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PSoC 5, 3 & 1

Anonymous
Not applicable

Hello Every body,

   

I am generating 3Vp-p from DAC inside PSOC 5LP. It looks very nice as long as i do not connect it with bridge (in order to excite the bridge). When i connect it, the voltage drops from 3 to 1/3 of that. I tried to place a voltage follower at the output of the DAC but as a result my signal becomes very noise and drops too (but not much).

   

I am failed to find the reason. Can any body have any idea?

   

I attached the snap shot of my design.

   

Looking forward for your suggestions.

   

Regards

   

Awais

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15 Replies
Bob_Marlowe
Expert II

When the WaveDAC voltage drops when connected to your design it is quite probable that the input resistance of your bridge is too low.

   

Connecting an amp/follower and getting more noise could be picked-up noise or something like a feed-back? Not quite sure.

   

 

   

Bob

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Anonymous
Not applicable

Hello Bob,

   

Thankyou for your reply.

   

The input resistance of the bridge is approx 350 ohms. The output voltage of the DAC Max of 3 V. So the current should be around 8.5mA. Even if we put the voltage follower inside the PSOC (the psoc opamp can deliver upto 25mA) we should be safe. But we are not.

   

Even in this configuration the voltage drops about 2/3.

   

Looking forward for your suggestion.

   

Regards

   

Awais

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Bob_Marlowe
Expert II

In my datasheet for the PSoC5 OpAmp I find 10mA as source or sink max current.

   

Moreover, the switch resistance internal to the PSoC5 to route the output of the opamp to the pin can be in the magnitude of your bridge resistance (depending on the choosen pin). Check this using the Ohm-meter int the analog viewe of the .cydwr-file.

   

 

   

Bob

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Anonymous
Not applicable

I measured the internal resistance but it show 750 ohms. If it is due to the internal resistance then it should not happend if i put the voltage follower outside the psoc. But it does. 

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ETRO_SSN583
Esteemed Contributor

You can see from this chart in datasheet output Z of an OpAmp is ~ 12 ohms.

   

That would not account, however, for the magnitude of the drop you are seeing.

   

 

   

The OpAmp is capable per datasheet +/- 25 mA.

   

 

   

Regards, Dana.

   

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ETRO_SSN583
Esteemed Contributor

Make sure in the Wavedac8 configurator, range selection, you select the buffered

   

parameter version. Also I assume you chose the Wavedac as a voltage output, not a

   

current output. If latter the output Z much higher.

   

 

   

Regards, Dana.

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Anonymous
Not applicable

Hello Dana,

   

I attached my configuration. Yes i selected buffered one. I want voltage as an output.

   

Regards

   

Awais

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Bob_Marlowe
Expert II

From Datasheet: (Picture does not embed, so attached)

   

The OpAmp follower may source/sink 25mA, but not at the level of the input voltage.

   

Be sure to use as output pin the defaulted pin for the amplifier, only this guarantees a Zero Ohm resistance (no switch needed)

   

 

   

Bob

   

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ETRO_SSN583
Esteemed Contributor

The OpAmp follower may source/sink 25mA, but not at the level of the input voltage.

   

 

   

Yes, it has a 12 ohm out put Z per the graph I posted. In fact NO OpAmp will follow exactly

   

the input V due to finite Aol and Output Z.

   

 

   

Be sure to use as output pin the defaulted pin for the amplifier, only this guarantees a Zero Ohm resistance (no switch needed)

   

 

   

The routing path is NOT 0 ohms. Its lower for the assigned pins, but not 0.

   

 

   

Regards, Dana.

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Bob_Marlowe
Expert II

Yes, that's right, Dana. Would be nice to have a Zero Ohms part in a PSoC, I would use it instantly for my modell maglev train ;-))

   

Nonetheless, in analog view the Ohmmeter shows ~0 Ohms.

   

 

   

Bob

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Anonymous
Not applicable

Oh Yes Bob and Dana, You are great :).

   

I got the solution :).

   

Now i am using the proper pin and my voltage is no more dropping any more :).

   

But can you tell me why i am getting a noisy signal if i attached an external voltage follower?

   

Thankyou so much Bob & Dana.

   

Regards

   

Awais

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ETRO_SSN583
Esteemed Contributor

The ohmmeter is a crude instrument as you know. Its not a Kelvin capable tool,

   

just a routing estimator from sysnthesizer/router.

   

 

   

Awais, the noise can be many things. Layout, coupling, ground bounce, poor

   

caps, bypassing.

   

 

   

How much noise are you experiencing ?

   

 

   

Use your scope on infinite persistence and look at grounds (referred to ground point entering board),

   

supply rails. Polymer Tants an order of magnitude better than standard tants, F vs Z, for bulk bypassing.

   

 

   

Regards, Dana.

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Anonymous
Not applicable

Its 2Vp-p.

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ETRO_SSN583
Esteemed Contributor

Look at its frequency to see if you can tie it to some timer, clock, counter you

   

are running.

   

 

   

If driving motor loads very suspicious of grounds and unclamped L loads.

   

 

   

2V P-P very large noise. Are you using large C loads, maybe the OpAmp

   

phase margin disappeared.

   

 

   

Your scope and probe set to the right attenuation when measuring noise ?

   

 

   

Regards, Dana.

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Anonymous
Not applicable

I am Driving a simple bridge circuit. Yes my scope and probes are setup at the right attenuation level. But i think you are right that this might be due to grounding.

   

 

   

Any ways, I will be using internal voltage follower for my signal.

   

Thankyou so much once again for helping me .

   

Regards

   

Awais

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