using a Digipot in place of the drain resistor in a common source amplifier

sorry i made a typo.. digipots are 5280.

Best regards, 

GZ

Please share the schematic.

How much current is expected to go through the drain resistor?

What are the supply requirements?

The supply is around 10V rail to rail  while the drain current is around 100uA.

The best option for me would be to remove IC303 and replace R304 with a 10K digipot (preferably an I2C) device acting both as a drain resistor and as a tap for the next stage.

For what I have seen in your portfolio there's no 10K I2c compatible with the supply, so I got to think about putting the two digipot within the 5282 in parallel, or putting 10K in parallel to 1/2 5282.  Which of these makes more sense if any?

If I wanted to exploit a 10K I2C which PN  do you suggest?

Is the LTC3260 adequate for this kind of application or you have a better proposal?

Thanks

G

You have a C304 and C307 in the resistor path, probably for low pass filtering... If you replace R304, are you removing C304 as well?

Can you share a simplified circuit ?

What is connected to Boost Out tap?

For Dual supply and I2C interface, only AD5280, AD5282 and AD5263 are recommended.

You have a C304 and C307 in the resistor path, probably for low pass filtering... If you replace R304, are you removing C304 as well?

Can you share a simplified circuit ?

What is connected to Boost Out tap?

For Dual supply and I2C interface, only AD5280, AD5282 and AD5263 are recommended.

Hello,

here's the simplified schematic for both options. Components have different names.

If i remove replace the drain resistance  R29 ( in the previous schematic would be  R304), i also remove the blocking capacitor. A 10K digipoit would do the Job, if this option is suitable for you.

The RC between the pot and the +5V supply has the sole purpose to lower the drain voltage to  suitable level thus improving the headroom.

I already successfully implemented and tested the first option with a 50K pot. Moving the pot in place of the drain resistor should be a better solution.

Two solutions are slightly different. The alternate solution will work as long as AD5282 resistance terminals are within the specified voltage ranges.

Alternatively, you can use LTSpice to check your schematic with AD5282 replaced by a resistor network. Or you can share the application details with the schematic for us to review.

In the actual solution, you have lowered the FET drain impedance by the AD5282, whereas in the alternate solution, it is not. Also, in the actual solution, AD5282 wiper can go from full range 0-V+, and in alternate solution, it is limited by the FET drain voltage at the lower end.

Second, you need to make sure that the AD5282 is powered up first or at the same time as the entire circuit.

they are always between the termial voltage ranges:VDD and VSS are tied in both cases to the amplifier's positive and negative voltage rail( +5V and -5V), so the voltage will never exceed the correct limit and ESD should not be impacted.

In the actual solution ( working) the wiper has one terminal tied to gnd and the other one that swings form positive to negative , while in the proposed solution one terminal is tied to  a voltage lesser than +5V ( because of the RC bias in series ) and the other can  swing up and down according to the drain voltage, but it will never exceed 5V.

the +3V3 VL voltage is generated via a separated LDO,  so there might be differencies with timings is this a problem? 

regards, 

G

All the A/W/B terminals will be in high impedance state before VDD/VSS/VL are powered up. So, the Boost stage buffer and the FET drain will be in floating condition for that duration. It will not be a problem for the AD5282 but you have to check whether the system is stable or not due to this.

Thank you, I will definitely attention this issue.

What About the PM?  Is the LTC3260  adequate?  do you have any better proposals ?

regards, 

G.

Let me know what your requirements are with for the power management.

There are online tools that can help you make a choice.

https://www.analog.com/en/products/power-management/switching-regulators/multitopology-dc-dc.html

https://www.analog.com/en/design-center/design-tools-and-calculators/power-management-tools.html

A am currently exploiting (in the first prototype)  the LTC3260-  for the generation of +-5V (100mA). Thisnobe seems a good low costnsolution. Noise is a big concern because this stage will be the first one of an high gain audio processor for guitar. So my question is wether you got a more adequate solution for my need.

Please raise your power management requirements as another query in the link https://ez.analog.com/power/

Our Power Management team will then look into it.

Thank you.