as clamp diodes for ESD and Console overdrive. So front end needs careful selection, with specs and test plant to verify linearity, monotonicity and missing codes.

Simple solution to some issues,but not all simple

Tony Stewart EE75
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    It's pretty useless to ask for more informations to that user, since he's disappeared 4 months ago. I've dug this post to provide some content about the topic, but you'll have to deal with the existing information – clabacchio May 15 '12 at 15:02