Digital potentiometer mixing matrix

Question

I need to mix 4 analog inputs with different weights into 4 analog outputs with audio application in mind. This can simply be made using potentiometers and opamps for buffering, like shown on the first pic for 3x3 matrix; I do understand that this schematic is not perfect in many ways (for example, you can't cut off any input completely), but it's good enough to demonstrate what am I trying to achieve.

However, I need to control the matrix digitally, say, using the I2C interface. This can be done using I2C digital resistors, but the schematic will just get huge and costly, as for a 4x4 matrix I will need 16 digital pots. And with even more I/O it just gets ridiculous.

So, my question is: Is there a way to make such a 4x4 controlled matrix (without D/A and A/D conversion!) more compact and cheap? Say, are there any ICs made specifically for this purpose? I could not find any.

EDIT: I've just realised that these digital pots are very rarely logarithmic, and that is kind of what I am looking for also.

Answer 1

I'm not saying this is a perfect solution for the OP's partially defined problem but it gives food for thought.

Analogue cross-point switches (as developed for the first digital telephone exchanges) give you the ability to do the signal routing and then what you are left with is controlling amplitude.

First the analogue cross-point switch (this one is 16 x 16): -

The one above is the ADV3205 from analogue devices but they also supply 8x8 types. So the problem is somewhat reduced to controlling the amplitudes of the inputs by 4 digital pots for 4 inputs.

If you want all four inputs to be weighted individually to each of the four outputs then you might need to use 16 digital pots and a 16x16 cross-point switch.

However, there is an alternative to digipots and that is the use of PWM to modulate the amplitude of an audio signal like this: -

So if you can generate several independent PWM outputs you can use simple analogue switches to chop up an analogue input to reduce its amplitude. You need to have PWM frequencies that are significantly higher than the analogue highest frequency of course.

Answer 2

One way to do this would to have the same circuit, but replace all of the pots with digital pots, such as this one:

The drawback is they are slightly noisier than pot's (at 10nv per root Hz), they are more expensive (of course). The other drawback is currently the best ones now only have 1024 resistive values.

You would also want to protect the inputs.

Another thing that might be valuable if you do go with a digital pot, is you can convert a linear pot to have a more logarithmic response, but a resistive divider might not be suitable for your application.

Source: https://www.maximintegrated.com/en/app-notes/index.mvp/id/838

Answer 3

How about a circuit of this type ?

You said no ADC/DAC but I suppose it's for the audio path, the MCP4728 is a 4-channel inexpensive DAC.

1. DC Offset the signal to stay positive. (signal P-P/2 + transitor junction).
2. Trim the signal
3. Decouple the signal to remove the offset.

^{simulate this circuit – Schematic created using CircuitLab}