Unity gain amplifier for switched capacitor circuits

[meghna]

Hi,
I have some doubts regarding the connections for the OTA connections used as buffers.
There are two inputs and two outputs of differential OTA. so can we directly
connect inputs to outputs (say in+ to out- and in- to out+) to act as a
buffer, or resisters are needed. If resisters are needed, what shud be there
values..is it that we need to have 4 resisters (2 in feedback path and 2 res
before the signal input)

Please clarify my doubts.

Regards
Meghna

 

[ezt]

Hi.
I think as a buffer structure, just connecting outputs to inputs (with inversed sign) is enough.

Regards,
EZT

 

[meghna]

Hi,
Sorry for the wrong subject of the mail. Actually my circuit is using swtiched capacitor cicuits, but for the buffer, it has nothing to do with switches. I just want to know a buffer connection without any switches or capacitor (to get less inaccuracies)

 

[noiseless]

two things needed to be consider
1. OTA gain. if directly connected, OTA will be lost. this can be seen by look at the output impedance.

2. slew rate.

 

[meghna]

thanks for the reply. Please explain me how the output impedance gets affected.
when I connect the buffer at the output of my circuit, the buffer itself changes that output. does that mean buffer itself is loading the main circuit?

 

[JPR]

If you are connecting the differential in/differential out as a buffer, you must be careful that there is not a contention with the common mode at the output. You could SIGNIFICANTLY load the previous section by your amplifier trying to set the common mode output voltage. Because of this, it would be much safer to connect using resistors in feedback.

 

[noiseless]

If you already put a buffer after the OTA, then that is fine.

 

[meghna]

Thanks JPR. It may be the cause of problem in my circuit. Could you tell me how to decide the values of resistors which ensures no loading.

Thanks noiseless. I am confused on how to decide which opamp should I use for using it as unity gain amplifier.
Actully I need to integrate two stages (menas output of first stage should be given to the next stage), but when I connect it then the output of first stage itself changes. I termed it as loading and thought of connecting a buffer. When I connect a buffer, it again loads the first stage. (say out+ and out- are the outputs of the first stage, then both the outputs tends to be the same)
Slew rate of my opamp is more than enough.