[SOLVED] Help in the choice of a switch.

[erikl]

I'm using a NMOS input. So, if I use a PMOS input it should work with the grounded positive input? Or should I use the VCM = 1.65V?

Both methods should work.

 

[brito.tb]

I'm using this configuration of the OTA:


And using this OTA, connecting the positive input and the switchs to gnd I have this:




And using this OTA, connecting the positive input and the switchs to vcm I have this:



If I use an OTA with the input PMOS can I use GND instead of VCM to achieve the second result?
Thanks.

 

[erikl]

PMOS input OTA's ICMR includes GND

If I use an OTA with the input PMOS can I use GND instead of VCM to achieve the second result?

Yes, I think so, as in this case the OTA's input common mode range (ICMR) will include GND .

 

[brito.tb]

Re: PMOS input OTA's ICMR includes GND

Yes, I think so, as in this case the OTA's input common mode range (ICMR) will include GND .

I'll try using a PMOS input then. Thanks for the tip.

 

[brito.tb]

Re: PMOS input OTA's ICMR includes GND

I used an OTA with the PMOS input and the result is the same. When I use VCM the configuration of integrator works, but if I use GND don't.
Using a rail-to-rail OTA, I'll obtain success in to achieve the desired result?
Thanks.

 

[erikl]

I used an OTA with the PMOS input and the result is the same. When I use VCM the configuration of integrator works, but if I use GND don't.

In this case the ICMR of your special OTA topology doesn't contain GND, i.e. if one of the inputs receives GND potential, the OTA doesn't work within its linear range any more.
This, e.g. is the case if you use feedback either directly or via a resistive divider from the output to the inverting input: if your output is unable to get completely down to GND, the OTA feedback loop cannot bring down the inverting input to GND either, hence the circuit gets out of its linear operation range. To avoid this, you'd have to redesign your OTA topology completely.

Using a rail-to-rail OTA, I'll obtain success in to achieve the desired result?

No, this wouldn't help. I suggest you use the VCM method; it's a virtual GND anyway.

 

[brito.tb]

In this case the ICMR of your special OTA topology doesn't contain GND, i.e. if one of the inputs receives GND potential, the OTA doesn't work within its linear range any more.
This, e.g. is the case if you use feedback either directly or via a resistive divider from the output to the inverting input: if your output is unable to get completely down to GND, the OTA feedback loop cannot bring down the inverting input to GND either, hence the circuit gets out of its linear operation range. To avoid this, you'd have to redesign your OTA topology completely.

I redesign the OTA like you said and the result is much better.


But I have another question. How can I start the integration from 0 V?
Thanks.

 

[erikl]

How can I start the integration from 0 V?

I think this isn't possible without using a negative voltage supply against GND .