INA1651 input resistors

[thor.zmt]

I played a bit with Bootstrap circuit with LtSpice for simulation.
I posted attached only one side of the balanced input I simulated just to show something I notice that I can't explain with certitude.

I note that you have chosen independent bootstrap networks, I would not recommend this as it causes more interaction.

On simulation I found that input impedance goes higher than 22kOhm (R11)...

Positive feedback is used in this circuit.

The phenomenon disappear if I disconnect C3/C4/R7, so I think it is comming from here but why?

Because you create phashift (R7/C4)

Also, there is a bump on the very low frequencies on the Output magnitude chart. How could we explain that?

Same way as above.

Third question, Are C1 and C9 needed to be audio capacitors or simple ceramic capacitors could be enough?

Most ceramics at these values will be X7R or worse. They are highly microphonic. Go either for film or in order to eliminate a lot of the phasing effects, use a much larger value bipolar electrolytic capacitor (they do come in SMT).

Thor

 

[sdvd]

I note that you have chosen independent bootstrap networks, I would not recommend this as it causes more interaction.

Yes because I understand that this structure doesn't need the matching resistors on bootstrap input to provide the common mode signal and on output to feed the differential input equally so I thought it was easier to get the expected result.
I understand it causes more interaction as it follows the signal constantly (Not only common mode signal compared to the other method)
Do you think it could oscillate or something? Is it a really bad idea?

What could be the consequence of unmached resistors on your common mode bootstrap schematic? (R1/R2, R4/R6, R8/R9)
In case of unmatched resistors, could the CMRR be worse than without the circuit or will it provide equal CMRR at least?

Thank you.
David.

 

[thor.zmt]

Yes because I understand that this structure doesn't need the matching resistors on bootstrap input to provide the common mode signal and on output to feed the differential input equally so I thought it was easier to get the expected result.

That is not what I meant. If you look at my generic input bootstrap schematic, everything is bootstrapped from one source, not several, as in your schematic.

In the real world "matched" resistors do not exist. Anything has tolerance.

The arguments for and against individual and common mode bootstrap are complex.

Personally I like using high input impedance where feasible, so I would tend towards individual bootstrap.

The industry however tends towards Bill Withlocks Common Mode Bootstrap, which you can get as Chip solution:

Getting the Most from THAT’s Balanced Line Drivers and Receivers

I suspect that THAT will be easier to get. Almost all Audio IC's by TI have zero stock at mouser and 104 weeks lead time.

Thor

 

[sdvd]

That is not what I meant. If you look at my generic input bootstrap schematic, everything is bootstrapped from one source, not several, as in your schematic.

Sorry for my poor English. I understand what you mean now. I used separated "sources networks" to isolate DC from the input. If I correctly understood your proposal, there is a resistive path between the two sides of the input DC capacitors which is not the case here.
What could be the downside of doing this?

Personally I like using high input impedance where feasible, so I would tend towards individual bootstrap.

I think the same.

 

[thor.zmt]

Sorry for my poor English. I understand what you mean now. I used separated "sources networks" to isolate DC from the input. If I correctly understood your proposal, there is a resistive path between the two sides of the input DC capacitors which is not the case here.

If this is a concern, you can use this circuit:



It also shows the "Birt" Circuit for a balanced in / balanced out design.

What could be the downside of doing this?

Complexity, extra phase-shifts.

Many ways of using Bootstrap.

Thor