Need help for this circuit

[deba_fire]

Hello everyone,

I have come across this circuit and I am bit confused about this.The circuit is below:

Iref_5uA is a pin on an IC.The circuit is used to give a 5uA reference pull down.Since its a pull down current this pin must be the source terminal of a NMOS current source.The idea is to measure the accurate resistance and adjust the voltage across the connector till the voltage corresponds to 5uA.This part is clear to me.

What I am not able to understand is why Iref_5uA is connected to VDDA by a capacitor?

Please provide some comments.

-DB

 

[checkmate]

If it's to generate a reference current, then "Iref_5uA" must be a reference voltage. If so, it makes sense to give it a reservoir cap. This reservoir cap is to Vdda, maybe such that a smaller cap can be used, or if Vdda is less noisy than gnda.

 

[plingane]

Hello everyone,

I have come across this circuit and I am bit confused about this.The circuit is below:

Iref_5uA is a pin on an IC.The circuit is used to give a 5uA reference pull down.Since its a pull down current this pin must be the source terminal of a NMOS current source.The idea is to measure the accurate resistance and adjust the voltage across the connector till the voltage corresponds to 5uA.This part is clear to me.

What I am not able to understand is why Iref_5uA is connected to VDDA by a capacitor?

Please provide some comments.

-DB

I suspect the capacitor is needed for stability of the reference, just the same as a voltage regulator that needs a minimum capacitance at it's output. Check the data sheet for the chip. Another possibility is that it is needed to provide a momentary pull up during power up. Perhaps both reasons. Try disconnecting it and see what happens when you power up. Or try disconnecting it after power up to see if the reference becomes unstable. If it does, then while power is still on, try connecting the capacitor to ground to see if the reference then becomes stable. Also, try powering up with the capacitor connected to ground instead of the supply.

 

[deba_fire]

Hi Checkmate,
Yes this pin is used to generate 5uA reference current for the IC.And why it has to be a reference voltage?In this case it is a 5uA sinking current.Which means it is the drain of pmos current mirror circuit.Instead of using a costly reference current chip a low cost poor man's current source is used.
I don't understand the meaning of reservoir cap?Can you please explain that to me?
Hi Plingane,
I will to the experiments once I have the board.