cancel
Showing results for 
Search instead for 
Did you mean: 

PSoC 5, 3 & 1 MCU

New Contributor II

Hi,

   

need to kno what is the actual minimum value of voltage Vssa in CY3866AXI-040. the datasheet specifies it as gnd +/- 0.5V but in the attached project of PGA the swing am seeing in the output is much below gnd at high gain value.Kindly see the attached file.

   

I've also seen the swing with a naked opamp in the same project, amplification is seen peak to peak an dnot above gnd.

0 Likes
Reply
10 Replies
Anonymous
Not applicable
        Hi neha, Where is description "specifies it as gnd +/- 0.5V" you said? 0.5V is too big. And "much below gnd", How much the voltage and your gain setting? As usually you can use internal Vref or a Vref module outside of PGA. I mean "OpAmp" voltage follower at Pin[0]-3. might be doesn't need. Need more detailed informations.   
0 Likes
Reply
Anonymous
Not applicable

1. What is the voltage you are giving in P0[3] pin?

   

2. Where is it specified that Vssa can be GND+/- 0.5V?

   

3. It is definitely not possible to get -ve voltage out of PSoC as the lowest potential for the device is 0V only. How much low are you seeing at the output?

   

4. Why don't you specify the voltage values you are seeing on different pins of your project?

0 Likes
Reply
Esteemed Contributor

If you have an ohmic path from Vssa to the point at which you observe the OpAmp

   

output refered to that ground point then you will see swings below ground. That

   

usually would be caused by poor layout, high current in Vssa ground path, poor

   

contact. The Vssa spec, as with virtually any CMOS pin, is controlled by worst

   

case threshold voltage of parasitic diodes attached to pins on all virtuall all CMOS

   

devices.

   

 

   

The reference spec is as follows -

   

 

   

0 Likes
Reply
Esteemed Contributor

Preferred pinout choices -

   

 

   

www.cypress.com/   AN72382

   

 

   

Regards, Dana.

0 Likes
Reply
New Contributor II

GND+/- 0.5 was a mistake, Pls rectify..its Vssd instead!

   

input is fixed frequency 5K triangular pulse which 280mV Vp-p( 140mv above and 140mV below gnd nearly). Gain settings can be seen in the main .c code which is attached with the project. an 8 bit variable can set PGA gain  values among unity, 2, 8, 16 and 24 through USBUART. I've observed that-

   

> at unity gain 100mV above gnd with retained peak and 40mv below gnd with peak clipped off

   

>at 2 times gain 200mV above gnd with retained peak and 80mV below gnd with peak clipped.

   

> at 24 times gain 2.5V above gnd and 1V below gnd peak clipped

0 Likes
Reply
New Contributor II

and for the opamp( opamp_3) in the schematic was used just to test the maximum swing in the output from Vdda to Vssa.  Same pulses as mentioned in OP were fed to PIN1_6 of the opamp which is non inverting terminal while the inverting is  external gnd.

   

the output is Vp-p 4.8V square wave with 2.4V above 2.4V well below gnd.

0 Likes
Reply
Anonymous
Not applicable

 Don't think you should have signal below 0V. We normally have a "virtual" ground which is 1/2 Vcc for analog signals. 

0 Likes
Reply
Anonymous
Not applicable
        OK, Please refer this modification. Annotation parts is out of PSoC.   
0 Likes
Reply
Esteemed Contributor

The OpAmp intrinsically cannot swing below its output Vssa rail, otherwise

   

it is a free energy generaor we should all be using. However input can common

   

mode below Vssa, but not much.

   

 

   

So if you have a signal, fed o Vssa, that swings blow ground, and you do not want it

   

clipped/distorted then you have  to meet both CMR on  input and output. You do this

   

by offsetting input so that it meets both input CMR and G x Vin output CMR.

   

 

   

Two resistors can accomplish this, see attached calculator spreadsheet and article.

   

http://electronicdesign.com/article/analog-and-mixed-signal/use-excel-to-calculate-a-d-level-shifter...

   

http://www.psocdeveloper.com/forums/viewtopic.php?t=3839

   

 

   

Regards, Dana.

0 Likes
Reply
New Contributor II
        Thnx dana, thnx evryone... gr8 help.. 🙂   
0 Likes
Reply